Polymerisation

Polymerisation
Chemical reaction in which monomer is converted to a polymer
Links monomer molecules into polymers
2 main polymerisation reactions
- Condensation polymerisation (step-growth)
- Addition Polymerisation (chain-growth)

Condensation (Step-Growth) Polymerisation

2 or more small molecules (monomer) of different chemical compositions combine to form chains of alternating chemical groups
Small molecules such as water are evolved (hence the name ‘condensation polymerisation’)
Repeat units are similar to the monomers

Can be divided mainly into 2 groups:
i) 2 different monomers with each possesses only 1 type of functional group
ii) A single monomer containing more than 1 type of functional groups
A number of chemical reactions may be used such as esterification, amidation and formation of urethanes
Usually proceeds by the reaction between 2 functional groups e.g. carboxyl, isocyanate and hydroxyl groups
Reaction proceeds in a stepwise manner
High molecular weight polymers are achieved when the polymerisation are kept until high conversion (98-99 %)
To achieve high molecular weight polymers, no side reactions shall occur and the formed condensates (such as water) must be removed

Examples of typical step-growth polymers:
i) Polyamide (nylon)

H2N – R – NH2 + HOOC-R’-COOH H-(NH-R-NH-CO-R’-CO)n-OH
+ H2O

ii) Polyester

HO-R-OH + HOOC-R’-COOH H-(O-R-O-CO-R’-CO)n-OH + H2O


iii) Polysiloxane

HO-SiR2-OH H-(O-Si-R2)-OH + H2O


iv) Polycarbonate

Addition (Chain-Growth) Polymerisation

Only one monomer species required
Reaction initiated using a suitable initiator
The initiator activates the monomer molecules by the breaking of a double bond between atoms
Reaction proceeds between reactive sites continues until the initiator is used up (at this point, no more bonding sites are created and the reaction stops)

3 types of chain-growth polymerisation:
- Free radical polymerisation
- Cationic polymerisation
- Anionic polymerisation

Monomers with electron-donating groups form stable carbenium ions and polymerise best with cationic catalysts
Conversely, monomers with electron-withdrawing substituents form stable anions and require anionic catalysts

Free radical polymerisation may be considered an intermediate case favoured by moderate electron withdrawal groups from the double bond plus conjugation in the monomer
Many monomers can be polymerised by more than one mechanisms
Typical chain-growth polymers include:

Monomer Repeat unit

i) Polyethylene CH2=CH2 -(CH2-CH2)-

ii) Polypropylene CH2=CH2-CH3 -(CH2-CHCH3)-

iii) Poly(vinyl chloride) CH2=CH2-Cl -(CH2-CHCl)-

iv) Polyisoprene CH2=CH-CCH3=CH2 - CH2-CH=CCH3-CH2-

iv) Poly(methyl methacrylate)

Free Radical Polymerisation
Chain-growth polymerisation in which the active sites are free radicals
Most common type of chain-growth polymerisation due to the wide variety of monomers that can be polymerised by this mechanism

The reaction involves 3 steps:
i) Initiation
ii) Propagation
iii) Termination

Initiation
Usually by thermal decomposition of an added thermal free radical initiator, commonly an organic peroxide, hydroperoxide or azo compound
Any other method which produces free radicals may be used such as ultraviolet (UV) light (photopolymerisation), high energy radiation or thermal activation of the monomer (thermal polymerisation)

Initiation
The initiation step is represented as follows:

I hv 2 R.

(where I is the initiator and R. is the radical formed by the decomposition of the initiator)

Two reactions commonly used to produce radicals for polymerisation are the thermal and photochemical decomposition of
i) Benzoyl peroxide
ii) Azobisisobutyronitrile (AIBN)
The primary radical R. initiates polymerisation by addition to monomer to form an activated monomer:

R. + M R – M. (where M is monomer)

Propagation
Successive monomers is added which propagates the chain
By rapid successive addition of further monomer molecules, a high molecular weight polymer molecule is rapidly formed
Propagation
R – M. M R – M – M. M R – M – M - M. , etc.

Or in general,

R – Mn – M. + M R – M n+1 M.

Addition of further monomer molecules is predominantly head-to-tail