SEARCH FOR NONOXIDATIVE, HYDROGEN-ABSTRACTING INITIATORS USEFUL FOR MELT GRAFTING PROCESSES

Using organic peroxide initiators in the melt grafting of monomers con taining amino groups onto polyethylenes is known to cause serious disc oloration of the resulting polymers. To eliminate the discoloration wh ile preserving the controlled degree of grafting, a nonoxidative and h ydrogen-abstracting initiator with appropriate thermal stability is ne eded. In this study, the hydrogen-abstracting capability of three azo initiators with suitable decomposition rates in the melt grafting temp erature ranges was evaluated using polyethylene cross-linking and poly propylene degradation experiments (called ''hydrogen-abstraction exper iments''). Among the three azo initiators tested, only a phenylazo ini tiator, 2-phenylazo-2,4-dimethyl-4-methoxyl-valeronitrile (V-19), demo nstrated strong hydrogen-abstracting capability from polymer backbones . These three azo initiators were used in the melt grafting of 2-(N'N- dimethylamino)ethyl methacrylate (DMAEMA) and 2-(N-t-butylamino)ethyl methacrylate (tBAEMA) onto a linear low-density polyethylene (LLDPE). In agreement with the results obtained from the hydrogen-abstraction e xperiments, only V-19 led to a considerable amount of grafting of both DMAEMA and tBAEMA onto LLDPE. As expected, polymers grafted with V- 1 9 showed significantly reduced discoloration compared with those graft ed with peroxide initiators. Further examination of the grafting resul ts indicated that the initiator efficiency, defined as the number of g rafted monomer units per radical generated from initiator decompositio n, was higher with the phenylazo initiator than with peroxide initiato rs. The hydrogen-abstracting capability of the phenyl free radical gen erated from the decomposition of V- 19 was estimated to be higher than peroxide and methyl free radicals produced in the decomposition of pe roxide initiators.