Radical-mediated modification of polyolefins: investigation of the synthesis of graft copolymers
Kaufman, Michael Steven
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A range of strategies exist for the production of polyolefin block copolymers with well-defined morphologies for the compatibilization of immiscible blends; however, high sensitivity to impurities and use of expensive transition-metal catalysts limit their economic viability. An alternative method is the solvent-free coupling of dislike polymers using peroxides and multifunctional coagents. This technique, however, does not distinguish between a blend of gelled homopolymers and the intended copolymer. The use of a solution-based approach, like the one described in this report, for the radical-mediated coupling of polyolefins with dislike polymers allows for the full characterization of the copolymer without the formation of gel. A one- and two-step synthesis for the coupling of two homopolymers in solution is demonstrated using triallyl trimesate. The one-step approach produced a copolymer in low yields due to the differences in reactivity between the homopolymers. The two-step synthesis consisted of the solvent-free grafting of the coagent to the less reactive polymer, followed by coupling in solution with the other polymer. Though this technique demonstrated improved yields over that of the one-step approach, the overall yields were limited due to the dilution effects of the high solvent levels needed for comiscibility. The coupling of polyethylene with poly(ethylene oxide) had a maximum graft yield of 9.5%, whereas the coupling between polypropylene and a thermoplastic polyolefin elastomer was negligible. It was found that the grafting of allylic ester coagents to polyethylene results in an uneven graft distribution. Given enough peroxide and coagent, the material will reach its gel point where there is a small, high molecular weight population with a disproportionately high graft content, while the remaining chains contained a modest concentration of bound coagent. Furthermore, a survey of the effectiveness of several common coagents in the radical-mediated crosslinking of polyethylene was investigated where it was found that allylic coagents had a greater contribution to the crosslink density of the resin than vinyl coagents.