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The document discusses the role of plastics in the circular economy, detailing their history, classification, and the science behind polymers. It covers the evolution of plastics from rare materials to ubiquitous consumer goods, highlighting significant milestones in polymer science. Additionally, it addresses the challenges and advancements in recycling and sustainable development of plastic products.

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Vincent Voet, Jan Jager, Rudy Folkersma

Plastics in the
Circular Economy

|
Authors
Dr. Vincent Voet Dr. Rudy Folkersma
NHL Stenden University of Applied Sciences NHL Stenden University of Applied Sciences
van Schaikweg 94 van Schaikweg 94
7811 KL Emmen 7811 KL Emmen
The Netherlands The Netherlands
vincent.voet@nhlstenden.com rudy.folkersma@nhlstenden.com

Dr. Jan Jager

NHL Stenden University of Applied Sciences
van Schaikweg 94
7811 KL Emmen
The Netherlands
jan.jager@nhlstenden.com

ISBN 978-3-11-066675-5
e-ISBN (PDF) 978-3-11-066676-2
e-ISBN (EPUB) 978-3-11-066713-4

Library of Congress Control Number: 2020950461

Bibliographic information published by the Deutsche Nationalbibliothek

The Deutsche Nationalbibliothek lists this publication in the Deutsche Nationalbibliografie;
detailed bibliographic data are available on the Internet at http://dnb.dnb.de.

© 2021 Walter de Gruyter GmbH, Berlin/Boston

Cover image: Thongchai Saisanguanwong / iStock / Getty Images Plus
Typesetting: VTeX UAB, Lithuania
Printing and binding: CPI books GmbH, Leck

2 Introduction to polymer science | 9

2.5.2 Processing of plastics | 69

3.3.23 Polyhydroxyalkanoates (PHAs) | 168

4 Recycling of plastics | 205

4.6.2 Plastic packaging | 276

4.6.3 Mechanical recycling | 277
4.6.4 Chemical recycling | 278
4.6.5 Sustainable plastic-product development | 278

Acknowledgment | 281

Bibliography | 283

Index | 287
1 Towards a circular economy
1.1 History of plastics
Many consumer goods that we use in our daily life are made from plastics. Today, we
cannot imagine a world that exists without these versatile materials. Plastics help to
keep us safe and healthy, and they improve the shelf life of food products. They make
our everyday lives convenient in many ways. Halfway through the last century, how-
ever, we hardly used any plastics. The availability of plastic materials was fairly lim-
ited as was the number of possible applications. How did plastics transform from be-
ing so rare to becoming all around us?
In general, plastics consist of large molecules, named polymers, mixed with addi-
tives such as plasticizers, colorants, or flame retardants that improve the properties of
the final product. Polymer science is considered a relatively new field of study. Never-
theless, polymeric materials have been applied for many centuries. In fact, more than
3,000 years ago, the Olmecs, an ancient Mesoamerican civilization in Mexico, played
their “pok-ta-pok” game with a ball made from natural rubber. They are believed to
be the first to actually process polymers from nature. It was not until the 1840s that
Charles Goodyear in the United States patented the vulcanization of rubber using sul-
fur. After this discovery, rubber was soon adopted for multiple applications, includ-
ing tires and footwear. The first synthetic polymer that was not derived from plants
or animals but from fossil fuels was invented in 1907 by Leo Baekeland. His phenol
formaldehyde resin was unique due to its hardness and heat-resistant properties, and
it was named Bakelite® , after his inventor.
In 1920, Hermann Staudinger published his paper entitled “Über Polymerisation”,
in which he proposed that polymers are molecules with a high molar mass, composed
of a large number of small building blocks. The fundamental understanding of this
new class of materials, which Staudinger called macromolecules, lead to the develop-
ment of a wide variety of new synthetic plastics and is therefore often considered to be
the birth of polymer science. A few decades later, Staudinger was awarded the Nobel
Prize in Chemistry for his pioneering work.
Today’s commodity plastics, such as polyethylene (PE), polyvinylchloride (PVC),
polystyrene (PS), and polyamide (PA), all became commercially available in the 1930s
(Fig. 1.1). The extensive needs of the military during the Second World War further in-
creased the industrial growth of plastics, and other polymers were developed includ-
ing polyester and Dupont’s polytetrafluoroethylene (PTFE), better known as Teflon® .
Synthetic polymers, applied in plastics, rubbers, and fibers, became increasingly
popular during the second half of the 20th century, when more and more products
found their way to the market. Styrofoam™ was invented, and the polyethylene bag
made its first appearance, as well as Tupperware® , developed by Earl Silas Tupper
who cleverly promoted his air-tight polyethylene food containers through a network of
housewives. In the late 1950s, LEGO® patented its block coupling–decoupling system,

https://doi.org/10.1515/9783110666762-001
2 | 1 Towards a circular economy

Figure 1.1: Timeline presenting the rise of the age of plastics.

and Mattel unveiled the first Barbie doll. Later on, acrylic paints were developed and
polyethylene terephthalate (PET) beverage bottles were introduced. By the end of the
1970s, plastics were the most common type of material worldwide.
Nowadays, polymers can be tailored with high precision to meet the require-
ments of modern society. Annually, millions of tons of plastics are being produced
worldwide. The applications are diverse: from low-cost products such as clothing,
food packaging, and children toys to high-performance plastics in composites used in
spacecraft, mobile phones, membranes, and medical implants such as artificial heart
valves. Synthetic plastics seem to last forever. Unfortunately, this advantage turns out
to be a serious disadvantage with respect to our environment. Since most plastics
decompose very slowly, if at all, plastic waste can stay in landfills for thousands of
years. Even more concerning is the part that leaks into our ecosystem, like the (mi-
cro)plastics that find their way to oceans forming floating junkyards better known
as plastic soup. These problems need to be addressed, and current efforts focus on
(micro)biologically degradable polymers, industrially compostable materials, and the
process of recycling plastics [1].

1.2 Introduction to the plastics economy

The largest fraction of industrial polymers is derived from petroleum. Crude oil that
comes out of the soil contains a multitude of components: various hydrocarbons,
mostly alkanes, cycloalkanes, and various aromatic hydrocarbons. In an oil refinery,
the crude oil is separated into different components, and large molecules are broken
down into smaller molecules. This is a process referred to as steam cracking. The petro-
chemical industry receives the refined oil and creates monomers through chemical
reactions and polymers through polymerization reactions. Next, the plastics industry
produces all sorts of products from those polymeric materials. The production process
is schematically depicted in Fig. 1.2. Plastics are usually not unambiguous materials.
There are many distinguishable types and numerous grades of plastics available on
the market, each with its own characteristics, properties, and applications. Most of-
ten, plastics are designed for a specific application, by choosing the right combination
1.2 Introduction to the plastics economy | 3

Figure 1.2: Schematic production process for plastics from crude oil.

of polymer and additive type, to meet the demands for that application in the most
efficient manner.
Plastics are produced in very large volumes. According to Plastics Europe, the as-
sociation of plastics manufacturers in Europe, 359 million metric tons of plastics were
produced worldwide in 2018 [2]. This enormous pile of materials equals 7,000 Titanic
ships, or circa 35,000 Eiffel Towers. These plastics include thermoplastics, thermosets,
elastomers, adhesives, coatings, sealants, and certain fibers. In past years, the global
production capacities of plastics increased enormously. In contrast, only 1.5 million
metric tons of plastics were produced worldwide in 1950.
The largest share of the plastics production volume is melt processable plastics,
so-called thermoplastics. Polyethylene (PE, 32 %), polypropylene (PP, 23 %), polyvinyl
chloride (PVC, 16 %), polystyrene (PS, 7 %), and polyethylene terephthalate (PET, 7 %)
comprised 85 % of the total global production volume of thermoplastics in 2015. As-
suming that the current world population is about 7.8 billion people (December 2019),
this means that on average more than 46 kg of plastics are produced annually per
capita of the world population. We can expect that the production volumes of plastics
will increase even further in upcoming years. Some recent studies even predict that the
total production volume will double in the next 20 years. It has been estimated that in
total about 8,300 million metric tons of virgin plastics have been produced globally to
date.
China is the largest producer of plastics in the world, producing 108 million metric
tons in 2018, which equals a 30 % market share. As shown in Fig. 1.3, about half of the
production volume of global plastics takes place in Asia, followed by North America
(18 %) and Europe (17 %).
According to Plastics Europe, the total converter demand for plastics amounts to
51 million metric tons in Europe. The European plastic converter demand includes both
thermoplastics and polyurethanes and some other plastics (thermosets, adhesives,
coatings, and sealants). In terms of market segments, almost 40 % of all plastics are
used in the packaging industry. Other important sectors are building and construc-
tion, automotive, electronics, household, and agriculture. In addition, there are a mul-
titude of other applications for which smaller quantities of plastics are used. Examples
are medical equipment, plastic furniture, and technical parts used for mechanical en-
4 | 1 Towards a circular economy

Figure 1.3: Distribution of the

global plastics production in
the world in 2018, according to
Plastics Europe [2].

Table 1.1: Plastics converter demand by market sector in 2017 [2].

Market sector Total European converter Percentage

demand by market sector in (%)
2017 (million metric tons)

Packaging 20.3 39.7

Building and construction 10.1 19.8
Automotive 5.2 10.1
Electrical and electronic 3.2 6.2
Household, leisure, and sports 2.1 4.1
Agriculture 1.7 3.4
Others (e. g., medical) 8.6 16.7

gineering. The total European converter demand by market sector in 2017 is presented
in Tab. 1.1.
Only a few types of plastics are used in the packaging industry. Various grades of
polyethylene (HDPE, MDPE, LDPE, LLDPE), polypropylene (PP), polyethylene tereph-
thalate (PET), polyvinyl chloride (PVC), and (expanded) polystyrene ((E)PS) are the
major players in the packaging industry (Tab. 1.2).
In 1988, the Society of the Plastics Industry (SPI) introduced the Resin Identifica-
tion Code (RIC) system in an effort to develop consistency in plastics manufacturing
and (recycled) plastics reprocessing. The Resin Identification Code assigns a number
from 1 to 7, with a “chasing arrows” symbol around the number, to a piece of plas-
tic to indicate its type. This coding system was developed for behind-the-scenes staff
performing recycling of plastic household waste, to improve the sorting and recycling
processes. Note that the coding system was never intended as a consumer communi-
1.3 The new plastics economy | 5

Table 1.2: Plastic types used in the European packaging industry, including their Resin Identifica-
tions Codes (RIC) [2].

Plastic type Packaging market Resin Identification

share in Europe (%) Code (RIC)

LDPE/LLDPE 31.1 4
PP 22.0 5
PET 18.9 1
HDPE/MDPE 18.3 2
(E)PS 5.7 6
PVC 2.1 3
Others 1.9 7

Figure 1.4: Resin identification codes (RICs) according to the coding system of the Society of the
Plastics Industry (SPI).

cation tool. Consumers, however, often assume that this code indicates that a certain
piece of plastic household waste is automatically recyclable, while that is not the case.
A piece of plastic with the code may or may not be recyclable, as shown in Fig. 1.4. The
Resin Identification Code is currently under the control by ASTM International and is
covered in a new international standard.

1.3 The new plastics economy

Clearly, plastics are an integral part of our global economy. Besides all the benefits, the
current plastics economy has led to serious (environmental) drawbacks as well. Mil-
lions of tons of plastics end up in the ocean each year. This equals the content of one
garbage truck every minute. According to the Ellen MacArthur foundation, our oceans
will contain more plastics than fish (by weight) in 2050. More than 30 years after the
launch of the RIC coding system (Fig. 1.4), still only 14 % of plastic packaging is col-
lected for recycling. Furthermore, at the current global production volume, about 6 %
6 | 1 Towards a circular economy

of available crude oil is employed in the production of plastic products. If the strong
growth of production volumes continues as expected, the consumption of crude oil by
the entire plastics industry will account for 20 % of the total crude-oil consumption by
2050.
At the beginning of 2016, a report was published with the title “The New Plastics
Economy— Rethinking the Future of Plastics”, issued by the Ellen MacArthur Foun-
dation [3]. It envisions a time when plastic materials never become waste. Instead,
plastics should re-enter the economy as valuable biological or technical feedstock,
which aligns with the principles of a closed-loop economy, better known as the circu-
lar economy. In 2018, the Ellen MacArthur Foundation launched a new initiative, the
New Plastics Economy Global Commitment. This initiative unites businesses, govern-
ments, and other organizations behind a common vision and targets to address plas-
tic waste and pollution at its source. Mid-2019, over 400 organizations have signed the
New Plastics Economy Global Commitment, like Apple, Phillips, Unilever, Coca-Cola,
and Walmart. Applying the principles of a circular economy, the New Plastics Econ-
omy brings together key stakeholders to rethink and redesign the future of plastics,
starting with packaging. The initiative advocates several ambitions, summarized in
the following actions:
1. “Create an effective after-use plastics economy by improving the economics and
uptake of recycling, reuse, and controlled biodegradation for targeted applica-
tions.” (This is the cornerstone of the New Plastics Economy and its first priority
and helps realize the two following ambitions.)
2. “Drastically reduce leakage of plastics into natural systems (in particular the
ocean) and other negative externalities.”
3. “Decouple plastics from fossil feedstocks by—in addition to reducing cycle losses
and dematerializing—exploring and adopting renewably sourced feedstocks
(biomass).”

