Branching in vinyl polymers

Branching in Vinyl Polymers.—It was pointed out in Chapter IV that a molecule of the monomer may sometimes perform the function of a chain transfer agent. If, as usually is the case, the chain transfer involves removal of an atom (often hydrogen) from the monomer, the ensuing growth from the transfer radical will produce a poly- 

Another chain radical may acquire the terminal unit of molecule XV in one of its propagation steps i.e., the terminal unsaturated unit may be incorporated as one of the units combined in another growing chain. When this occurs a branched polymer molecule XVI is produced.  

The same result might follow from chain transfer by disproportionation between chain radical and monomer according to reaction (32) of Chapter IV. 

The reaction of a chain radical with a unit of a previously formed polymer represents an additional possible chain transfer process not previously considered in Chapter IV. The point of attack might again be located in the substituent X, or it might involve removal of the tertiary hydrogen on the substituted chain carbon. The following sequence of reactions, in which the latter alternative has arbitrarily been assumed, would then lead to a branched polymer molecule as indicated.  

The immediate result of the intervention of the chain transfer process indicated in the first step is the termination of a growing chain and the reactivation of a polymer molecule, which then adds monomer to gener- 

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