Free-Radical Grafting Reactions to Polymers with Double Bonds
Carbon to carbon double bonds, either in the backbone or in the pendant groups, are potential sites for free-radical attacks. In addition to the double bonds, the hydrogen atoms on the neighboring carbons are allylic and potential sites for chain transferring. Because rubbers, natural and synthetic, possess such unsaturations, they are used extensively as backbones for various grafting reactions. Whether the reactivity of the initiating radical is important in determining grafting efficiency is not completely established. Graft copolymers of poly (methyl methacrylate) on gutta-percha, however, form in good yields when the initiator is benzoyl peroxide. Yet, when azobisisobutyronitrile is used, only a mixture of homopolymers forms. Work with 14C-labeled initiators shows that the primary radicals react both by addition to the double bonds and by transfer to the methylene group.
Grafting reactions to polybutadiene, however, proceed via chain transferring from the growing chain radical to the backbone . Nevertheless, strong evidence also shows that the initiator radicals can interact directly with polymeric backbones . When graft copolymers of polystyrene to natural rubber form, the chain length of the attached branches equals to the chain lengths of the unattached polystyrene homopolymer that forms simultaneously . This led to the following conclusions :
1. As the concentration of rubber increases, the length of the grafted branches diminishes, while their number remains the same.
2. When the concentration of the initiator increases, the length of the branches diminishes, but the number of branches increases.
3. When the concentration of monomers increases, the length of the branches also increases, but their number diminishes.
4. When the polymerization time increases, the length of the branches remains the same, but their number increases.
