FUNCTIONALIZED-NITROXYLS FOR USE IN PEROXIDE-INITIATED MODIFICATIONS OF POLYMERS
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Delayed-onset crosslinking of commodity polymers is accomplished by peroxide-initiated reactions of 4-acryloyloxy-2,2,6,6,-tetramethylpiperidine-N-oxyl (AOTEMPO) under solvent-free conditions. Consistent with previous studies of polyethylene cure formulations, AOTEMPO provides predictable induction periods for ethylene-rich copolymers, such as poly(ethylene-co-propylene) (EPR) and poly(ethylene-co-vinyl acetate) (EVA), without compromising ultimate crosslink densities. However, cure yields for AOTEMPO formulations of 1,2-polybutadiene (vinyl-BR) and 1,4-butadiene (cis-BR) fell well below those of nitroxyl-free formulations, due to lost backbone C=C group oligomerization during the induction period. Quenching a radical in these butadiene-rich materials eliminates multiple crosslinks, creating a deficit that acrylate oligomerization cannot overcome. As a result, AOTEMPO can only delay the onset of polybutadiene cures at the expense of crosslink yield. A new extension of AOTEMPO chemistry offers a means of crosslinking polymers that cleave when treated with peroxides alone. The severe degradation incurred by poly(ethylene oxide) (PEO) and polypropylene (PP) when heated with dicumyl peroxide (DCP) is arrested by nitroxyl trapping during the induction phase of the cure. Upon complete conversion of nitroxyl, oligomerization of pendant acrylate groups dominates scission of the polymer backbone, yielding the desired thermoset. Residual peroxide initiator beyond that required to convert macromonomer groups serves only to degrade the polymer network, necessitating the optimization of peroxide and AOTEMPO concentrations. Experiments conducted on PEO, PP, polyisobutylene (PIB), and poly(isobutylene-co-isoprene) (IIR) are discussed in terms of known principles of H-atom abstraction efficiency, acrylate oligomerization, and macroradical scission. The importance of H-atom transfer to AOTEMPO formulation efficacy has motivated the study of a technique for measuring the yield of macroradicals produced by peroxide thermolysis in a polymer melt. DCP decomposition in polymer compounds containing 4-(1-naphthoyloxy)-2,2,6,6-tetramethylpiperidine-1-oxyl (NTEMPO) generates polymer-bound alkoxyamines whose concentrations can be measured by fluorescence spectroscopy. Measurements of alkoxyamine yields are comparable to gas chromatography measurements of cumyl alcohol, which is an initiator byproduct of H-atom abstraction from the polymer.