Reactivity of radical centres during polymerization

An active centre is formed by the addition of a primary radical (for example from an initiator) to the monomer. A polar primary radical may considerably affect the electronic configuration of the centre, and this effect will decay with the number of added monomers. The first additions may proceed by a different mechanism and may even be more rapid or slower than additions at later stages, when the inductive effect has been suppressed. 

Dependence of activation energy difference on the ratio of pre-exponential factors of transfer (tr) and propagation (p) for the addition of MMA to a radical with the 

Barson and Ensor [34] have studied the temperature dependence of the apparent transfer constant in telomerization of MM A with CBr4. They found dramatic differences in the Arrhenius parameters between the first, second, etc. monomer additions to the CBr radical (Table 2.) The authors explain these results by electron transfer from radical to monomer, yielding the combining ions 

In this way, the activation energy of addition can be reduced with respect to the reaction which is unaffected by the polar end. The rate of reaction of the active centre with the telogent CBr4 also changes with chain length, of course. 

Another type of reactivity change of the centres was observed in the polymerization of acetylene and its derivatives. The generated chain with conjugated double bonds makes possible a far-reaching delocalization of the unpaired electron. The reactivity of the active centre decreases with chain length [35, 36], so that the number of monomer additions is limited. 

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