In contrast to a linear economy that refers to a society that takes, makes, and disposes,
the circular economy is both restorative and regenerative by design. In Fig. 1.5, the
butterfly model as proposed by the Ellen MacArthur Foundation is shown. The gen-
eral principle is to optimize resource yield by circulating products and materials at all
times.
According to the United Nations (UN), the concept of circular economy, in which
natural systems are regenerated, materials are kept in use, and waste does not exist,
can contribute significantly to the implementation of 2030 Agenda for Sustainable De-
velopment (Fig. 1.6), translated into the 17 Sustainable Development Goals (SDGs) [4].
A closed-loop economy holds promise for achieving SDG 7 (Affordable and Clean En-
ergy), 8 (Decent Work and Economic Growth), 11 (Sustainable Cities and Communi-
ties), 12 (Sustainable Consumption and Production), 13 (Climate Action), and 14 and
15 (Life below Water and on Land). Clearly, the transition from a linear to a circular
economy requires the joint effort of companies, policy makers, and research institutes.
1.3 The new plastics economy | 7

Figure 1.5: The circular-economy model, proposed by the Ellen MacArthur Foundation, representing
a closed-loop system with biological and technical cycles of nutrients [3].

In June 2019, the World Economic Forum (WEF) published ten emerging technolo-
gies [5] that have the potential to provide major benefits to the global society and econ-
omy. Prior to this, an international steering committee of leading technology experts
evaluated dozens of proposals from experts around the globe. In the end, the “Top
10 Emerging Technologies 2019” were identified. The topmost emerging technology
was Bioplastics for a Circular Economy. According to the WEF, the development of bio-
based and/or biodegradable plastics can contribute to the goal of a closed-loop plastic
economy in which plastics are derived from and converted back to biomass feedstock.
Even more recently, in December 2019, the European Commission adopted the so-
called European Green Deal. The European Green Deal is a set of policy initiatives with
the overarching aim of making Europe climate neutral in 2050. In fact, it is the new
roadmap for making the EU’s economy sustainable. The Green Deal also includes a
“circular-economy action plan” [6] that identifies initiatives along the entire life cycle
of products, targeting, for example, their design, promoting circular-economy pro-
cesses, fostering sustainable consumption, and aiming to ensure that the resources
used are kept in the EU economy for as long as possible. This document states that
“The Commission will develop requirements to ensure that all packaging in the EU
market is reusable or recyclable in an economically viable manner by 2030, will de-
velop a regulatory framework for biodegradable and bio-based plastics, and will im-
plement measures on single use plastics.”
8 | 1 Towards a circular economy

Figure 1.6: The Sustainable Development Goals (SDGs), as proposed by the United Nations in the
2030 Agenda for Sustainable Development. Image used under license from Shutterstock.com,
ID 1538140988.

1.4 The story continues

The closed-loop economy model presented in Fig. 1.5 distinguishes between biolog-
ical (or green) and technical (or blue) cycles. In the biological cycle, food and bio-
based materials are designed to return through processes like anaerobic digestion and
composting. A biological cycle regenerates living systems that provide renewable re-
sources. On the other hand, the technical cycle aims to recover and restore materials
and products. This is achieved through strategies such as reuse, repair, refurbish, and,
eventually, recycling.
The structure of this book follows the circular economy model and its two cycles.
First, an introduction to polymer science is given in Chapter 2 to communicate ba-
sic knowledge on the structure and behavior of plastic materials. It discusses various
ways to make polymers and explores the unique structure–property relationship of
macromolecules. Bio-based and biodegradable plastics are the main subject of Chap-
ter 3. This chapter explores the processing of renewable feedstocks into biochemicals
and ultimately bioplastics. In other words, the role of plastics in the biological cycle is
discussed. Chapter 4 focuses on the mechanical and chemical recycling of plastics, as
well as circular product development. The recycling of plastic products is part of the
technological cycle. To summarize, in this book we will discover the role of “Plastics
in the circular economy”.
2 Introduction to polymer science
2.1 Classification
The success of plastics can be attributed to their low costs and wide range of prop-
erties that can be changed by tweaking the macromolecular structure. This unique
structure–property relationship will receive specific attention in the remainder of this
chapter. We will start by discussing various ways to categorize polymer materials.
The term polymer is derived from the Greek words πoλυς́ (polus = “many”) and
μερoς
́ (meros = “part”). Polymers can be defined as large molecules, sometimes re-
ferred to as macromolecules, composed of a large number of repeating units. The exact
number required to meet the definition of a polymer depends on the size and chemical
structure, but a good rule of thumb is at least 50 repeating units. Smaller molecules
are named oligomers, which translates to “few parts”.
The building blocks that form a polymer molecule are called monomers, literally
meaning “one part”. Monomers can undergo polymerization, in which they react to-
gether to form a polymer chain or network structure. Polymerizations are the subject
of Section 2.3.
Considering the definition of a polymer, a tremendous amount of possibilities
arise: flexible or stiff polymers, polymers derived from nature or those made in fac-
tories, polymers that absorb or repel water, polymers applied in airplanes or used
in clothing, etc. To create some order, polymers are classified based on various cat-
egories, as displayed in Fig. 2.1.

2.1.1 Origin

A wide variety of macromolecules is present in nature. Polysaccharides like starch and

cellulose are formed in plants, while proteins such as silk and casein can be found in
animals. Polynucleotides, known as DNA and RNA, play a crucial role in storing, cod-
ing, expressing, and regulating genetic information. While some natural polymers can
be used directly as technical materials when harvested, other polymers need to be
modified. For example, cellulose can be transformed into cellophane films or rayon
and viscose fibers via the xanthate process. The hydroxyl groups in cellulose are mod-
ified in a basic environment, and cellulose is regenerated from viscose using acid.
Also, technical rubber is produced by crosslinking latex from rubber trees via a process
named vulcanization. Macromolecules made by chemically treating natural polymers
are named semi-synthetic polymers [7].
Nevertheless, most polymers have a fully synthetic origin. They are created by the
polymerization of monomers into macromolecules. One monomer polymerizes into a
homopolymer, while two or more chemically different monomers make a copolymer

https://doi.org/10.1515/9783110666762-002
10 | 2 Introduction to polymer science

Figure 2.1: Classification of polymers based on origin, chemistry, (physical) properties, polymeriza-
tion route, and application.

(Section 2.2.1). Polystyrene (PS), polyethylene (PE), polyethylene terephthalate (PET),

and acrylonitrile-butadiene-styrene (ABS) are all examples of synthetic polymers.

2.1.2 Chemistry

Based on the chemical building blocks along the polymer chain, polymers can be sub-
divided into two classes: organic and inorganic macromolecules. In general, organic
polymers are based on the element carbon (C). Their skeletal structure mainly consists
of C, H, O, and N atoms. The vast majority of polymers is considered organic, including
most synthetic polymers such as polyolefins, polyamides, polyesters, and acrylics.
In the most distinctive definition, inorganic polymers have a backbone that does
not include carbon atoms at all. For instance, polydimethyl siloxane, better known as
silicone rubber (when crosslinked), is composed of a –Si–O– backbone with methyl
side groups. On the other hand, polyphosphazenes carry P and N atoms in the re-
peating unit. Polymers containing both inorganic and organic components, like DNA
having organophosphate functionalities, are officially named hybrid polymers [8].
2.1 Classification | 11

2.1.3 Properties

Related to the molecular structure, some polymer materials have the ability to flow at
elevated temperatures. Thermoplastics are composed of separate linear or branched
polymer chains. Approximately 90 % of all plastics belong to this category, like
polyethylene terephthalate (PET), polyethylene (PE), polymethyl methacrylate
(PMMA), and polyamides (PAs). Upon heating, these materials can be processed
in the molten state. Upon cooling, they return to their solid state. This cycle can be
repeated, which enables the (mechanical) recycling of thermoplastic materials.
On the other hand, thermosets are crosslinked systems. Formation of the network,
the so-called curing reaction, is typically initiated by heating or UV irradiation. Due to
the crosslinked chains, a cured thermoset cannot be molten or dissolved. This type of
materials is less suitable for recycling in a conventional way. The first synthetic poly-
mer Bakelite® is a thermosetting phenol formaldehyde resin.
In between thermosets and thermoplastics, elastomers constitute weakly
crosslinked systems. Elastomers, commonly known as rubbers, are recognized by
their elastic behavior. Depending on the nature of the crosslinks, we can distinguish
thermoplastic elastomers (TPEs) and thermoset rubbers, as will be explained in Sec-
tion 2.2.2.

2.1.4 Polymerization

Chemists prefer to define macromolecules based on the manner in which the polymer
chains are constructed during polymerization. Step-growth polymerization usually in-
volves a condensation reaction. Classic examples are the reaction between multifunc-
tional acids with multifunctional alcohols, or amines, resulting in the formation of
polyesters and polyamides, respectively. The corresponding products are referred to
as condensation polymers. Most chain-growth polymerizations occur via continuous
addition of double bonds, for instance, during polymerization of ethylene (E), propy-
lene (P), or styrene (S). Hence, polyethylene (PE), polypropylene (PP), and polystyrene
(PS) are all addition polymers.

2.1.5 Application

Defined by the polymer industry, the most common areas of application are plastics,
fibers, and rubbers. The plastics industry comprises packaging, consumer goods, coat-
ings, and adhesives. Technical or textile fibers can originate from natural resources,
like semi-synthetic rayon (or viscose) fibers, or fossil resources, such as synthetic
polyester yarn. The rubber industry comprises both thermoplastic as thermoset elas-
tomers.
12 | 2 Introduction to polymer science

2.1.6 Nomenclature

The ultimate system of classification is, of course, naming. However, various termi-
nologies have been developed in the polymer science community that can be rather
confusing for outsiders. Table 2.1 provides an overview of the most common ways of
nomenclature.
In general, macromolecules can be named for the typical functional groups
within their repeating units, usually combined with the prefix “poly-”. For instance,
polyamide essentially refers to any polymer material with amide functionalities in
the main molecular chain. In that sense, both proteins and nylons are designated as
polyamides. Similar, the name fluoropolymer can refer to either polytetrafluoroethy-
lene (PTFE), polyhexafluoropropylene (PHFP), or polyvinylidene fluoride (PVDF). In
other words, naming polymers by functionality is rather unspecific.
While (low molar mass) organic chemical compounds are systematically named
by IUPAC nomenclature [9], another more practical terminology has been developed
in polymer science. The basic principles of polymer nomenclature is founded on the
original building blocks, for example, polymerization of ethylene leads to polyethy-
lene, abbreviated as PE. In a similar way, methyl methacrylate (MMA) polymerizes
into PMMA: polymethyl methacrylate. In certain cases, the repeating unit is used in-
stead of the monomer. While the industrial synthesis of silicon rubber starts with
dimethyldichlorosilane, PDMS stands for polydimethylsiloxane.
Occasionally, nomenclature based on building blocks is rather impractical for
daily use. A good alternative in that case is the introduction of trade names. For in-
stance, poly(para-phenylene terephthalamide) (PPTA) is better known worldwide as
Kevlar® or Twaron® , the product names of PPTA given by DuPont and Teijin Aramid,
respectively. Other examples include Perspex® (PMMA) from ICI and Teflon® (PTFE)
from DuPont.

2.2 Structure of macromolecules

The previous paragraph already sketched the large diversity in the world of macro-
molecules. The chemical building blocks in the polymer backbone, the length of the
chain, and the architecture of the macromolecules, all factors that influence the per-
formance of polymers. It creates a valuable toolbox for polymer chemists and engi-
neers to selectively change the properties by controlling the structure of the macro-
molecular chain.
2.2 Structure of macromolecules | 13

Table 2.1: Nomenclature of polymers, based on the building block(s), general structure, or industrial
trade name.

Structural formula Specific name Abbreviation General name Trade name

Polyethylene or PE
Polyethene

Polystyrene PS

Polyethylene PET Polyester Terylene®

terephthalate

Polydimethyl PDMS Silicone rubber

siloxane

Polymethyl PMMA Acrylic Perspex®

methacrylate Plexiglass®

Polycaprolactam PA Polyamide Nylon

Poly(para- PPTA Aramid Kevlar®

phenylene Twaron®
terephthalamide)

Polytetrafluoro- PTFE Fluoropolymer Teflon®

ethylene

2.2.1 Chain composition

A polymer is composed of building blocks that repeat along the (linear) chain. A chain
that consists of only one single monomer type is called a homopolymer. The structural
formula of homopolymers can be represented by depicting its repeating unit placed
14 | 2 Introduction to polymer science

Figure 2.2: Schematic representation of ho-

mopolymer with “n” repeating units.

within square brackets (Fig. 2.2). The number of repeating units, also referred to as the
degree of polymerization, is indicated by the subscript “n”.
Macromolecules composed of at least two chemically distinct monomers are
called copolymers. An infinite number of different copolymers can be synthesized
by altering the types of monomers, as well as the chain sequence (Fig. 2.3). For in-
stance, if two monomers are polymerized in a statistically determined manner, a
random copolymer (or statistical copolymer) is realized. Random copolymers are of-
ten denoted as poly(A-co-B), where A and B are two different monomers. The exact
placement of monomers along the chain is determined by the A-to-B ratio and their
relative reactivity, as will be explained in more detail in Section 2.3.3.

Figure 2.3: Schematic representation

of various copolymers with different
monomer sequences.

