Radical chain polymerization autoinhibition

Allylic transfer is also variously named degradative chain transfer, autoinhibition, or allylic termination. The stable radical derived from the monomer by reactions like (6-90) are slow to reinitiate and prone to terminate. Low-molecular-weight products are therefore formed at slow rates and small concentrations of allyl monomers can inhibit or retard the polymerization of more reactive monomers. 

An especially interesting case of inhibition is the internal or autoinhibition of allylic monomers (CH2=CH—CH2Y). Allylic monomers such as allyl acetate polymerize at abnormally low rates with the unexpected dependence of the rate on the first power of the initiator concentration. Further, the degree of polymerization, which is independent of the polymerization rate, is very low—only 14 for allyl acetate. These effects are the consequence of degradative chain transfer (case 4 in Table 3-3). The propagating radical in such a polymerization is very reactive, while the allylic C—H (the C—H bond alpha to the double bond) in the monomer is quite weak—resulting in facile chain transfer to monomer... 

The first of these is an example of induced decomposition of benzoyl peroxide (see Section 1.3.1). The second shows chain transfer to toluene, which is a solvent commonly used for solution polymerization. The last reaction is characteristic of allylic monomers and is prevalent to the extent that homopolymerizations of allylic monomers yield only chains with degrees of polymerization below about 20 (i.e. oligomeric chains). The high incidence of chain transfer to allylic monomers results from the high resonance stability of the allylic radical produced and the reaction is often referred to as autoinhibition. 

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