Overall Scheme of Radical Chain Polymerization

Radical chain polymerization, as noted above, is a chain reaction consisting of a sequence of three steps—initiation, propagation, and termination. The initiation step is considered to involve two reactions. The first is the production of free radicals. There are many ways to accomplish this, but the most common method involves the use of a thermolabile compound, called an initiator (or catalyst), which decomposes to yield free radicals. The usual case is the homolytic dissociation of an initiator I to yield a pair of radicals R- 

Initiation is followed by chain propagation, which consists of the growth of Ml by the successive addition of large numbers of monomer molecules. Each addition creates a new radical which has the same identity as the previous one, except that it is larger by one monomer unit. The successive additions may be represented by 

At some point, the propagation reaction is terminated due to annihilation of the radical center of the propagating chain. The annihilation of the radical centers occurs by bimolecular reaction between radicals. Two radicals react with each other by combination (also known as coupling). 

These two different modes of termination can be represented in general terms by 

Termination can also occur by a combination of coupling and disproportionation. However, since both the reactions result in the formation of dead polymer molecule(s) one can also express the termination step by 

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