If two monomers are built into the chain in an alternating fashion, the resulting macro-
molecule is an alternating copolymer, or poly(A-alt-B). Although commonly referred to
as a homopolymer, polyethylene terephthalate (PET) is better described as an alternat-
ing copolymer resulting from the polycondensation of ethylene glycol and terephthalic
acid. Block copolymers on the other hand are composed of clusters of identical repeat-
ing units along the backbone. Those clusters are referred to as blocks. Poly(A-block-
B) is called a diblock copolymer, while poly(A-block-B-block-A) and poly(A-block-B-
block-C) are triblock copolymers. Block copolymers can show intriguing phase behav-
iors at the nanoscale due to their well-defined molecular structure. Finally, if polymer
blocks are attached as side chains to a different polymer, the resulting structure is
called a graft copolymer or poly(A-graft-B). There are various ways to synthesize graft
copolymers.
Copolymers synthesized from three chemically distinct monomers are named
terpolymers. A famous example is acrylonitrile-butadiene-styrene (ABS), industrially
made by polymerizing acrylonitrile (A) and styrene (S) in the presence of polybutadi-
ene (B). The result is a graft copolymer having a polybutadiene backbone with side
chains of a styrene-acrylonitrile copolymer. ABS combines the chemical inertness and
2.2 Structure of macromolecules | 15

heat resistance of acrylonitrile with the toughness of butadiene, while styrene adds
to processability and rigidity. By varying the building blocks, the properties of ABS
can be tuned as desired. Leaving out one of the comonomers leads to poly(styrene-co-
acrylonitrile) (SAN), poly(styrene-co-butadiene) rubber (SBR), or poly(acrylonitrile-
co-butadiene) rubber (NBR).

2.2.2 Chain architecture

Besides chemical diversity, the properties of polymers can be influenced by their

(macro)molecular architecture. Thermoplastic polymers consist of either linear or
branched macromolecules, or both (Fig. 2.4). For instance, starch, a polysaccharide
that is produced by plants as energy storage consists of both linear amylose and
branched amylopectin. Synthetic polyethylene can be linear or branched depending
on the polymerization method. While free radical polymerization leads to a highly
branched structure referred to as low-density polyethylene (LDPE), coordination
polymerization results in high-density polyethylene (HDPE) with a more linear chain
architecture. Methods of polymerization are explained further in Section 2.3.

Figure 2.4: Schematic representation of linear, branched, and network chain architectures.

The molecular architecture of thermoset polymers is best described as a network. In

principle, such a covalent network is in fact one large macromolecule. The network
structure can be formed in various ways. One route is the covalent binding of sep-
arate linear chains (prepolymers) by reactive groups, so-called crosslinking agents
(Fig. 2.5(a)). The vulcanization of rubber using sulfur, invented by Charles Goodyear,
follows this mechanism. The density of the crosslinked matrix depends on the con-
centration of crosslinking agents.
Network formation can also be achieved via polymerization in the presence of
monomers with a functionality of three or more (Fig. 2.5(b)). For instance, polycon-
16 | 2 Introduction to polymer science

Figure 2.5: (a) Polymer network formation via (pre)polymerization followed by crosslinking reactions,
and (b) polymer network formation via copolymerization in the presence of trifunctional monomers.

densation of terephthalic acid with (trifunctional) glycerol instead of (difunctional)

ethylene glycol leads to a macromolecular network structure.
Not all network polymers are thermosetting materials. Depending on the type of
crosslinks, a system may show thermoplastic behavior as well. While conventional
rubbers rely on a covalently bonded network with so-called chemical crosslinks, elas-
tomers such as thermoplastic polyurethane (TPU) and polystyrene (TPS) rubber con-
tain physical crosslinks. At lower temperatures, specific chain segments cluster in hard
domains that behave as crosslinks. However, in contrast to covalent crosslinks, those
domains are not stable at higher temperatures. In fact, the material becomes soft
again, making a thermoplastic elastomer reprocessable. Upon cooling, the physical
crosslinks return, i. e., the network structure of thermoplastic elastomers is reversible.

2.2.3 Chain regularity

The configuration along a polymer chain will influence the spatial (three-dimensional)
arrangement and consequently the thermal and mechanical properties of a material.
Like small molecules, macromolecules can exist as isomers. In other words, two poly-
2.2 Structure of macromolecules | 17

mer chain segments can be represented by the same molecular formula, but different
structural formulas. Below we discuss a few examples.

2.2.3.1 Constitutional isomers

In constitutional isomerism, or structural isomerism, the atoms are connected in
a different order. Considering the polymerization of a vinyl monomer like propy-
lene, the monomer can attach to the growing chain in two manners. When it re-
acts in the same orientation as the previous monomer, a head-to-tail connection
(–CH2 -CHR–CH2 -CHR–) is formed (Fig. 2.6). Head-to-tail polymerization leads to a
regular chain configuration, in which every second carbon atom along the chain pos-
sesses a methyl side group (R). Head-to-head (–CH2 -CHR–CHR-CH2 –), or tail-to-tail
(–CHR-CH2 –CH2 -CHR–) connections may also occur and result in structural isomers.
Nevertheless, head-to-tail polymerization is usually favored, especially in case of
bulky side groups [10].

Figure 2.6: Examples of structural isomers and stereoisomers in macromolecular chains.

2.2.3.2 Stereoisomers
In case of stereoisomerism, or spatial isomerism, the polymer segments have the same
sequence of bonded atoms—that is, the same constitution—but differ in spatial form.
If the polymer is partially unsaturated, the chain can adopt either a cis- or trans con-
figuration at the double bonds (Fig. 2.6). This seemingly small difference at the molec-
ular level can have a surprisingly large effect on the macroscopic scale. For example,
18 | 2 Introduction to polymer science

poly(cis-1,4-isoprene), better known as natural rubber, is soft, sticky, and elastic. In

contrast, poly(trans-1,4-isoprene), or gutta percha, is a much harder and non-sticky
material due to the more dense (crystalline) structure of the chains.
Enantiomers are a specific type of stereoisomers that originate from the presence
of asymmetric carbon atoms. Let us examine the head-to-tail polymerization of propy-
lene once more. Every second carbon atom along the chain is asymmetric. The methyl
side group at such a chiral center can be oriented in two directions with respect to the
polymer backbone. The two options are referred to as the R- and S-configuration. Rep-
etition of an R/S configuration can result in different stereoisomers of polypropylene
(PP): isotactic, syndiotactic, or atactic (Fig. 2.6). In isotactic PP (PP-it), all asymmet-
ric carbon atoms are configured in the same way. For instance, by this definition, a
natural protein is isotactic with all side groups in the S configuration. Syndiotactic PP
(PP-st) reveals an alternating orientation of methyl side groups, while in atactic PP
(PP-at) the distribution of R and S configuration is completely random. Again, small
differences at the molecular scale have important macroscopic consequences. While
both PP-it and PP-st are (semi)crystalline materials with melting temperatures in the
range of 150–170 °C, the irregular structure of PP-at cannot form a crystal lattice and is
therefore an amorphous material without a melting point. The tacticity in a polymer
chain can be directed by applying so-called stereospecific polymerization methods,
as will be discussed in Section 2.3.2.3.

2.2.4 Chain length

The length of a polymer chain is quantified by the degree of polymerization (P). P is

the number of repeating units in a macromolecule. By this definition, a monomer has
a degree of polymerization of 1, while for a dimer P equals 2. In general, molecules are
named macromolecules or polymers, when P is about 50 or higher.
Another parameter that is used to define the length, or to be more specific the size
of a polymer molecule, is its molar mass (M), in g/mol. The molar mass, in polymer
science traditionally referred to as molecular weight, equals the product of the degree
of polymerization and the molar mass of one repeating unit (m0 ), in g/mol, as depicted
in equation (2.1).

M = m0 ⋅ P. (2.1)

Clearly, molar mass increases with increasing degree of polymerization. The size of
the macromolecule affects the thermophysical properties of the material. Consider the
polymerization of ethylene as a model reaction (Tab. 2.2). The dimer is a gas, like the
monomer itself. However, when increasing the molar mass, the polymerization prod-
ucts become liquid (P = 3–8) and eventually solid (P > 8).
In practice, polymerization often leads to a mixture of macromolecules with dif-
ferent chain lengths and architecture [11]. In this case, the distribution of chains can
2.2 Structure of macromolecules | 19

Table 2.2: The length of a polymer chain can be expressed in degree of polymerization or molar
mass (molecular weight). The molar mass is calculated directly from the degree of polymerization by
equation (2.1).

Molecular formula Degree of Molar mass Chemical name State of matter at

polymerization (g/mol) ambient conditions

CH2 =CH2 1 28 Ethylene Gas

CH3 -CH2 -CH2 -CH3 2 56 Butane Gas
CH3 -(CH2 -CH2 )3 -CH3 4 112 Octane Liquid
CH3 -(CH2 -CH2 )7 -CH3 8 224 Hexadecane Liquid
CH3 -(CH2 -CH2 )15 -CH3 16 448 Dotriacontane Solid
CH3 -(CH2 -CH2 )31 -CH3 32 896 Tetrahexacontane Solid
(CH2 -CH2 )n >50 >1,400 Polyethylene (PE) Solid

Figure 2.7: A mixture of chains can be defined by various averages: (a) number average molar mass
and (b) weight average molar mass. The molar mass of the repeating unit (m0 ) is 50 g/mol.

be defined by an average value. Let us imagine a hypothetical polymer sample of five

molecules (Fig. 2.7): four chains with a molar mass of 250 g/mol (P = 5) and one
chain with a molar mass of 1,000 g/mol (P = 20). The molar mass of the repeating
unit is 50 g/mol. An intuitive manner of calculating the average is adding all molec-
ular weights together and dividing by the total number of molecules. The answer of
400 g/mol is referred to as the number average molar mass (Mn ), in g/mol.
∑ Ni Mi
Mn = = ∑ ni Mi . (2.2)
∑ Ni
The formula to calculate Mn is contained in equation (2.2). Here, Ni is the number of
molecules having a molar mass Mi . The number average molar mass Mn can also be
calculated using the mole fraction (ni ), like in the example of Fig. 2.7(a).
Besides the number average, there are other valid methods to calculate averages.
For instance, taking into account the mass of each chain provides another perspective
20 | 2 Introduction to polymer science

for our polymer sample. Half of the mixture consists of macromolecules with a molar
mass of 1,000 g/mol, and the other half consists of polymer chains with a molar mass
of 250 g/mol (Fig. 2.7(b)). The resulting average of 625 g/mol is referred to as the weight
average molar mass (Mw ), in g/mol.
The weight average molar mass Mw (equation (2.3)) equals the sum of the products
of the mass fraction (wi ) and molar mass (Mi ).

∑ Ni Mi2
Mw = = ∑ wi Mi . (2.3)
∑ Ni Mi

By definition, Mn ≤ Mw . Other averages can be calculated, but in practice Mn and Mw

are most often used. As indicated, both of them provide unique information about the
distribution of polymer chains in a sample. The (unitless) quotient of weight and num-
ber average molar mass, named polydispersity (D), is a measurement for the broadness
of the molar mass distribution (equation (2.4)).

Mw
D= . (2.4)
Mn

A large polydispersity relates to a broad distribution of polymer chains. In the example

in Fig. 2.7, the polydispersity is 1.6. If all macromolecules in a sample have the exact
same length (molar mass), Mw and Mn are equal. In that case, the polydispersity equals
1 and the sample is named monodisperse. For synthetic polymers, this is very unlikely
to occur. However, natural polymers such as DNA and specific proteins can be exactly
reproduced within organisms, making them monodisperse.
Similar to molar mass, an average for the degree of polymerization can be de-
termined. Following from equation (2.1), the number average Pn (equation (2.5)) and
weight average Pw (equation (2.6)) relate to the respective molar masses, in g/mol.

Mn = m0 ⋅ Pn , (2.5)
Mw = m0 ⋅ Pw . (2.6)

The molar mass and polydispersity of polymers is strongly influenced by the method
of polymerization, as will be discussed in Section 2.3.

2.2.5 Characterization of macromolecules

Clearly, the structure and size of macromolecules greatly affect the polymer proper-
ties. Detailed characterization of composition, configuration, and molar mass (distri-
bution) is of great importance to predict product performance.
2.2 Structure of macromolecules | 21

2.2.5.1 Structure determination

Spectroscopic methods are a powerful tool to determine the chemical structure and
chain regularity in polymer samples. Nuclear magnetic resonance (NMR) spectroscopy
measurements can reveal the composition of chemical groups along the polymer
chain in great detail. For instance, NMR is used to analyze the copolymer ratio in ran-
dom copolymers. Furthermore, the extent of head-to-tail coupling can be determined,
as well as isotactic and syndiotactic sequences along the polymer chain. Additionally,
Fourier transform infrared (FTIR) spectroscopy can be used to determine parameters
such as degree of branching, crystal structure, cis/trans configuration, and tacticity.

2.2.5.2 Molar mass determination

Various techniques have been developed to calculate the molar mass of polymers.
A popular method to do so is end group determination. Quantitative analysis of end
groups can be done either chemically, via titration, or by spectroscopic methods such
as NMR. For the latter, the intensity of the signal(s) corresponding to the end group(s)
is compared to the intensity of the signal(s) from the repeating units of the polymer
chain. From the ratio between both signals, the number average degree of polymer-
ization can be calculated (Fig. 2.8). The Mn follows from equation (2.5). Note that end
group analysis is only applicable when the chemical structure and number of end
groups per chain are known.

