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

Abstract

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 O. 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, ...

Performance of Manganese(III) Acetylacetonate in Solvent-Borne and High-Solid Alkyd Formulations

“…Note that the lower final hardness is a common feature of the alkyd coatings treated with cobalt alternatives [29]. Furthermore, the long-term stability of the active species, originating from cobalt carboxylates, has also some negative aspects, such as a tendency to early embrittlement and aging as the polymer degradation processes, which are also based on autoxidation and can be promoted by primary driers [30].…”

“…The C-H stretching band at 3008 cm −1 is very suitable for the investigation of autoxidation kinetics as it follows consumption of the active substrate (Figure 2a). Based on our previous studies [6,30], the autoxidation of a thin layer of alkyd resins behaves as a reaction of pseudo-first-order until~50% conversion, which is related with sufficient mobility of the system. In later phases of the process, the system becomes solid and the autoxidation slows down more rapidly than the pseudo-first order reactions as it no longer satisfy the rule of a stirred liquid, which becomes evident as deviation from linearity in the logarithmic plots.…”

Connecting Gas-Phase Computational Chemistry to Condensed Phase Kinetic Modeling: The State-of-the-Art

“…Due to light, heat, humidity, air, or a combination of those, many polymers undergo degradation [ 313 , 314 , 315 , 316 , 317 , 318 , 319 , 320 , 321 ], as also shown in Figure 7 . It can be either an undesirable process that alters the polymer processing and causes the decrease in the (average) molar mass, or it can be a targeted process if polymer recycling through depolymerization is considered.…”

Fast Oxygen Scavenging of Macroporous Poly(Norbornadiene) Prepared by Ring‐Opening Metathesis Polymerization

“…This small discrepancy might have several reasons as described in the publication introducing the oxygen partial pressure method 10. Additionally, the formation of volatile oxygen bearing species, for example water, is conceivable11 and might be responsible for underestimating the oxygen consumption obtained from mass balancing. Samples prepared with Span 80 as the surfactant showed a similar reactivity against oxygen (Figure S10, Supporting Information).…”

“…Further, ≈25 ppm Ru species are present which might act as the initial radical source. Once polymer centered radicals are formed the classical autoxidation mechanism11 is followed: Oxygen reacts with the (polymer centered) radicals forming peroxyl radicals which rapidly propagates via energetically easily feasible hydrogen atom abstractions from double allylic tertiary carbon centers in pNBD leading eventually to products similar to that formed during oxidation of pDCPD 3,4…”