Mechanism of some transfers

A very efficient transfer agent in polymerizations of styrene, vinyl acetate and methyl methacrylate is CBr4. 

Barson and Ensor assume that transfer is stimulated by electron migration from an active centre to the transfer agent, i. e. by a contribution of non--bonded resonance of the type [34] 

Not only carbon-halogen bonds but also halogen with other atoms, for example with E are efficient sources of transfer, especially when a resonance-stabilized radical is generated by the transfer. 

The value of Ctr decreases in the chlorophosphine series Ph2PCl PhPCl2 PC13. The differences are thought to be due to the ability of the phenyl ring to stabilize the transition complex [22] 

A carbon-halogen bond in the monomer, polymer or solvent is always a centre of transfer reactions. Transfer to monomer controls chain length in polyvinyl chloride so efficiently that the molecular mass of the products is independent of the amount of initiator over a rather wide concentration range [35], Also, vinylidene chloride lowers the polymerization degree of its own polymer by transfer [36]. 

The presence of a carbon-halogen bond is not absolutely essential for the occurrence of tranfer to monomer. Moore et al. [37] studied styrene polymerization with y-irradiation. They measured thermodynamic quantities, particularly the Gibbs energy, enthalpy, entropy and volume changes by the method of rotating sectors and found that transfer to monomer is negli- 

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