Figure 2.8: Quantitative analy-

sis of end groups can be used
to determine degree of poly-
merization (and molar mass)
of a polymer sample.

Dissolution of a polymer in solvent results in changes of properties such as the freez-

ing point, boiling point, vapor pressure, and osmosis. These are together referred to as
colligative properties [12]. In a laboratory, colligative properties are used to determine
the molar concentration of the solute, in this case, the polymer. If the mass of the poly-
mer is also known, its molar mass can be determined. In fact, membrane osmometry
is an important method to provide Mn .
Other absolute methods to determine molar mass are (static) light scattering and
ultracentrifugation. From light scattering experiments, the Mw can be calculated. Ul-
22 | 2 Introduction to polymer science

tracentrifugation enables determination of molar mass distributions, and thus poly-

dispersity, as well.
In contrast to the absolute methods just described, relative methods for molar
mass determination need calibration by monodisperse samples with a known molar
mass. Nevertheless, those characterization techniques are still very relevant and ap-
plied often. A famous example in polymer science is viscometry. When the concentra-
tion of the polymer in solution increases, it leads to a higher viscosity of that solution.
However, the viscosity also increases for polymers with a higher molar mass. This ef-
fect is used in viscometry measurements. Using glass capillary viscometers, such as
Ostwald or Ubbelohde, the intrinsic viscosity can be directly derived from the elution
time of the polymer solution (measured in a concentration series). The viscosity aver-
age molecular weight (Mv ), in g/mol, can be derived from the intrinsic viscosity ([η]),
in cm3 /g, via the Mark–Houwink equation (equation (2.7)).

[η] = KMva . (2.7)

Here, K and a are the so-called Mark–Houwink parameters. Both are dependent on
the polymer, solvent and temperature, which means Mv is not an absolute quantity
as Mn and Mw . Clearly, during viscometry measurements, temperature needs to be
controlled precisely.

2.2.5.3 Determination of molar mass distribution

Molar mass distributions can be obtained via techniques that separate the polymer
sample based on size of the macromolecules. The most widely used method in poly-
mer science is size exclusion chromatography (SEC), sometimes referred to as gel per-
meation chromatography (GPC). A polymer solution is introduced on a column packed
with a porous gel. The smallest macromolecules within the sample will enter many
pores when passing through the column and travel a relatively long distance. As a
result, they will exit the column at a later stage than the larger macromolecules. De-
tection takes place at the end of the column, usually by refractive index (RI) or UV/VIS
detectors. The resulting SEC curve (Fig. 2.9(a)) depicts a size distribution. In order to
relate elution time (or volume) to the molar mass, a calibration with narrow disperse
polymers of known molar mass can be performed. This enables determination of both
Mn and Mw and the polydispersity of the sample. Calibration is no longer needed when
SEC is combined with static light scattering, an absolute method.
In particular, a very sensitive method is MALDI-ToF mass spectroscopy. MALDI-
ToF is short for “matrix assisted laser desorption ionization time-of-flight”, which ba-
sically describes the technology involved. The method requires isolation of polymer
molecules in a crystalline matrix.
The matrix can absorb energy from a laser pulse, leading to an explosive tran-
sition of matrix and embedded polymers into the gas phase. At the same time, the
matrix also facilitates ionization, resulting in charged macromolecules. The ionized
2.3 Synthesis of polymers | 23

Figure 2.9: Schematic representation of (a) Size exclusion chromatography (SEC), and (b) MALDI-ToF.
Both techniques can be carried out to determine the molar mass distribution of polymer samples.

polymers are accelerated in an electrostatic field. After a certain time-of-flight (ToF),

typically a few ms, the ions reach the detector. The relationship of mass to electric
charge, denoted as m/z, is calculated from the flight time. Polymers with small molar
mass reach the detector first, followed by larger ones. The resulting mass spectrum
(Fig. 2.9(b)) depicts a molar mass distribution, which enables absolute determination
of Mn , Mw , and D. In contrast to SEC, which leads to a continuous distribution, in a
MALDI-ToF mass spectrum individual signals can be identified. This means, in prac-
tice, Pn = 50 is distinguishable from Pn = 51. In fact, the distance between the two
peaks equals the molar mass of the repeating unit. This high level of accuracy also
allows determination of the molar mass of end groups.

2.3 Synthesis of polymers

Macromolecular architecture, whether it is a linear chain, branched molecule, or net-
work structure, is constructed via polymer synthesis. The process in which monomers
react together to form polymers is named polymerization. Polymerization can be clas-
sified by the mechanism of polymer-chain formation [13]. Two main polymerization
mechanisms are distinguished: step-growth and chain-growth polymerization.
In step-growth polymerization (Fig. 2.10(a)), all particles can react with one an-
other during the reaction time, and their reactivity is independent on the length of
the chain. Typically, a low molar mass byproduct is formed during the reaction. How-
ever, chain-growth polymerizations (Fig. 2.10(b)), are initiated by specific reactive
molecules. Monomers can only react with the initiated (growing) chains. Termination
of chain-growth occurs after a short time period, while, at the same time, new chains
24 | 2 Introduction to polymer science

Figure 2.10: A schematic representation of the mechanisms for (a) step-growth and (b) chain-growth
polymerization.

are initiated and start growing. Both mechanisms are discussed in more detail in the
following paragraphs.

2.3.1 Step-growth polymerization

Polymerization requires monomers with at least two functional groups that can re-
act with each other. Linear chains are formed via reaction of difunctional monomers,
while polymerization in the presence of monomers with a functionality of three or
higher results in branched or network structures (Fig. 2.5(b)).
In case of step-growth polymerization, complementary functional groups can be
united in the same monomer. Let us consider the polymerization of lactic acid. Lactic
acid contains both a hydroxyl and carboxyl group in its chemical structure: a hydroxy
acid. Those groups can react together to form ester moieties and water molecules
(Fig. 2.11). The term condensation polymerization, or polycondensation, refers to the
formation of water as a byproduct. The esterification reaction is in equilibrium, indi-
cated by the double arrow, since water can hydrolyze the ester group, forming the start-
ing compound(s). The final product of the polycondensation is PLA, short for polylac-
tic acid, part of the polyester family.
Like every step-growth polymerization, the polycondensation of lactic acid takes
place step by step. When two lactic acid monomers react, the resulting dimer contains
again one hydroxyl and one carboxyl functionality. The dimer can react with another
monomer, dimer, or oligomer to form a longer chain. The dimer has the same reactivity
2.3 Synthesis of polymers | 25

Figure 2.11: Polycondensation of lactic

acid, containing both a hydroxyl (–OH) and
carboxyl (–COOH) group, leads to polylac-
tic acid (PLA).

as the original monomer. The same holds for the oligomer. In other words, the reactiv-
ity of a functional groups seems independent on the length of the chain to which it is
attached. This phenomenon is known as Flory’s principle of equal reactivity [14].
Alternatively, two distinct difunctional monomers can undergo step-growth poly-
merization, as well. For instance in the polycondensation of ethylene glycol, a di-ol,
and terephthalic acid, a di-acid (Fig. 2.12). The monomers can polymerize in an alter-
nate fashion to form polyethylene terephthalate (PET). A high degree of polymeriza-
tion can only be obtained when starting with stoichiometric amounts, meaning both
monomers are equally present in the reaction mixture. Again, the esterification is an
equilibrium reaction since hydrolysis can occur. Somewhat confusing, the common
name of PET is simply “polyester”. Technically speaking, however, PET is just one
type of polyester, like PLA.

Figure 2.12: Polycondensation of ethylene glycol (di-ol) and terephthalic acid (di-acid) leads to
polyethylene terephthalate (PET).

By definition, the degree of polymerization (Pn ) is the average number of repeating

units (n) per polymer molecule. This relates to the initial number of molecules (N0 )
and the number of molecules at time t (Nt ), as presented in equation (2.8).

N0
Pn = . (2.8)
Nt

For instance, when examining the simplified polymer sample in Fig. 2.7, initially 40
monomers were available (N0 = 40). After polymerization, at time = t, five polymer
chains have been formed (Nt = 5). As a result, the average degree of polymerization
equals eight, according to the previous equation.
26 | 2 Introduction to polymer science

The conversion of the condensation polymerization (equation (2.9)) at t is related

to N0 en Nt as well, which results in an equation for Nt (equation (2.10)).

N0 − Nt N0 Nt N
p= = − =1− t, (2.9)
N0 N0 N0 N0
Nt = (1 − p)N0 . (2.10)

Subsequently, Pn can be written as a function of conversion. Equation (2.11) is known

as Carothers’ equation.

N0 N0 1
Pn = = = . (2.11)
Nt (1 − p)N0 1 − p

Thus, the degree of polymerization can be controlled by conversion. Carothers’ equa-

tion implies actual polymers are formed only at high conversion rates, above ca. 98 %,
as indicated by the graph in Fig. 2.13. In practice, it means that water has to be removed
from the reaction to shift the equilibrium in Figs. 2.11 and 2.12 to the right, leading to a
polymer product. Note that Carothers’ equation holds solely for linear condensation
polymers and that its simplicity is based on Flory’s principle of equal reactivity.

Figure 2.13: Degree of polymerization as a func-

tion of conversion: Carothers’ equation. Polymers
(Pn ≥ 50) are formed only at very high conversion
(p ≥ 0.98).

In addition to the number average Pn , the weight average degree of polymerization

(Pw ) can be written as a function of p as well (equation (2.12)). Combining both re-
lations with the definition of polydispersity (equation (2.4)), leads to equation (2.13).
Since the conversion can reach up to 100 % (pmax = 1), it follows that the polydisper-
sity for a polycondensation has a maximum value of 2.0. The molar mass distribution
resulting from condensation polymerization is commonly referred to as the Schulz–
Flory distribution.

1+p
Pw = , (2.12)
1−p
M P 1 + p/1 − p
D= w = w = = 1 + p. (2.13)
Mn Pn 1/1 − p
2.3 Synthesis of polymers | 27

2.3.1.1 Linear polycondensates

Polyethylene terephthalate has the highest share of the polyester market. As presented
in Fig. 2.12, PET can be synthesized through polycondensation of ethylene glycol with
terephthalic acid. Alternatively, the aromatic polyester is produced via transesterifi-
cation with the dimethyl ester of terephthalic acid (Fig. 2.14(a)). PET is known for its
high stiffness and hardness and is used in textile fibers and in packaging, particularly
in plastic bottles. The latter is due to the good O2 and CO2 barrier properties of PET.
If ethylene glycol (1,2-ethanediol) is substituted for 1,4-butanediol, polymerization re-
sults in polybutylene terephthalate (PBT). PBT is applied in the automotive industry
and household appliances.

Figure 2.14: Synthesis of (a) partially aromatic PET and (b) aliphatic PES. Polymerization can pro-
ceed either via polycondensation of terephthalic and succinic acid (R = H) or transesterification of
dimethyl terephthalate and succinate (R = CH3 ). Starting from 1,4-butanediol rather than ethylene
glycol (1,2-ethanediol) leads to PBT (instead of PET) and PBS (instead of PES).

Despite their excellent properties and recyclability, aromatic polyesters are not bio-
logically degradable. Recently, aliphatic polybutylene succinate (PBS) and polyethy-
lene succinate (PES) have gained renewed interest due to the increasing demand for
bioplastics. Both polymers can be synthesized via polycondensation or transesterifi-
cation (Fig. 2.14(b)), similar to PET and PBT. PBS is both bio-based and biodegradable
(Section 3.3.21) and has been marketed as the environmental friendly alternative to
common plastics, such as polypropylene (PP) [15].
Among the family of polyesters, polylactic acid is one the most common bioplas-
tics. The monomer can be produced from renewable resources like fermented plant
starch. Figure 2.11 shows the polycondensation route towards PLA, however, it typ-
ically leads to low molar masses and competes with ring formation. The industri-
28 | 2 Introduction to polymer science

ally more favored route is via ring opening polymerization of the cyclic diester of lac-
tic acid, named lactide. Ring opening polymerization (ROP) follows a chain-growth
mechanism, as will be discussed in Section 2.3.2.
The vast majority of the polyamide market is occupied by Nylon 6 and Nylon 6,6.
Both polyamides have similar properties, can be applied in mechanical equipment,
and are used extensively as textile fibers in clothing and carpets.
The numbering of polyamides refers to the number of carbon atoms in the
monomer(s). Analogue to polyesters, polyamides can be synthesized from one
monomer containing amine- and carboxyl functionalities, or by reacting diamines
with diacids. A linear aliphatic polyamide originating from 6-aminohexanoic acid
is named Nylon 6 (Fig. 2.15(a)). Instead of the proposed reaction scheme, Nylon 6
however is rather produced by ring opening polymerization (ROP) of caprolactam
(Section 2.3.2.4).

Figure 2.15: Possible synthesis routes of (a) Nylon 6 and (b) Nylon 6,6. The latter can be produced via
an intermediate product called AH salt or by interfacial polymerization.

