Polymerization : The process by which the monomers molecules are linked to form a big polymer

molecule, is called polymerization.

Classification of polymerization:

Chain polymerization:

Chain polymerization is characterized by a self addition of the monomer molecules to each other very
rapidly through a chain reaction no bioproduct is formed the produce has the same elemental
composition as that of the monomer. The bifunctionality is provided by the double bonds present in the
monomer. Compounds containing reactive double bond can, therefore, undergo a chain polymerized
reaction.

Typical Example:

1. Vinyl compounds:
a. Vinyl alcohol (CH2=CHOH)
b. Vinyl Chloride (CH2=CHCl)
c. Vinyl ethyl ether (CH2=CHOC2H5)
2. Allyl compounds
a. Allyl alcohol (CH2=CHCH2OH)
b. Allyl Chloride (CH2=CHCH2Cl)
3. Olefines
a. Ethylene (CH2=CH2)
b. Propylene (CH2=CHCH3)

Since the majority of there monomers fall under the vinyline group chain polymerization is also called
vinyl polymerization.

Example of chain polymerization:

Chain polymerization consists of three major steps:

a. Initiation
b. Propagation
 c. Termination

These process can be brought about by a free radical, ionic or coordination mechanism.

Free Radical Polymerization:

The initiation of the polymer chain growth is brought about by free radicals produced by the
decomposition of compounds called initiation. The term chain growth represents a process involving a
continuous and very rapid addition of the monomer units to form polymer molecules or polymer chains.
As more and more monomer units are added, the length of the polymer chains increases continuously
and the chains grow rapidly.

Initiator:

The Initiators are thermally unstable compounds and decompose into products called free radicals. If R-
R is an initiator and the pair of electron forming the bond between the two R’s can be represented by
dots, the initiator can be written as

The decomposition of the initiator to form free radicals can be induced by heat energy, light energy or
catalysts.

A host low molecular weight compounds comprising mainly azo compounds, peroxides, hydroperoxides
etc are useful initiators.

Chemical name Chemical structure Free radical formed

The thermal decomposition takes place can be illustrated by the following example:

The rate of decomposition of these initiator depends apart from their chemical nature, reaction
temperature and the solvent used.

Polymer stages:
Initiation: A free radical contains a lone electron.A lone electron is like a prospective bachelor, always
looking for another lone electron to couple with and established. A free radical is therefore highly
reactive and can attach any molecule which either has a lone electron or prepared, to part with one of
its electron.This is what happens in the process of initiation.

The free radical R* attacks the double bond in the monomer molecule, resulting in the following
chemical change.

The π electros are not very close to the nucleus and produced a little away from the axis of the molecule
and tend to be in far away from each other so if get favorable condition then break.

This process of forming the active radicals is called initiation.

Propagation:

After initiation comes propagation.

In the propagation step, the radical site at the first monomer unite attacks the double bond of the fresh
monomer molecule. This results in the linking up of the second monomer unit to the first and the
transfer of the radical site from the first monomer unite to the second, by the unpair electron transfer
process.

The mode of addition of the incoming monomer to growing chain can be:

a. Head to tail
b. Tail to tail
c. Tail to head
d. Head to head

If we call the –CH2- and the CH-X parts of monomeric unit its head and tail respectively, the four modes
of addition can be represented as follows:

a. Head to tail

b. Tail to tail
 c. Tail to head

d. Head to head

Termination : Afterpropagation comes termination. The process of stopping the further addition of
monomer unit to the growing chain is called termination.

There are four kinds of termination:

a. Termination by coupling
b. Termination by disproportion
c. Termination by chain transfer
d. Termination by inhibitors

Termination by Coupling:

In this case, the two growing chains unite by the coupling of lone electron present in each chain to
for an electron pair and thus nullify their reactiveress. Since this process involves the coupling of the
two lone electrons, this kind of termination by coupling:

Termination by disproportion: In this case, one H from one growing chain is abstracted by their
growing chain is abstracted by the other growing chain and utilized by the lone electron for getting
established, while the chain, which had donated the H, gets established by formation of double
bond:

Termination by chain transfer: In the case of transfer reaction, the growth of one polymer chain is
stopped or arrested, forming a dead polymer, there is the simultaneous generation of a new free
radical capable of initiating a fresh polymer chain growth. This reaction takes place by the
abstraction of a hydrogen atom or some other atom from the initiator, monomer or from any other
species present in the system. This represented by
The growing chain is now terminated, but a new free radical R’ is formed. R’ will now initiate the
polymer chain growth a fresh, which will followed by the chain propagation. There the termination
of one chain growth and the initiation of a new one takes place simultaneously.

Termination by inhibition: Inhabitiors are chemical substances capable of inhibitory or killing the
chain growth by combining with the active free radicals and forming either stable products or
inactive free radicals. Hydroquinone, nitrobenzene, dianitrobenzene are some of the inhibitors
customarily used in the polymer industry. The inhibitory action of thier compounds can be potted by
the following example

[The free radical nature of the end group is however powerful enough to recombine with the radical
of another growing chain and terminate the growth of the latter:

In this case, we can see that a single molecule of the inhibitor has killed two growing chain.]

Atmospheric oxygen is good inhibitor. The inhibiting action of oxygen is due to its biradical nature.

This is why radical polymerization is generally carried out under a nitrogen atmosphere, try to avoid
contact with atmospheric oxygen.

Chain transfer Agent: The compound with which chain transfer reaction takes place is called chain
transfer agents.

Eg. CCl4,CBr4, H2 gas etc

Chain transfer Reaction: In certain case, the chain termination occurs by an interfering reaction
called chain transfer reaction.

Chain transfer reaction is done by chain transfer agents like CCl4, CBr4 etc.
Inhibitor:

Inhibitor: The chemical substances capable of inhibiting or killing the chain growth polymerization by
coupling with the active free radicals are known as inhibitors.

Eg. Nitrobenzene, Dinitrobenzene etc.

Retarder: The compounds which slow down the chain growth in polymerization is called reacter. It
also decreases the molecular wt. of the polymer.

Eg-Aromatic nitro-compounds and metal salts.

Polymerization is a process which allows simple low molecular weight compounds to combine and
form a complex high molecular weight compound. For this each molecule of the compound should
have the capability to react with other molecules of the same or some other molecules. In other
words, they should have the functionality of at least two. The functionality of a compound depends
on the number of reactive sites it has. A compound assumes functionality because of the presence
of reactive functional groups like –OH, -COOH, -NH2 etc. The number of such functional groups per
molecule of the compound defines its functionality.

Acetic Acid CH3COOH -COOH Monofunctional

Some compounds, however do not contain any reactive functional group, but the presence of
double or triple bonds in the molecule bestow polyfunctionality on them. For example, ethylene or
of a halogen has a functionality of two because of the presence of the double bond,

CH2=CH2+H2 → CH3-CH3

CH2=CH2+H2 →CH2Cl-CH2Cl

Low molecular weight compounds having a functionality of two or more are called monomers. For
them to polymerize, we have only to induce suitable chemical reactions between them. Then these
monomer molecules combine (at the reactive sites or through the functional groups) to form higher
molecular weight molecules.
Depending on the functionality of the monomers used, we get linear, branched or three dimensional
cross linked polymer.

For example, if we take bifunctional monomers, each molecule can only add two or more molecule
can only add two or more molecules and give a linear polymer as shown.

If we take trifunctional monomers, we get a cross linked polymer.