The mechanism of dehydrochlorination has been studied by examining the degradation of polychloroprene/poly(methyl methacrylate) blends, using thermal volatilization analysis and infrared spectroscopy; the behaviour has been compared with that previously found for PVCPMMA blends. Unlike the latter system, the potychloroprene blends did not show any increased production of methyl methac~'late monomer in the early stages of breakdown. The stabilization effect on PMMA due to reaction of ester groups with hydrogen chloride, on the other hand, is much more evident in the case of polychloroprene blends than for PVC. PVC dehydrochlorination is retarded by the presence of PMMA, but evolution of hydrogen chloride from polychloroprene is unaffected to any significant extent. It is concluded that the dehydrocNorination of polychloroprene is not a radical chain process. A unimolecular mechanism is suggested.
IN THE tWO previous papers in this series, the stability of various samples of polychloroprene have been compared, and kinetic parameters have been evaluated ;(1~ the products of degradation of the polymer have been investigated/-'~ Polychloroprene (PC) and poly(vinyl chloride) (PVC) show certain similarities. Thus, both polymers break down in two main stages, the first yielding hydrogen chloride and the second involving fragmentation of a polyene chain. There are also considerable differences in behaviour between the two polymers. PVC has a much lower energy of activation, and undergoes dehydrochlorination in a lower temperature range under pro~ammed heating conditions. The products of degradation of PC are more complex, including some chlorinated hydrocarbons; methane is formed in greater amounts than from PVC Conjugation is much less extensive in the polyenes formed in degraded PC samples, and some methyl groups have been found.
The investigations reported so far have given little indication of the mechanism of dehydrochlorination, although the fact that long polyene sequences are not readily formed suggests that a radical chain mechanism might be less likely in PC degadation than for PVC.
McNeiU and Neil Ca'*~ studied the degradation of PVC/poly(methyl methacrylate) blends and found interesting interactions to occur in the degradation of the mixed sampies. PMMA samples, prepared in such a way as to be stable to over 250 Β° when heated alone under programmed heating conditions (I0Β°/min), broke down to yield monomer in substantial quantities from 200 Β° when blended with PVC in a film. This interaction was investigated in detail and it was concluded that it could best be explained in terms of initial chlorine radical attack on the PMMA, followed by chain scission and depolymerization. These mixture experiments, therefore, gave a strong indication that