In contrast, Nylon 6,6 is synthesized from two monomers, hexamethylene diamine

(HMD) and adipic acid (AA) (Fig. 2.15(b)). The two digits refer to the presence of six
carbon atoms in both starting compounds. Stoichiometric requirements are met by
first crystallizing the monomers into an ammonium/carboxylate salt, a so-called AH
salt (nylon salt). After purification, both constituents are present in an exact molar ra-
tio of 1:1. Next, the nylon salt is successfully converted at high temperatures into Nylon
6,6. This procedure was developed in 1930s at DuPont by Carothers and his team.
Another preparation method of Nylon 6,6 involves the removal of polymer prod-
uct during the reaction. First, hexamethylene diamine (HMD) is dissolved in water and
2.3 Synthesis of polymers | 29

adipoyl dichloride (ADC) in an organic solvent, such as cyclohexane. At the interface

between the aqueous and organic phases, a polyamide film is formed via a so-called
interfacial polymerization. A nylon fiber can be continuously drawn from the inter-
face, and in the meantime new polymer is formed. The synthesis method is a popular
demonstration experiment, known as the nylon rope trick, however, it is not commer-
cially viable.
Polyaramides, or aramids, are the aromatic equivalent of nylons. The polymeriza-
tion is more challenging with respect to aliphatic polyamides. Poly(para-phenylene
terephthalamide) (PPTA), with trade names as Kevlar® (DuPont) and Twaron® (Tei-
jin Aramid), is prepared by polycondensation of para-phenylene diamine (PPD) and
terephthaloyl dichloride (TDC). Aramid fibers are spun from a PPTA solution in con-
centrated (fuming) sulfuric acid and show superior mechanical performance and heat
resistance. The molecular structure in aramid fibers is highly ordered (Fig. 2.16), which
explains the directional dependence of their mechanical strength. Due to the high
costs of Kevlar® and Twaron® , those fibers are only applied when strictly needed, for
instance, for safety in bullet-proof vests and as reinforcing agents in sports car tires.

Figure 2.16: Highly ordered struc-

ture of PPTA macromolecules in
aramid fibers. Dashed lines rep-
resent intermolecular hydrogen
bonding between chains.

Polycarbonates are basically polyesters of carbonic acid. The most important polycar-
bonate is prepared from diphenyl carbonate and bisphenol A (Fig. 2.17). Alternatively,
diphenyl carbonate can be substituted by phosgene. Its high transparency makes poly-
carbonate applicable in eye-protection and car windows.
Inorganic polysiloxanes are obtained by hydrolysis of chlorosilanes and subse-
quent polycondensation. The polycondensation towards linear polydimethyl siloxane
(PDMS), displayed in Fig. 2.18, competes with the formation of cyclic siloxanes. PDMS
is a viscous liquid and is used in hydraulic oils and lubricants, due to its excellent heat
30 | 2 Introduction to polymer science

Figure 2.17: Synthesis of polycarbonate from diphenyl carbonate (X = Ph) or phosgene (X = Cl) and
bisphenol A, giving hydrochloric acid (HCl) or phenol as a byproduct.

Figure 2.18: Synthesis of PDMS: hydroly-

sis of dimethyl dichlorosilane, followed by
polycondensation.

resistance. When crosslinked, polysiloxanes are referred to as silicone rubber. Silicon

rubber is applied in seals and medical implants.
Table 2.3 depicts an overview of the most important linear condensation poly-
mers, discussed in this paragraph.

2.3.1.2 Polycondensate networks

As described in Section 2.2.2, polymer networks can be formed via various mech-
anisms. For example, epoxy resins are produced in a similar fashion as shown in
Fig. 2.5(a). First, low molar mass polymers with epoxy end-groups are synthesized. In
the next step, these epoxy prepolymers are transformed into thermosets by reaction
with crosslinking agents such as anhydrides or amines (Fig. 2.19(a)). Conversion from
a resin to a thermoset is called curing. Epoxy resins are often applied in coatings and
adhesives, or combined with fibers to fabricate fiber-reinforced plastics, a composite
material used in automotive and construction industries.
Another successful application of network polymers is the family of poly-
urethanes, well known from their application in foams for insulation panels and
mattresses. Polyurethanes result from the reaction of isocyanates with alcohols. The
use of monomers with more than two functional groups leads to network formation.
An example is the polymerization of diisocyanates with multifunctional alcohols
(Fig. 2.19(b)). The reaction follows network formation as presented in Fig. 2.5(b). It
has to be noted however that often polyols are used, which can be considered as pre-
polymers. The molar mass of the polyol can be varied, thereby controlling the density
of the network. As a result, either rigid (with low-Mn polyol) or flexible (with high-Mn
polyol) polyurethane foams are produced. Clearly, it is a powerful tool to vary the
properties of the final product.
2.3 Synthesis of polymers | 31

Table 2.3: Overview of most important linear polycondensates.

Polymer Condensate Abbreviation Chemical structure

structure polymer
Polyethylene PET
terephthalate

Polybutylene PBS
succinate
Polyesters

Polylactic PLA
acid*
(Polylactide)

Nylon 6* PA6

Polyamides
Nylon 6,6 PA6,6

Polyaramides Kevlar®
PPTA
(Aramids) Twaron®

Polycarbonates Polycarbonate PC

Polysiloxanes Polydimethyl PDMS

siloxane

* PLA and Nylon 6, however, are typically produced by ring opening polymerization of lactide and capro-

lactam, respectively. Ring opening polymerization will be discussed in Section 2.3.2.4.

32 | 2 Introduction to polymer science

Figure 2.19: (a) Epoxy thermosets are formed by curing the epoxy resin prepolymers with amines (or
anhydrides) as crosslinking agents. (b) Polyurethane networks can be formed via polymerization of
diisocyanates and multifunctional alcohols.

Other relevant crosslinking systems are phenolic and melamine resins, prepared by
condensation of formaldehyde with phenol or melamine, respectively. Curing is nor-
mally achieved at elevated temperatures. Both have similar properties, but the use of
melamine formaldehyde can be preferred since it is colorless. Phenol formaldehyde
has been the first synthetic polymer commercially available, under the trade name
Bakelite® .

2.3.2 Chain-growth polymerization

In the case of chain-growth polymerization, macromolecules are typically formed by

addition to a reactive group, mostly a carbon–carbon double bond, or by the opening
of a ring. Crucial is the formation of an active center that can be transferred along the
2.3 Synthesis of polymers | 33

chain at the moment a new monomer unit is added. Depending on the chemical nature
of the monomer, different chain-growth mechanisms can occur.

2.3.2.1 Radical polymerization

Most chain-growth polymerization reactions proceed via continuous addition to dou-
ble bonds. The resulting polymers are named addition polymers. When radicals are
involved during initiation and chain growth, it concerns a radical polymerization. Dur-
ing conventional free radical polymerization, the chain growth process passes through
several stages. Let us examine the polymerization of a typical vinyl monomer, such as
vinyl chloride.
It all starts with the availability of a radical source, often an initiator molecule. The
initiator dissociates into radical species under the influence of (UV) light or by heat-
ing. Common initiators are peroxides or azo compounds, like azo-bis-isobutyronitrile
(AIBN). Decomposition of AIBN is depicted both schematically and by structural for-
mulas in Fig. 2.20(a).

Figure 2.20: (a) Decomposition of AIBN, yielding two radical species and nitrogen gas. (b) Initiation
of vinyl chloride. The rate constants of initiator decomposition and initiation are kd and ki , respec-
tively.

Initiation of vinyl chloride takes place by addition of the radical species to the dou-
ble bond of the monomer. As a result, a new radical center is formed (Fig. 2.20(b)).
The new radical can add additional monomers, a process referred to as propagation,
thereby extending the PVC chain (Fig. 2.21). During propagation, the polymer chain
remains active. The radical center continuously shifts to the end of the chain when
extended by a new monomer unit.
Chain termination ceases the formation of reactive intermediates. Depending on
the reaction conditions, termination takes place by radical combination of growing
34 | 2 Introduction to polymer science

Figure 2.21: Propagation of

vinyl chloride monomers. The
rate constant of propagation is
denoted as kp .

chains (Fig. 2.22(a)), or by disproportionation in the form of a hydrogen atom transfer

(Fig. 2.22(b)). Note that chain combination results in one chain per termination reac-
tion, while disproportionation gives two chains, of which one has an unsaturated end-
group. Typically, both termination reactions occur, but disproportionation is favored
at higher temperatures.

Figure 2.22: (a) Termination of chain growth by combination of radicals. (b) Termination of chain
growth by disproportionation. (c) Chain transfer reaction to vinyl chloride monomer. The rate con-
stants of termination by combination, termination by disproportionation, and chain transfer are ktc ,
ktd , and ktr , respectively.

Another way to end the growth of a single polymer chain is by transferring the radical
center to another species, a so-called chain transfer agent (CTA). In turn, the new radi-
cal species can initiate new chain growth. CTAs can be added intentionally to regulate
chain growth. A higher concentration of chain transfer agent, in that case, lowers the
average molar mass. Chain transfer can take place to a solvent or to the monomer it-
self. In our example, vinyl chloride is known to be an effective chain transfer agent
2.3 Synthesis of polymers | 35

(Fig. 2.22(c)). In addition, transfer may occur to another polymer as well. In that case,
it introduces branches on the polymer backbone, leading to a higher molar mass in-
stead.
The degree of polymerization (Pn ) of macromolecules prepared from radical poly-
merization is dependent on the kinetics of the possible reactions already described.
The average chain length is related to the probability of chain growth (propagation)
versus the probability of chain termination (by disproportionation). In other words,
Pn equals the ratio of the rate of propagation (vp ) to the rate of termination (vtd ), as
stated in equation (2.14).
vp
Pn = . (2.14)
vtd

From Fig. 2.21, we learn that the rate of chain growth is dependent on the rate constant
(kp ), and the concentration of monomers ([M]) and concentration of radical chain
ends ([P⋅]) (equation (2.15)). In case of termination (Fig. 2.22(b)), the rate of termi-
nation relates to the rate constant (ktd ) and the concentration of radical chain ends
(equation (2.16)). This leads to equation (2.17).

vp = kp [M][P⋅], (2.15)
vtd = ktd [P⋅][P⋅], (2.16)
kp [M][P⋅] kp [M]
Pn = = . (2.17)
ktd [P⋅][P⋅] ktd [P⋅]

The radical concentration ([P⋅]) is dependent on the rate constant of initiator decom-
position (kd ), the initiator concentration ([I]) and the rate constant of termination,
according to equation (2.18).

2kd [I]
[P⋅] = √ . (2.18)
ktd

Subsequently, Pn can be written as a function of monomer and initiator concentration

(equation (2.19)). This equation only holds when termination occurs through dispro-
portionation. When termination takes place via combination, two chains join, and the
degree of polymerization is doubled (equation (2.20)).

kp [M] [M] kp
Pn = = ⋅ , (2.19)
√2kd [I]ktd √2kd ktd √[I]
kp [M]
Pn = 2 ⋅ . (2.20)
√2kd ktc √[I]

It becomes clear that the average chain length obtained in radical polymerization is
dependent on the concentration of monomer and initiator, at a specific time inter-
val. During polymerization monomers are consumed, so the monomer concentration
36 | 2 Introduction to polymer science

drops. Consequently, the Pn lowers during the reaction time. So, in the beginning of
the polymerization reaction, longer chains are formed, while chains become shorter
when the reaction proceeds. Therefore, free radical polymerization generally leads to
higher values for polydispersity, in contrast to step-growth polymerization having a
Dmax of 2.0 (equation (2.13)).
A better control over polydispersity can be achieved by suppressing termination
reactions. Let us compare the rate of propagation (equation (2.15)) with that of ter-
mination (equation (2.16)) once more. The former is a first-order reaction in radical
concentration ([P⋅]), while the termination rate is second-order ([P⋅]2 ). This means re-
ducing the radical concentration has more influence on termination than on propaga-
tion (and thus polymerization). After all, termination only occurs when two radicals
meet.
Controlled radical polymerization (CRP) uses this phenomenon by setting up an
equilibrium between an active and inactive state (Fig. 2.23). A stable radical (X⋅) can
react with a growing chain (Pn ⋅) and form an inactive macromolecule (Pn -X). This reac-
tion is, however, reversible, and the “dormant species” can split into the active state.
Chain growth can take place by reaction of Pn ⋅ with monomers (M). After a few propa-
gating steps, Pn ⋅ deactivates into its dormant state by reacting with the stable radical,
while other chains may activate. Since kdeact > kact , the majority of chains are dormant
during polymerization. In other words, the concentration of radical chain ends is sig-
nificantly lowered. As a result, termination by combination or disproportionation is
heavily suppressed and chain length can increase linearly with conversion.

Figure 2.23: Controlled radical polymeriza-

tion: equilibrium between active and dormant
species lowers the concentration of radical
chain ends, thereby suppressing termination
reactions. Active species can continue propaga-
tion (polymerization) until they are deactivated
once more.

