Hydrogen Transfer Reactions of Nitroxides in Free Radical Polymerizations

Abstract

The kinetics of radical polymerization of styrene with nitroxide stable radicals and the corresponding hydroxylamines and secondary amine were investigated at 90 °C. The amine derivative was found to be inert. The hydroxylamine causes retardation of polymerization. The rate constant of termination of the styrene propagating radicals by hydroxylamines, C x, was found to be 0.18−0.43, depending on the structure of the nitroxyl. Nitroxide caused an induction period and retarded the rate after the induction period ended. Both retardations are ascribed to hydrogen transfer. In the case of hydroxylamines it is the result of hydrogen abstraction from the hydroxyl group of the hydroxylamine by the polystyryl radical. In the case of nitroxides, it is suggested that retardation is caused by hydroxylamine formed as a byproduct during the induction period when the nitroxide reacts with the propagating radical. This is generally a minor side reaction of the propagating radical scavenging by nitroxyl radicals, but this reaction may become important at elevated temperatures when the adduct of the styrene radical with nitroxide is unstable and a reversible reaction prevails. When these two reactions of retardation occur simultaneously, it makes termination of the propagating radicals by nitroxides a catalytic reaction.

Nitroxide-controlled free radical polymerization of a sugar-carrying acryloyl monomer

“…The gradual increase of the M -w /M -n ratio with time is ascribed to side reaction(s). These results are similar to those for the tertbutyl acrylate/DBN system, in which a decomposition of the active polymer chain end occurs through the b-proton abstraction by the nitroxyl and the subsequent hydrogen transfer reaction 13,18) . The present system possibly undergoes similar side reactions.…”

Kinetic Modeling as a Tool to Understand and Improve the Nitroxide Mediated Polymerization of Styrene

“…[1,5] Recently, the focus of NMP research has shifted to the development of other nitroxides, such as N-(2-methyl-2-propyl)-N-(1-diethylphosphono-2,2-dimethylpropyl)-N-oxyl [DEPN, also known as SG1; Figure 2(b)], [9,[26][27][28][29] that lead to higher polymerization rates and that also enable the controlled NMP of other monomers, such as acrylates. [1] Several side reactions can interfere with the NMP activation/deactivation, such as alkoxyamine decomposition yielding terminally unsaturated polymer and hydroxylamine, [30][31][32] chain transfer to the thus formed hydroxylamine, [33] transfer reactions involving the nitroxide, [34] and nitroxide decomposition. [9,35] For styrene, the NMP is typically performed at temperatures above 373 K, and a further complication arises due to the occurrence of thermal initiation through the Mayo mechanism ( Figure 3) by which initiator radicals are continuously generated.…”

“…Transfer reactions from radicals to TEMPO-H, regenerating the nitroxide, have been studied by Gridnev [33] and Zubenko et al [32] Based on simplified kinetic models, these authors reported values of 3: [33] and 0:2 m 3 Á mol À1 Á s À1 . [32] Using kinetic modeling, the importance of transfer reactions involving hydroxylamine has been assessed and found to be insignificant in the conditions studied in this work.…”

Synthesis of poly(chloromethylstyrene-b-styrene) block copolymers by controlled free-radical polymerization

“…The primary benzylic alkoxyamine is known to be quite stable (low dissociation rate) 34 and, therefore, leads to polydisperse polymers for styrene. 17 Because TEMPO is known to be a good hydrogen abstractor, [35][36][37] it can react with chloromethyl groups to yield the corresponding hydroxylamine TEMPOH (eq 5). Similarly, with consideration of the chlorine atom [bond dissociation energy (BDE) for C 6 H 5 OCH 2 OCl ϭ 68 kcal/mol, BDE for C 6 H 5 OCH 2 OH ϭ 85 kcal/mol), 38 the chloromethyl groups could also react with TEMPO to yield the corresponding oxoaminium chloride TEMPOCl (eq 6).…”