Chain Transfer and Reaction Inhibitors

In practice, another type of reaction sometimes occurs in free-radical addition polymerizations. These chain-transfer reactions kill a growing chain radical and can start a new one in its place (as long as the radical transfers to another monomer)  

chain transfer results in shorter chains, and if the reactions in Equation 9.32a are not too frequent compared to the propagation reaction and do not have very low rate constants, chain transfer will not change the overall rate of polymerization appreciably. 

The compound R H in the above reaction is known as a chain-transfer agent. They are also sometimes referred to as chain terminating agents, because the free-radical on the growing polymer Px is removed. Under appropriate conditions, almost anything in the reaction mass may act as a chain-transfer agent, including initiator, monomer, solvent, and dead polymer. 

Example 9.3 Show, using dots to represent the electrons involved, how chain transfer to a dead polymer leads to long-chain branching in polyethylene. 

Example 9.4 Show, as above, how short branches arise in polyethylene when a growing chain bites its own back, that is, the radical transfers to an atom a few [5-8] carbon atoms down the chain. 

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