In recent years, several CRP mechanisms have been developed that apply the con-
cept described above. Most common are nitroxide-mediated polymerization (NMP),
atom transfer radical polymerization (ATRP), and reversible addition fragmentation
and transfer (RAFT) [16]. In contrast to free radical polymerization, controlled radical
polymerization leads to narrow molar mass distributions (D ≈ 1.1). In addition, it en-
ables the synthesis of well-defined polymer architectures such as block copolymers.
Nevertheless, the technology is rather demanding and therefore expensive. Another
method to synthesize block copolymers is via living anionic polymerization, described
in the following paragraph.
2.3 Synthesis of polymers | 37

2.3.2.2 Ionic polymerization

Ionic polymerization involves a chain-growth mechanism with cations or anions as
active centers, rather than radicals. As stated before, most chain-growth polymeriza-
tions occur via continuous addition to double bonds. Whether a monomer can un-
dergo cationic or anionic polymerization depends on its electron negativity, which
is largely dependent on the side group(s). Monomers with electron donating groups
favor cationic polymerization. The electron density on the double bond is increased,
which facilitates electrophilic addition and stabilizes a cationic center. Examples are
isobutylene and ethyl vinyl ether.
Figure 2.24(a) presents the cationic polymerization of isobutylene, initiated by a
Brønsted acid (H+ X− ). Alternatively, Lewis acids such as BF3 , TiCl4 , and AlBr3 can be
employed as initiators. Chain growth occurs via electrophilic addition to isobutylene
monomers. The counterion (X− ) can act as a free ion, or may exist as an ion pair with
the active chain end. Since the growing chains are identically charged, they will not
terminate by reacting with one another, in contrast to radical polymerizations. Nev-
ertheless, termination of chain growth may still take place via deprotonation or, for
example, chain transfer to the monomer. Those unwanted side reactions can be sup-
pressed, but the reaction mixture needs to be very clean. Water and other protic com-
pounds should be absolutely absent. In addition, polymerizations have to be carried
out at low temperatures, typically below −100 °C, if a high degree of polymerization is

Figure 2.24: (a) Cationic polymerization of isobutylene, initiated by a Brønsted acid. Termination by
combination or disproportionation is not possible due to positively charged chain ends. Neverthe-
less, alternative termination reactions can occur. (b) Anionic polymerization of acrylonitrile, initiated
by an organometallic initiator. Termination by combination or disproportionation is not possible due
to negatively charged chain ends. This enables living polymerization.
38 | 2 Introduction to polymer science

desired. Those extreme requirements make cationic polymerization less suitable for
industrial application.
Monomers with electron withdrawing groups favor anionic polymerization. The
electron density on the double bond is decreased, which facilitates a nucleophilic at-
tack and stabilizes the anionic center. Examples are acrylonitrile and methyl metha-
crylate (MMA).
Figure 2.24(b) presents the anionic polymerization of acrylonitrile, initiated by
sec-butyl lithium. Organic metal compounds are popular nucleophilic initiators for
anionic polymerizations. They are commercially available and have good solubility
in a wide range of organic solvents. Alternatively, electron transfer reactions can be
used. Chain propagation takes place via nucleophilic addition to the acrylonitrile
monomers. Again, termination via combination of active chain ends does not occur.
Anionic polymerizations are usually carried out below room temperature. As for
cationic polymerization, traces of water and alcohols must be avoided to prevent un-
wanted termination reactions. By doing so, an ideal anionic polymerization can be
achieved. When the rate of initiation is significantly larger than the propagation rate,
all chains start growing at the same time. And, when termination and transfer reac-
tions are absent, propagation continues until all monomers have been consumed. The
growing chain remains active, and the polymerization is referred to as a living polymer-
ization [17].
During living anionic polymerization, the chain length increases linearly with
conversion. In the case of complete initiation, the average degree of polymerization
(equation (2.21)) at full monomer conversion is determined by the ratio of initial
monomer concentration ([M]0 ) and initial initiator concentration ([I]0 ). An increase
in initiator concentration leads to lower chain lengths, and vice versa.

[M]0
Pn = . (2.21)
[I]0

Essentially, living polymerization leads to polymer products with a very narrow molar
mass distribution, referred to as the Poisson distribution. The products are monodis-
perse, with values for polydispersity below 1.1. Higher molar mass leads to lower poly-
dispersity (equation (2.22)).

1
D=1+ . (2.22)
Pn

Since the chains remain active, even when all monomer has been consumed, adding
a different type of monomer to the reaction mixture results in the formation of well-
defined block copolymers (Fig. 2.25). Eventually, living anionic polymerization can be
stopped by intentionally adding a protic compound, such as methanol.
2.3 Synthesis of polymers | 39

Figure 2.25: Schematic overview of polymer products from free radical polymerization versus liv-
ing/controlled polymerization. Free radical polymerization leads to polydisperse samples. Longer
chains are formed at the start of the reaction, while shorter chains are formed when less monomer
is available. Via controlled radical polymerization, or living anionic polymerization, narrow distri-
butions can be obtained. Block copolymers can be synthesized by sequentially adding different
monomers.

2.3.2.3 Coordination polymerization

Chain growth reactions that are catalyzed by transition metals are generally called
coordination polymerizations. About half of all polymers worldwide are produced by
this specific type of polymerization. Propylene, for example, cannot be polymerized
via radical, cationic, or anionic polymerizations. It was only after the discovery of
titanium-based catalysts by Ziegler and Natta in the 1950s that industrial production
of polypropylene (PP) became possible [18].
Ziegler–Natta polymerization enabled the production of polyolefins at ambient
conditions. Figure 2.26 depicts the polymerization of ethylene. First, ethylene coor-
dinates with a transition metal catalyst to form a so-called π-complex. The ethylene
molecule is inserted into the alkyl ligand, and a new coordination site becomes avail-
able. By repeating this process, a polyethylene (PE) chain is produced. Termination
can take place through hydride elimination.
If chain migration occurs during the process (as in Fig. 2.26), the next monomer
will coordinate in the exact same way as before, due to the stereospecific catalyst. Poly-
merization with Ziegler–Natta catalysts can result in stereospecific polymers, which
was a great achievement at that time and awarded the Noble Prize for Chemistry in
1963.
A few decades later, in the 1990s, Brintzinger discovered metallocene catalysts de-
rived from zirconocenes [19]. Those homogenous catalysts introduced a higher level
of microstructural control in comparison to the traditional systems from Ziegler and
40 | 2 Introduction to polymer science

Figure 2.26: Chain growth via Ziegler–Natta polymerization: coordination of ethylene, followed by
insertion and migration. By repeating the process, high-density polyethylene (HDPE) is obtained.

Natta. By catalytic polymerization of propylene in the presence of isoselective metal-

locene, isotactic PP (more than 98 %) can be obtained. Alternatively, syndioselective
metallocene results in highly syndiotactic PP (Section 3.3.2).

2.3.2.4 Ring opening polymerization

Polymerization that proceeds via ring opening is applied to produce several impor-
tant commercial polymers. Ring opening polymerization (ROP) follows a chain-growth
mechanism, and can be initiated via radical, ionic, or coordinative pathways, as de-
scribed in the previous paragraphs. Suitable monomers for ROP are cyclic ethers,
amides (lactams), esters (lactones), olefins, and siloxanes.
As pointed out before, the industrially favored route to produce PLA is not via
step-growth polymerization of lactic acid, but through ring opening polymerization of
cyclic lactide (Fig. 2.27(a)). In a similar fashion, polycaprolactone (PCL) and Nylon 6
are produced by ionic ROP of caprolactone (Fig. 2.27(b)) and caprolactam (Fig. 2.27(c)),
respectively.
Cyclic olefins can undergo a special type of ring opening polymerization. For
instance, cyclooctene is polymerized via a metathesis reaction. Ring opening metathe-
sis polymerization (ROMP) yields polycyclooctene (Fig. 2.27(d)), marketed under the
name Vestenamer® . The mechanism of ROMP follows the same principles as pre-
sented in Fig. 2.26. During the insertion step, the ring is opened. ROMP of cyclic
olefins has enabled the polymerization towards formally unknown architectures,
such as polynorbornenes.
2.3 Synthesis of polymers | 41

Figure 2.27: Ring opening

polymerization of (a) lactide
to PLA, (b) caprolactone to
PCL, (c) caprolactam to Ny-
lon 6 (PA6), (d) cyclooctene
to Vestenamer® .

2.3.2.5 Important addition polymers

The majority of commercially available polymers are vinyl polymers, based on vinyl
monomers (CH2 =CHX). Polyethylene (PE), polypropylene (PP), polystyrene (PS), and
polyvinylchloride (PVC) together share ca. 80 % of the plastics market worldwide.
Free radical polymerization of ethylene leads to low-density polyethylene (LDPE).
LDPE is a flexible material extensively applied in plastic films for packaging pur-
poses. Ziegler–Natta polymerization of ethylene results in less branched high-density
polyethylene (HDPE). HDPE has a better mechanical performance, leading to appli-
cation in containers and pipes. A specialty PE is Dyneema® , an ultra-high molecular
weight polyethylene (UHMWPE). Dyneema® fibers demonstrate excellent resistance
to impact and wear and are used in high-end applications, such as ballistic protection
and extreme sports.
Polypropylene (PP) is only produced by coordination polymerization. By carefully
selecting the catalyst, chain tacticity can be varied, and isotactic, syndiotactic, or at-
actic PP is obtained. Depending on the specific grade, PP may be used in carpets, pack-
aging, and molded parts for cars.
The term styrenics refers to polystyrene (PS) and its copolymers. The latter will be
discussed in the next paragraph. PS is mostly produced via radical polymerization,
although chain growth can take place via ionic propagation as well. Polystyrene is
easily processable above its glass transition and used to fabricate various children’s
toys. Particularly important are polystyrene foams. Styrofoam™ (Section 2.5.4) is ex-
panded polystyrene (EPS) used in food containers, coffee cups, and packaging.
Polyvinylchloride (PVC) is a very popular thermoplastic polymer, with a great num-
ber of applications. It can be processed via extrusion, injection molding, blow mold-
42 | 2 Introduction to polymer science

ing, pressing, and calendering. Credit cards, squeeze bottles, and transparent tapes
are made from PVC. Recycled PVC is extensively applied in tubes and pipes. When
produced with large amounts of plasticizers, soft PVC is obtained. This rubbery com-
pound is used in flooring and cables.
While acrylonitrile is a crucial comonomer in ABS and SAN plastics, its homopoly-
mer polyacrylonitrile (PAN) can be spun into synthetic fibers for rugs and blankets.
Spun PAN is the number one precursor for high-quality carbon fibers as well. Carbon
fibers can be applied in fiber-reinforced plastics or composites for aerospace and au-
tomotive industry.
In addition to vinyl polymers, polymers based on vinylidene monomers
(CH2 =CXY) are quite popular on the plastics market. Polyisobutylene (PIB) is produced
on a large scale via a challenging cationic polymerization of isobutylene (Fig. 2.24(a)).
Commercial applications include chewing gum and skin-care products. One of the
hardest plastics is polymethyl methacrylate (PMMA), well-known by its tradename
Perspex® . PMMA has excellent transparency and is therefore used in spectacles,
optical fibers, traffic signs, and solar panels.
Fully fluorinated ethylene can be radically polymerized to polytetrafluoroethy-
lene (PTFE). PTFE shares 60 % of the fluoropolymer market. It is better known un-
der DuPont’s trade name Teflon® . This fluoropolymer is chemically inert and anti-
adhesive, resulting in its application in non-stick coatings for cookware.
Isoprene and butadiene can polymerize via 1,4-addition into polyisoprene rubber
(PIR) and polybutadiene rubber (PBR). Alternatively, 1,2 linkages may also be formed.
Both materials can be produced via radical, anionic, or coordination polymerization.
Polybutadiene is crosslinked for application in car tires and dampers. Polyisoprene
occurs in nature as natural rubber (cis configuration) or gutta percha (trans configura-
tion). Like polybutadiene, it is crosslinked before practical use in shoes and gloves.
Table 2.4 depicts an overview of the technically most important addition poly-
mers, discussed in this paragraph.

2.3.3 Copolymerization

Section 2.2.1 explored the large diversity of macromolecular structures, including var-
ious types of copolymers. An infinite number of copolymers can be synthesized by
altering the type of monomers, as well as the chain sequence (Fig. 2.3). The material
properties are dependent on the chemical nature of building blocks and their ratio
along the polymer chain. Copolymerization allows the synthesis of tailor-made macro-
molecules by tuning those variables.
Let us consider the radical copolymerization of acrylonitrile (A) and butadiene
(B). During chain growth, several reactions may occur. When monomer A reacts with
a growing chain, an active chain of A is formed (—a⋅). This active chain may react with
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over until it seemed as if she never would come back.
When half way up he paused a few seconds, to turn his head aft and
get a free breath, for water was smiting him at every step. He saw a
signal wig-wagged to him from the after mast. It was from Dan
Davis, who was going up on the same duty.
"I'll race you to the top," signaled Davis.
"Go you!" answered Sam, starting up the ladder at a lively clip. Dan
was not caught napping. He was off with Sam. Every little distance
up these masts is a landing made of woven leather strands, and a
person mounting to the top has to cross each one of these, taking a
ladder on the other side.
The Battleship Boys barely struck the high places in crossing the
landings. It seemed as if they surely must fall.
"Look careful, aloft there!" roared a voice from the bridge.
"Aye, aye, sir," floated back the reply from Hickey.
They had reached next to the last landing, far up there in the spray-
laden air, when a shout attracted all eyes aft.
A man was seen hanging from the platform by his feet. With each
roll of the ship his body would swing far out from the mast, as he
hung suspended between sea and sky.
"Man the main mast!" thundered an officer, his voice being heard
above the roar of the storm.
Half a dozen jackies sprang for the mast.
"Who is the man aloft there?" demanded the captain.
"It's Gunner's Mate Davis, sir," answered the executive officer.
The captain groaned.
"He'll be lost. Look alive there, men! Quick! Quick!"
Sam had seen and understood, but he did not halt. He was under
orders to go to the top, and to the top he went as fast as his feet
and hands would carry him. Not until he had reached the swaying
platform at the top of the cage mast did he venture to look astern.
The lad's heart fairly leaped into his throat as he saw his
companion's terrible peril.
In running across the landing, Dan had been caught by a sudden
violent lurch of the ship and thrown forward. He felt his head and
shoulders going through between the braces of the mast. With quick
instinct he spread both legs, turning his toes outward.
Nothing else saved him from plunging a hundred feet into the sea.
And there he clung by his feet, every muscle in his body strained to
its utmost tension. With each roll of the ship he felt that he would be
unable to hold on through another.
"Hold fast!" shouted a voice far below him.
"Hold Fast!" Shouted a Voice Below.

