The Fate of the Peroxyl Radical in Autoxidation: How Does Polymer Degradation Really Occur?

Bolland and Gee's basic autoxidation scheme (BAS) for lipids and rubbers has long been accepted as a general scheme for the autoxidation of all polymers. This scheme describes a chain process of initiation, propagation, and termination to describe the degradation of polymers in the presence of O2. Central to this scheme is the conjecture that propagation of damage to the next polymer chain occurs via hydrogen atom transfer with a peroxyl radical. However, this reaction is strongly thermodynamically disfavored for all but unsaturated polymers, where the product allylic radical is resonance-stabilized. Paradoxically, there is no denying that the autocatalytic degradation and oxidation of saturated polymers still occurs. Critical analysis of the literature, described herein, has begun to unravel this mystery. One possibility is that the BAS still holds for saturated polymers but only at unsaturated defect sites, where H transfer is thermodynamically favorable. Another is that peroxyl termination rather than H transfer is dominant. If this were the case, tertiary peroxyl radicals (formed at quaternary centers or quaternary branching defects) may terminate to form alkoxy radicals, which can much more readily undergo chain transfer. This process would lead to the creation of hydroxy groups on the degraded polymer. On the other hand, primary and secondary peroxyl radicals would terminate to form nonradical products and halt further degradation. As a result, under this scenario the degree of branching and substitution would have a major effect on polymer stability. Herein we survey studies of polymer degradation products and of the effect of polymer structure on stability and show that indeed peroxyl termination is competitive with peroxyl transfer and possibly dominant under some conditions. It is also feasible that oxygen may not be the only reactive atmospheric species involved in catalyzing polymer degradation. Herein we outline plausible mechanisms involving ozone, hydroperoxyl radical, and hydroxyl radical that have all been suggested in the literature and can account for the experimentally observed formation of hydroperoxides without invoking peroxyl transfer. We also show that oxygen itself has even been reported to slow the degradation of poly(methyl methacrylate)s, which might be expected if peroxyl radicals are unreactive toward hydrogen transfer. Discrepancies between the rate of oxidation and the rate of degradation have been observed for polyolefins and also support the counterintuitive notion that oxygen stabilizes these polymers against degradation. We show that together these studies support alternative mechanisms for polymer degradation. A thorough assessment of kinetic studies reported in the literature indicates that they are limited by their propensity to use models based on the BAS, disregarding the chemical differences intrinsic to each class of polymer. Thus, we propose that further work must be done to fully grasp the complex mechanism of polymer degradation under ambient conditions. Nonetheless, our analysis of the literature points to measures that can be used to enhance or prevent polymer degradation and indicates that we should focus beyond just the role of oxygen toward the specific chemical nature and environment of the polymer at hand.M.L.C. gratefully acknowledges the Australian Research Council (ARC) for a Georgina Sweet ARC Laureate Fellowship and Dr. Anya Gryn’ova, Dr. Richmond Lee, and Professor Francoise Reyniers for many stimulating discussions ̧ about autoxidation mechanisms

Another Way of Saying Enough: Environmental Concern and Popular Mobilization in Kyrgyzstan

This article is a foray into the understudied issue of environmental protest politics in Central Asia. Specifically, it uses Kyrgyzstan as a case study to test the argument that environmental concerns mobilized people to engage in protest and in ways different from other kinds of protest. This essay presents the first systematic study of public opinion about the environment in Kyrgyzstan. It includes results from a 2009 nationwide survey, over 100 expert and elite interviews, and newspaper content analysis. Furthermore, it spatially analyzes these results to identify geographical variation in public perception and political event occurrence patterns. Protest engagement is a complex process determined by the interaction of several factors, and is not explained solely by affluence, rationality, or grievances. Eco-mobilization - collective political action about the environment - represents a class of protest events that offers a different view into mass discontent in the former Soviet Union and neo-patrimonial societies. The study finds that these political actions about the environment are not necessarily elite driven; there is a basic foundation of national concern and salience of these issues, and demonstrated environmental beliefs do help to explain protest behavior