Chain growth mechanism
Chain growth mechanism:
The mechanism involves three steps.
I. Chain initiation II. Chain propagation III Chain termination
I. Chain Initiation: This involves two steps.
i) Decomposition of the initiator to form free radicals
ii) Addition of the first vinyl monomer molecule to the free radical to form intermediate (I)
R CH2 = CH R – CH2 – CH
Initiator added to the monomer undergoes dissociation to form free radicals. Initiators commonly used are acetyl peroxide (CH3CO– O – O – COCH3), benzoyl peroxide (C6H5COO-O-COC6H5) and azobis –isobutyronitrile (AIBN).
The intermediate (free radical) immediately reacts with a monomer molecule to form the reactive species which is responsible for the growth of the polymer chain.
Termination reaction may occur due to a coupling reaction or a disproportionation reaction.
- In coupling or combination reaction two reactive or, free radical polymer chains combine together to form a dead polymer.
- The disproportion reaction involves the transfer of a hydrogen atom from one of the free radical polymer chain to another free radical giving rise to one saturated and one unsaturated polymer molecules. Disproportionation also terminates the propagation or growth of the polymer chain resulting in a dead polymer
- The chain can be terminated by reaction with the solvent molecules like carbon tetrachloride.
- Chain can be terminated by the reaction of growing polymer chain with inhibitors like hydroquinone and trinitrobenzene.