"Hold fast—they're coming!" howled Sam Hickey from his perch high
in the air. His voice was lost on the roar of the gale, but he did not
know it.
"Where's that confounded waterspout?" he muttered. "Oh, I see it.
The thing is going to come pretty close to the ship, I'm afraid. But I
don't care. I'm too high up to get hit by it."
His mind turning from the waterspout to Dan Davis, Sam wheeled,
steadying himself by holding tightly to the railing that extended
around the top. Every lurch of the ship was like "cracking-the-whip"
at school. It seemed to make every bone in one's body snap.
Sam groaned as he saw Dan swaying back and forth.
"Oh, why doesn't he grab the mast? Why doesn't he?"
Sam did not know that Dan was making desperate efforts to do this
very thing, but thus far had been unable to.
All at once the lad's feet slipped out of position.
"He's going! He's going overboard!" yelled Hickey in a voice that was
heard on the bridge and to the stern of the superstructure.
Sam shut his eyes and stood there trembling. He had forgotten
waterspout, raging sea and all—all save the fact that his companion
was falling.
A yell aroused him. The yell was different from the rest. It was a yell
of joy. Sam opened his eyes, blinked, rubbing the salt water out of
them, then gazed aft through the mist.
CHAPTER XII
IN THE COILS OF A "TWISTER"
"There he goes! Oh, that's too bad!" groaned the captain.
He had seen the boy's body shoot outward.
"No, he's struck something. He's caught a stay," cried the executive
officer.
"He'll never hang there. He'll surely go over now."
Dan was hanging with desperate courage to the rope that he had
caught.
"Such grit! What a pity!"
By this time the jackies had reached the platform, but they could be
of no assistance to their shipmate. Dan was hanging twenty feet out
from where they were.
He seemed to have lost his bearings, and, for the moment, appeared
not to realize where he was. Little by little his power of reasoning
returned to him, while all hands were watching him with breathless
interest. The stay to which he was clinging extended forward to the
foremast, running from the middle of the mainmast to the middle of
the foremast.
Hand over hand the plucky lad began moving along the rope brace.
It was slow progress at best. At last he was directly over the huge
funnels. Hot, suffocating smoke, belching from the funnels, hid him
from the view of those on deck. The smoke and coal gas well-nigh
strangled the boy, but he kept on. A cheer reached his ears as he at
last emerged from the cloud of black smoke.
"Keep it up, Dynamite! Keep it up!" howled a dozen voices.
"Steady now! Hold to your course. You're on the last lap!"
"Come on, Dan!" howled Sam Hickey, dancing about on his insecure
foothold, almost beside himself with excitement.
On the other hand, at that moment, Dan Davis was perhaps the
least excited of all that ship's company. He was in full command of
himself, though his arms ached and he had to exert great self-
control to keep from letting go. Now and then he would pause,
hanging by one hand to rest the other arm, then he would go on
again, moving more rapidly than before.
"Bridge, there!" roared Sam.
"Aye, aye."
"Can't somebody come aloft to give Davis a hand when he reaches
the foremast?"
"Get aloft, there!" bellowed the executive officer.
"Yes, the boy Hickey has more sense than all the rest of we officers
down here," exclaimed the captain.
Men ran up the ladders in a squirming white line, and quickly
clambered out into the steel rigging. As Dan neared them they
stretched forth their hands.
"Only a little way further, matey," they encouraged. "That's the boy!
You'll make a tight-rope walker one of these days, only you want to
learn to walk with your feet instead of your hands."
"Grab me!" called Dan.
"Got him!" yelled a jackie at the top of his voice.
The word carried to the bridge and to the superstructure, where a
hundred or more sailors were crouching trying to peer up into the
mist. They broke forth into a wild yell of applause.
In the meantime strong hands had grasped Dan, pulling him in
among the steel supports of the cage mast, where they held him
while he rested from his great ordeal.
Sam Hickey was dancing a jig on the top of the military mast, yelling
as if he had suddenly gone mad.
"The boy is safe, sir," announced the executive officer.
"Thank God!" breathed the captain. "Aloft, there!"
"Aye, aye."
"Is Davis all right?"
"Yes, sir."
"Send him below as soon as he is able."
"Aye, aye, sir."
"I'm able now," said Dan. "I'm going below. I've got to get back to
my station."
"All right, matey. Want any help?"
"No; I can get down alone."
Dan's arms ached, and his muscles were pretty well stiffened, as he
started to make his way down the rocking mast.
At last he reached the foot of the mast, which was the navigating
bridge of the ship, and started to run down the steps to return to his
post.
"Davis!" The voice was sharp and commanding.
"Aye, aye, sir," answered the boy, halting and saluting.
"Where are you going?"
"To my post, sir," he answered, as he faced the commanding officer.
"You need not return to your post. There are enough men aloft in
the mainmast now. You have done quite enough. How did you
happen to fall?"
The boy explained, not omitting the fact that he and Sam were
running a race for the tops.
The captain did not rebuke the boy for this, perhaps realizing that
Dan had already been severely punished for his foolhardiness.
"That is all for the present. Aloft, there!"
"Aye, aye, sir."
"How about that waterspout!"
The seas were engulfing the ship so that the officers could not see
the waterspout at all. They had wholly lost sight of it.
"Yeow! Wow!" yelled a voice far above their heads.
Looking up, they saw the red-headed Sam dancing again, shouting
lustily and pointing off the starboard bow.
"Aloft, there, what is it?"
"Waterspout! Waterspout!" howled Hickey.
"Where away?"
"It ain't away at all."
"Where away? Answer, you lubber!"
"Right off the starboard bow, sir. Look out, she's going to hit us! Lo-
o-o-o-k out! Ye-ow!"
"Hard aport!" shouted the captain. "Hold fast on the bridge! Look
alive, men aft, there! Waterspout coming aboard. Every man look
out for himself!"
All tried to do so, but not all were quick enough to get under cover.
Only a few of them succeeded.
With a terrifying roar the waterspout swept down on the ship. It
towered above them like a huge mountain, bearing to the northeast.
It struck the battleship on the starboard bow, sending a shiver
through the ship, hurling to the deck every man who was not
clinging to some support.
The twister recoiled after sending tons of water over the ship—
recoiled as if to gather strength for a final crushing blow. The
quartermaster, who had been holding the steering wheel, had been
wrenched from the wheel and hurled down a flight of steps to the
spar deck. Not an officer on the bridge was on his feet.
Dan Davis, who had crept up the companionway to get a better view
of the waterspout, was huddled against the cage mast, clinging to
one of its supports.
All at once he discovered that no one was at the wheel. Without
waiting for an order, he leaped forward. Grasping the wheel, he
swung it sharply to port. The thought suddenly occurred to him that
the best way to meet the twister would be head-on. He did not know
what the result of such a meeting might be, nor did he have time to
think. As it was, the ship was laboring in the trough of a terrific sea,
and might be swamped.
The bow of the ship pierced the base of the waterspout. With a
mighty roar the towering column of water suddenly collapsed. The
sound was like thunder, as tons upon tons of water beat down on
the decks. The whole ship seemed to be under water. Everything
movable was moving. The officers lay prone upon the narrow
navigating bridge, clinging to its stanchions for their lives.
At the wheel a hatless boy, fairly swimming in salt water, was
working to get a foothold that would enable him to swing the ship.
At last he managed to wrap both legs about the wheel frame, and
there he clung, tugging at the wheel with all his strength.
Very slowly, at first, the ship began to respond. First the battleship
seemed to shake itself, trying to throw off the great weight of water
upon its decks; then its blunt, stubborn bow rose clear of the seas. A
moment, and the shining decks themselves cleared the water, every
scupper discharging a green salt flood overboard, every deck below
soaked with brine.
The captain was the first to regain his feet. He sprang up, his eyes
taking in the after part of the ship in one sweeping, comprehensive
view. Then his eyes rested on the man at the wheel.
"Davis, is that you?"
"Yes, sir."
"You weren't at the wheel before we were struck?"
"No, sir."
"How did you happen to get there?"
"I guess I must have been washed here, sir.
"Where is the quartermaster who was at the wheel?"
"I saw him falling down the after companionway, sir. I think you will
find him on the spar deck, sir."
"You steered us out?"
"Yes, sir."
"Where is the spout?"
"I smashed it, sir."
"You what?"
"Smashed it."
"How?"
"I steered the ship into it."
"You did that?"
"Yes, sir," answered Dan, now expecting that he was in for a severe
rebuke.
"Explain."
"I saw, immediately after the wheelman had been swept away, that
the ship was in a bad position. The waterspout was going to hit us,
quartering on the starboard bow. It seemed to me that the best
thing to do would be to split it. I didn't know whether I could do it or
not, but I made up my mind to try. There was no one to ask, nor
time to do so. I had to do something in a hurry."
"So you rammed the waterspout, eh?"
"I did, sir."
"What do you think of that, Coates?" as the executive officer picked
himself up, wet, capless, very much the worse for his encounter with
the waters of the twister.
"What is that, sir?"
"Davis rammed the twister."
The captain then went on to relate in detail what had happened
while they were on their faces, holding fast to the bridge stanchions
to keep from going overboard.
"Davis, I shall have to commend you again and for this—perhaps
saving the ship—I shall send your name in to the department.
Quartermaster, here!"
"Aye, aye, sir."
"Man the wheel!"
CHAPTER XIII
TWO ARE MISSING
Night came on; dark, heavy clouds were hanging low in the sky, the
wind shrieking dismally.
The jackies, however, were happy. They were not disturbed by the
roar of the gale. So rough was the sea, however, and so heavy the
roll of the ship, that it was decided not to set the mess tables for the
evening meal. The men sat around on the lower decks, legs crossed,
balancing themselves and their plates of food, joking and laughing
over the little mishaps of their companions.
Down in the captain's quarters matters were little better. Most of the
time the commanding officer was holding to his own table with both
hands. A plate of hot soup had just turned turtle, landing in his lap,
soiling the spotless uniform that he had put on after returning from
the bridge. The officers in the ward room, where all the other
commissioned officers eat, were having their own troubles.
All at once there was a yell. Some tumbled over backwards in their
chairs, while others sprang up and scrambled out of harm's way, as
a huge object came hurling through the air. It landed full force on
the mess table, the table going down beneath it with a mighty crash.
The dark object was the ward-room's upright piano. The captain,
hearing the crash, rushed in from his quarters adjoining.
"What's wrong?" he shouted.
"Nothing, captain. There's music in the air, that's all," answered the
ship's surgeon. This put all hands in good humor, even though a
quantity of china had been utterly ruined.
China was not troubling the jolly tars forward, nor were they
disturbed over the wet decks on which they were sitting. Every man
of them was soaked with salt water.
In the galley kettles were sliding across the range, and from there
out on to the deck. Food was everywhere, except where it should
have been.
Suddenly the jackies on the seven-inch gun deck set up a yell of
delight. A steward descending a ladder carrying a kettle of hot beans
suddenly lost his hold.
With a howl, he plunged headlong. Sam Hickey chanced to be right
in the path of the human projectile. The kettle of boiling hot beans
turned turtle just as it was hovering over the red-headed boy's head.
Down came kettle, beans and all over Sam's head. Part of the
contents scattered, catching other unlucky jackies who were sitting
near him.
Hickey's yells could be heard above the roar of the storm, as he
scrambled madly to his feet, tugging at the kettle to get it off his
head. The handle had dropped down under his chin.
Shipmates sprang to his rescue, else Sam would have been seriously
burned. As it was, his face was red and swollen, his hair was matted
with beans and his eyes glared angrily.
"You did that on purpose," he howled, starting for the unlucky
steward.
"Yes, of course he did," urged several voices. "He ought to be
dumped overboard for the fishes."
"No; he's too tough, they wouldn't eat him."
The steward himself settled the question of his disposal, by
scrambling up the companionway as fast as he could go. He knew
the jackies well enough to be aware that they would like nothing
better than having some sport with the "sea cook," as they call every
man connected with the kitchen department.
"Hello, Sam, what's the matter?" questioned Dan Davis, as he shot
across the deck head first, having lost his grip on the frame of the
water-tight door where he had been standing for a moment.
"Look out! Here comes the dynamite projectile!" warned a voice.
Dan landed among a group of sailors, and what food they had in
hand was scattered all over that part of the deck. The next second
he found himself sprawling in the middle of the deck, where they
had hurled him.
Hickey grinned.
"What's the matter with you?"
"I must have been fired with a charge of smokeless powder, as I
don't see any smoke," laughed Dan. "Well, you are a sight! What
happened to you?"
"Beans!" jeered the jackies.
"I thought you looked like one of the fifty-seven varieties," laughed
Dan Davis, at which there was a loud uproar.
"Throw him overboard. It's them kind of jokes that causes
waterspouts and earthquakes. Don't you ever dare say anything like
that again, Dynamite, or we'll forget you're a shipmate and bounce
you!"
"You had better begin right now, then," retorted Dan defiantly. "I'm
ready for any kind of a row you want to start. It's a good night for a
rough-and-tumble. We haven't anything else to do. Come on, if you
