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

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 ß-fragmentation, releasing alkyl radicals and affording imines, which in turn are susceptible to allylic H-abstraction and further ß-fragmentation leading to dialkylpyridines as the ultimate degradation products.

Radicals and Polymers.

This article describes some selected results of my 30 years as an industrial researcher. During this time, I had the chance to work on many very different projects. About two thirds of them were dealing with the fascinating interaction of radicals and polymers. Of the more than 350 million tons of industrial polymers produced worldwide every year, about 50% are made by the radical polymerization of monomers that are often stabilized against undesired premature polymerization by addition of polymerization inhibitors. Stabilizers are necessary to protect the majority of polymeric materials during their service life from radical degradation processes triggered by oxygen, heat or light. Modification of the polymeric architecture can be easily achieved via radical polymer analogous reactions. One of the most important developments in polymer science in the last 25 years is controlled radical polymerization. Recently, radical bearing redox-active polymers emerged as promising in energy storage applications, for example, in organic radical batteries. Our contributions to the fields mentioned above are described with examples of: a) novel benzofuranone stabilizers for polymers; b) the serendipidous discovery of novel dyestuffs; c) eco-friendly polymerization inhibitors; d) novel nitroxides and alkoxyamines for the first industrial realization of controlled radical polymerization; e) novel and safe radical initiators; f) nitroxide radicals bearing polymers for electrochemical applications.

Nitroxide-Mediated Polymerization at Elevated Temperatures.

A new alkoxyamine based on a highly thermally stable nitroxide is used for the controlled polymerization of styrene and butyl acrylate at temperatures up to 200 °C. High monomer conversions are reached in a few minutes with a linear increase in polymer chain-length with conversion, a final dispersity (D) of ∼1.2, and successful chain-extension of the resulting material. The alkoxyamine concentration was altered to target various chain lengths, with autopolymerization dictating the polymerization rate of styrene regardless of alkoxyamine concentration. Controlled polymerization of methacrylate monomers and acrylic acid was successful with the addition of styrene. The new material opens the possibility to increase the range of specialty products made for applications in coatings, inks, overprint varnishes, and adhesives.

Reaction of benzopinacol with non-ionic bases: reversing the pinacol coupling.

The reaction of benzopinacol with the non-ionic bases butyllithium and phosphazene P4 leads to the formation of the corresponding ketyl radical anions, which have been characterized by EPR/ENDOR spectroscopy. This conversion has a high efficiency. Such a reversed pinacol reaction can be used for a controlled release of ketyl radicals. Moreover, the nature of the base has a marked effect on the association of the ketyl radical anion and the counterions. This illustrates the importance of ion pairing for reductive coupling.

Novel thione-thiol rearrangement of dithiocarbonic acid O-(2,2,6,6-tetramethylpiperidin-1-yl) ester in the context of controlled radical polymerization.

Addition of the lithium salt of N-hydroxy-2,2,6,6-tetramethylpiperidine 10 to carbon disulfide and subsequent methylation affords the rearranged dithiocarbonic acid S-methyl-S'-(2,2,6,6-tetramethylpiperidin-1-yl)-ester 9 rather than the expected xanthate ester S-methyl-O'-(2,2,6,6-tetramethylpiperidin-1 -yl)-ester 8. n-Butylacrylate could be polymerized with the novel compound 9 even though the polymerization was not controlled.