TUTORIAL 14
Unit 4: Carbon Compounds
PRODUCTS FROM OIL
POLYMERS
Polymers are very large molecules (sometimes called macromolecules). Each polymer molecule is
a long chain of (mainly carbon) atoms. Polymers are made from many smaller molecules,
called monomers.
These monomers (the starting materials) are often alkenes.
1. The use of a polymer depends on its physical properties which are derived from its structure.
2. One can chemically modify polymers to change their physical properties to suit a particular use
or application:
If you increase the chain length of the polymer molecules you increase the
intermolecular forces between the chains so it is stronger and less flexible and has a
higher softening/melting point.
If you shorten the average chain length, the polymer has lower softening/melting and is
easier to shape, and the plastic is more flexible.
Cross-linking, i.e. adding a cross-linking agent to the monomer/polymer mixture which
forms strong chemical bonds between polymer chains is discussed below and this
modification is most important when comparing thermosoftening plastics and
thermosetting plastics
POLYMERIZATION
The process (chemical reaction) that turns monomers into polymers is called polymerisation. The
conditions used for polymerisation are heat, pressure and a catalyst. Polymers produce a very
wide range of materials with many uses. New materials are often used to replace older ones because
the new material has superior properties. The properties of the material depend on what
the polymer is made from and its structure.
The general public know polymers as plastics. In science, the word plastic refers to a specific type
of deformation. A polymer may show plasticity, or it may not. Call it a polymer, not a plastic.
ADDITION POLYMERIZATION
In addition polymerisation there is no other product except the polymer itself because all the
molecules are used up to form longer and longer polymer chains with the same repeating unit based on
the original monomer (see the example below).
E XAMPLE : P OLY ( ETHENE )
The equation for the formation of the polymer poly(ethene) from the monomer ethene is shown
below. It is often commercially called 'polythene' or written incorrectly as polyethene or polyethylene.
The equation for the polymerisation of ethene is shown in several ways below:
To indicate the repeating nature of the molecule, bond dashes go through the brackets giving the
'shorthand' or 'abbreviated' representation of the structure of poly(ethene) -(-CH2─CH2-)- where n is
the very large number, and can represent hundreds or even thousands of repeating units in one
polymer molecule, in this case, poly(ethene).
Above is the displayed formula style equation for poly(ethene) formation, or a simpler molecular
formula style equation below:
n CH2=CH2 ===> -(-CH2─CH2-) n-
The repeating unit shown in brackets for poly(ethene) is (CH2─CH2) and n is a very large number, and
can represent hundreds or thousands of repeating units in one molecule.
-(-CH2─CH2-) n- is a sensible way of writing these huge molecules!
CONDENSATION POLYMERIZATION
Condensation polymerisation involves monomers with two functional groups (one at each end of
the molecule). When these types of monomers react they join together (polymerise), small molecules
such as water are eliminated in the process, and so the reactions are called condensation
reactions, hence the process is called condensation polymerisation.
E XAMPLE : P OLYESTER
A polyester can be made from ethane diol (an alcohol with two hydroxy groups two -OH's)
and hexanedioic acid (a carboxylic acid with two -COOH groups).
These are the two starting monomers prior to polymerisation and both must have a reactive group at
each end.
the ethanediol monomer molecule HO-CH2-CH2-OH
the hexanedioic acid molecule monomer molecule HOOCCH2CH2CH2CH2COOH
Alcohols react with carboxylic acid to form esters with the elimination of water. In this case the ester
linkage is formed at both ends of each molecule with the elimination of water molecules.
Polyesters are condensation polymers because of how they are formed - by this condensation reaction
that eliminates a small molecule to form the bond between the monomers.
Where n is the very large number of monomer molecules, the condensation polymerisation of ethane
diol and hexanedioic acid can be represented as...
n HO-CH2-CH2-OH + n HOOCCH2CH2CH2CH2COOH ===>-(-CH2-CH2-OOC-CH2CH2CH2CH2-COO-) n- + n H2O
or more simply: n HO-[][]-OH + n HOOC-[][][][]-COOH ==> -(-[][]-OOC-[][][][]-COO-)n-+ 2n H2O
where n is a very large number!
(Rectangles represent rest of the carbon chain in each unit)