are looking for trouble."
Dan squared off as if ready for a fight. Just then the ship gave a
heavy lurch. The Battleship Boy disappeared under one of the big
guns. His messmates hauled him out by the feet, amid shouts of
laughter, and began tossing him about as if he were a ball.
Davis took his rough treatment good-naturedly.
"Thought you were going to fight?" jeered the jackies.
"No; like Sam Hickey, I've changed my mind," laughed Dan.
"Hark!"
"What is it?" All hands stopped to listen.
"It's the bugle. They're piping some squad to quarters. I wonder
what's up now?"
"That's the whaleboat crews they're piping up," nodded Dan. "I
guess the boats are being washed away."
"There goes another call."
"Starboard seven-inch gun crew called to quarters!" shouted
Gunner's Mate Davis. "Jump for it, boys!"
There was a rush of those of the gun crew who were on the deck
with Dan. They well knew that something was wrong at their station.
For all they knew they might have been called to work the gun; still
such a call was hardly to be looked for during the mess hour.
Reaching the seven-inch turret, they found the place flooded with
salt water. With every lurch of the ship a great column was forced in,
as if through a gigantic hose. The first charge of this caught Sam
Hickey, sweeping him clear out into the corridor.
Sam came back, choking and coughing, yelling at every one in his
excitement.
"Attention!" roared the gun captain.
"Attention!" repeated Dan Davis. He saw instantly what had
happened.
"The steel buckler plates have been wrenched loose!"
These buckler plates are employed to cover the opening in the side
of the ship about the guns. Without them the ship would be flooded
in heavy weather.
It was not an easy task that had been set for the gun crew. Every
man knew that.
"Who will volunteer to do the work outside?" demanded the gun
captain.
"I'll attend to that," answered Dan promptly.
"Me, too," added Sam, without hesitation. "I can't get any wetter
than I am."
"You'll get something besides wet," said the captain. "Very well, you
two go out. Hold fast! Look out for yourselves."
The Battleship Boys were climbing from the turret ere the words
were out of his mouth.
"Don't try any tricks, Sam," advised Davis.
"Better take that advice to yourself. If I remember rightly you were
running a race, or something, when you fell off the cage mast to-
day. Woof!"
A heavy sea smashed into them, laying them flat on the deck. The
boys hung on until the sea had rolled over them. They were high up
on the superstructure, where the seven-inch guns are located. Not a
thing could they see in the darkness, but they knew their way about
as well as if it had been broad daylight.
The buckler plates were thrust in from the inside of the turret, the
duty of the lads outside being to make fast the catches which were
employed to hold the buckler plates in position in heavy weather.
Under ordinary conditions it was not necessary to set these
emergency catches. It had not been done in this instance,
consequently the plates were battered in, flooding the deck and all
that part of the ship.
"All ready out here!" shouted Dan.
With a grating sound the bucklers were shoved into position.
"Click!"
The catches snapped into place.
"Right!" bellowed Hickey, placing his lips close to the side of the
muzzle of the gun.
"Come, let's get out of here," called Dan.
"Look out for yourself. Duck! Grab!" roared Sam.
"Wha—what——"
Dan did not complete the sentence. A wall of water struck the turret
with a report like that of the three-inch forward rifles.
From the depths of the great green wave came a muffled yell. Sam
Hickey's grip had been wrenched loose from the guard rope at the
side of the muzzle of the seven-inch.
At the same instant both lads felt themselves lifted from their feet.
Then down, down they dropped. It seemed to them that hours were
consumed in that terrible drop. They felt themselves falling into an
abyss of the sea. Such was not the case, however, though their
situation was, at that instant, every bit as serious as if they had in
reality been falling into the sea. As it was, they were being swept
toward it.
The smash of the wave having carried them from their feet, rolled
them along the upper or spar deck, dropping them down some
twenty feet to the quarter-deck, that was all awash. Fortunately the
water below caught them, or they might have been killed in the
twenty-foot fall to the quarter-deck.
Suddenly Sam came into violent contact with something that he
gripped anxiously. That something did not give way. Dan met with a
similar experience, and there the lads hung, neither knowing what
had become of the other, seas smiting them, threatening every
second to hurl them on and into the sea itself.
In the meantime those of the gun crew had returned to the gun
deck to dry their clothes. The gun captain, however, waited for the
return of the boys who had gone outside.
"I wonder what has become of those boys," he mused, peering out
through the hatchway that he opened the merest crack. There was
neither sight nor sound of them.
"Davis! Hickey!" he bellowed.
His effort brought no answer.
The gun captain knew no personal fear. He stepped out, closing the
hatch behind him quickly. He clung there, watching, listening, then
shouting. All at once he turned and hurried back to the gun deck.
Sending word to the executive officer, he informed that officer of the
absence of the two boys.
The captain heard the news a moment later, and a stir ran all
through the ship.
"They're overboard. Nothing could save them, sir," advised the
executive officer.
"Man the searchlights. Both tops!" commanded the captain, now all
activity. "Pipe all hands to stations!"
CHAPTER XIV
DOWN THE AMMUNITION HOIST
The searchlights flashed out over the troubled sea. Nothing but
water—angry, foaming water—could be seen. Not a sign that looked
as if it might be a man were they able to pick up.
"They're trying to find us. They think we have gone overboard,"
muttered Dan Davis. He uttered a loud shout.
At that instant there sounded another shout close by him. At first he
thought it was the echo of his own voice. All at once he made the
discovery that some one else was near.
"Hello!" shouted Dan.
"Hello yourself!"
"Is that you, Sam?"
"No, it's only part of me. Most of me has been blown overboard.
That you, Dan?"
"Ye-e-e-s," answered Davis in a choking voice. "Yell, Sam, if you've
got any voice left. Yell for your life. They don't see us."
Hickey uttered a lusty howl. Dan saw at once that the men in the
tops were unable to depress the searchlights enough to sweep the
quarter-deck with the light rays.
"They don't see us, Sam. Yell louder."
"I'll have to borrow a stomach pump to jerk the salt water out of
me, before I can yell any more at all. I'm afloat, inside and out, and
not a compass to guide me. Where are we?"
Dan felt about him cautiously.
"I think we are astern somewhere. Judging from the position of the
searchlights, I think we must be somewhere on the quarter-deck."
"How'd we get here?"
Another wave made it impossible for Davis to answer for a minute or
so. When finally he had gotten his breath he said:
"I think we must have been washed here. But——"
"Say, let's get out of here, Dan."
"But how we ever dropped from the topside to the quarter-deck
without being killed is more than I can figure out."
"I'm going to try to cross the deck."
"Don't do it, Sam. You will be swept into the sea instantly. Wait! I
have a plan."
"What is it?"
"Can you work your way along the rope railing to where I am?"
"I can swim over to you."
"Come on, then, but keep tight hold of the rail."
"Here's the flagstaff," shouted Sam. "I've got my bearings now."
"You will need something more than that to get you out of this
scrape. Come up close to me and I'll tell you what to do."
"Here I am. Where are you?"
Dan reached out a hand, grasping the arm of his companion.
"There ought to be a rope right at the foot of the staff, here. Yes,
here it is. Hold fast to me, so I don't go overboard, while I untie the
knot."
"What are you going to do?"
"I'll show you in a minute."
Dan made the rope fast to a cleat on the after stanchion, then took
a twist about his own arm with the free end.
"Now, I want you to stand right here until I give three tugs on the
rope."
"What are you going to do?"
"I don't know what I am going to do, but I'm going to try to get to
the twelve-inch turret with this rope."
"You'll have to swim for it, then."
"I expect to have to swim part of the way, but leave that to me.
When I give three long tugs on the rope you start working along it."
"But where will we go? The water-tight doors are fastened on the
inside; we can't get in. We shall be swept from the deck. I guess I'll
stay where I am, and hang on until morning."
"No; you can't do it. You will be washed overboard. Watch the rope.
I may go over, too, but you can tell by the feel of the rope, and if
you think I'm going over, haul in. I'll yell, too. The wind is this way
and you can hear me. Now, don't bother me. I'm going in a minute."
Dan hung to the rail, rope in hand, watching the roll of the ship,
which he was obliged to observe not by sight, but by the sense of
feeling.
All at once, as the stern rose into the air, he darted forward. He was
in water nearly up to his waist, but as the quarter-deck rose the
water rushed to the sides of the ship in a raging flood.
Suddenly Dan felt himself being drawn backward. At first he could
not understand the meaning of it. Then he realized. Sam was
hauling him in.
"Stop it! Stop it!" yelled Davis.
Sam kept on hauling. Losing his foothold on the slippery deck, Dan
went down. At the same time the quarter-deck shipped a big wave
and Dan was swimming blindly. Through it all he managed to keep
hold of the rope with one hand. He was being dragged along the
deck so fast that he could not get to his feet, even after the water
had receded a little.
Finally, yelling at the top of his voice, Hickey finished his work,
grabbed Dan from the deck and slammed him against the rail.
"I got you! I got you! I saved your life, didn't I?"
"Sam—Sam Hickey, you're the biggest fool I ever bumped into in all
my life!"
"A fool—a—see here, is that all I get for saving you——"
"What did you haul me back for?"
"Because you yanked on the rope."
"I did nothing of the sort."
"You did."
"I didn't."
"We—we won't argue the question. I—I haven't enough breath left
in me to argue. Now, next time, don't you pull on the rope until you
hear me yell, or until the rope swings way over to port. I am going
to run quartering so that if I get caught by another wave I will be
washed toward the twelve-inch turret. Understand?"
"Sure, I understand."
Waiting until the stern rose again, Dan made another dash. This
time he had, as he had planned to do the other time, reached a spot
opposite the turret before the deck sank under another wave. He
was washed right up against the turret when the wave did come.
The instant the wave left him, he took a turn about a big ring-bolt
on the turret.
"Sam! Sam!"
A faint "hello" was wafted to him on the gale.
"Come on!"
Dan waited and waited, but no Sam came. He began to grow
worried.
"Sam!"
"Yeow!"
"Come on. I'm waiting for you."
A strain on the rope told Davis that his companion had started, and
a few minutes later Sam Hickey stood beside him.
"What's the matter, Sam?"
"Nothing, except that I'm wet."
"Why didn't you come when I called you?"
"I was watching the sparks up there on the wireless aerials. Say, it is
just like a lot of lightning bugs. Did you ever watch the sparks at
night?"
"Yes, but not when I was trying to save my life and another's. I don't
believe it was half worth the effort. I am beginning to think that
there doesn't much of anything matter, so far as you are concerned.
Let's get inside now."
"How are you going to do it?"
"We will climb up under the turret, through the manhole."
"I never thought of that."
Dan unfastened the opening on the under side of the turret
projection, and, sending Sam ahead, climbed in after, closing the
opening behind them. It was intensely dark in the turret and the
room was so small that it was with difficulty that the boys could find
their way through.
For a minute or so they were engaged in climbing up to get into the
enclosure from where a ladder led down into the lower part of the
turret.
"Now, Sam, be very careful that you don't fall. This is a bad place to
be fooling around in when it is dark. I wish I could turn on the
electric lights here, but I don't know where the button is."
"Shall I light a match?"
"No, sir!"
"Why not?"
"Supposing there should chance to be some powder scattered on the
floor, and——"
"Wow! That would be a nice thing, wouldn't it? There'd be an
explosion, eh?"
"There might be. Better take the chance of bumping our heads——"
"Say, Dan, where are you going?"
"I am going to follow you. Come here. Give me your hand."
"What for?"
"Get in here. Make yourself as small as possible."
Hickey crawled into the small opening, though he did not know
where he was.
"What is this place you're stowing me in?" he demanded.
"It's the ammunition hoist," answered Dan, as he began to pull
down on a rope.
The ammunition hoist for the twelve-inch guns is a sort of dumb
waiter that is raised and lowered by pulling on a rope attached to its
top and bottom.
A few minutes later the guard on duty in the magazine corridor was
startled by a creaking and groaning sound. After listening a moment,
he traced the sound to the ammunition hoist.
All at once the hoist came down with a bang, spilling Hickey full
length on the floor of the corridor. The guard made a grab for the
newcomer, and, at the same instant, Sam Hickey wrapped both arms
about the legs of the marine who was on guard duty.
That worthy went down on top of Sam. For a minute there was a
lively tussle, but ere it had come to an end, the ammunition hoist
shot down again and Dan Davis leaped out into the passageway. He
gazed in astonishment at the two men on the floor.
"Get up, Sam! What in the world are you trying to do?"
Sam threw the guard off.
"This chocolate candy soldier jumped on me when I came down. Let
me at him——"
Davis pulled his companion away.
"You'll have to come with me," announced the guard. "I shall be
obliged to arrest you. Your conduct is suspicious."
"Well, I like that!" grumbled Sam. "First you get tossed overboard
and then you get arrested because you didn't go drown yourself. I
won't be arrested."
"Take us to the master-at-arms; he understands," said Dan.
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