Mechanistic Insights into N-Acyloxyamine-Initiated Controlled Degradation of Polypropylene: The Unexpected Role of Keto-Enol Tautomerization in Carboxylate Radical Chemistry.

Abstract

Controlled degradation of polypropylene (PP) is used industrially to improve the properties of crude PP. While this degradation is traditionally initiated by organic peroxides, N-acyloxyamines are now preferred due to their greater stability. However, their mechanism of action remains unclear. Using high-level ab initio calculations, we show that N-O homolysis is the most likely fragmentation pathway available to N-acyloxyamines, in contrast to the more usual C-O homolysis observed for the closely related N-alkoxyamines. This would, in theory, generate aminyl and carboxylate radicals, with the latter undergoing decarboxylation to generate methyl radicals. However, the enol forms of N-acyloxyamines are significantly less thermally stable, having bond dissociation free energies that are over 50 kJ/mol below those of their keto equivalents. Under conditions where keto-enol tautomerism is feasible, enol N-O homolysis, which forms the more stable acetic acid radical, would be the dominant degradation pathway. This reveals the crucial and underappreciated role that polar impurities play in the initiation process of enolizable initiators and may explain contradictory observations in the experimental literature. The product aminyl radicals are susceptible to β-fragmentation, releasing alkyl radicals and affording imines, which in turn are susceptible to allylic H-abstraction and further β-fragmentation leading to dialkylpyridines as the ultimate degradation products.