Radiochemical studies of free‐radical vinyl polymerizations. Part II. The polymerization of vinyl monomers in the presence of polyisoprenes: Use of C14‐labeled initiators to determine the mechanism of graft‐interpolymer formation

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Abstract
Polymerization of methyl methacrylate at 60°C. by benzoyl peroxide in benzene solutions of polyisoprenes (e.g., natural rubber and gutta‐percha) yields polymethyl methacrylate and a graft interpolymer having polymethyl methacrylate side chains attached to the polyisoprene backbone. Use of azobisisobutyronitrile (AZBN) as initiator in comparable systems gives free polymethyl methacrylate, but no graft interpolymer. The cause of this specific initiator effect, and the mechanism of graft polymerization have been determined by using C14‐labeled initiators. Benzoyl peroxide initiates graft polymerization by prior reaction of the derived phenyl and benzoyloxy radicals with the polyisoprene by addition to the double bond, and by abstracting α‐methylenic hydrogen atoms to give polyisoprenic alkyl and alkenyl radicals, respectively, which act as loci for methyl methacrylate polymerization. In the system gutta‐percha‐methyl methacrylate, at 60°C., 35–45% of the grafted vinyl polymer side chains are initiated by hydrogen abstraction, and about 90% of the initiator radicals undergoing addition to the double bond are benzoyloxy. The modes of termination of the grafted and free polymethyl methacrylate radicals have been evaluated semiquantitatively, and it is found that, in the graft interpolymer, 13–27% of the polymethyl methacrylate chains have both ends attached to gutta‐percha chains (i.e., have an “I” structure), while 73–87% are attached at one end only to gutta‐percha chains (i.e., are “T” structures). The reliability of the separatory procedure previously devised8 to resolve the bulk polymer obtained in the graft‐polymerization system has now been verified by use of the isotopedilution technique. The inability of AZBN to initiate graft polymerization is attributed to the markedly inferior capacity of the resonance‐stabilized Me2Ċ(CN) radicals, relative to C6H5· and C6H5COO·, to engage in double bond addition and hydrogen abstraction reactions.

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