The role of mid‐chain radicals in acrylate free radical polymerization: Branching and scission

Abstract

The past 5 years have seen a significant increase in the understanding of the fate of so‐called mid‐chain radicals (MCR), which are formed during the free radical polymerization of monomers that form highly reactive propagating radicals and contain an easily abstractable hydrogen atom. Among these monomers, acrylates are, beside ethylene, among the most prominent. Typically, a secondary propagating acrylate‐type macroradical (SPR) can easily transfer its radical functionality via a six‐membered transition state to a position within the polymer chain (in a so‐called backbiting reaction), creating a tertiary MCR. Alternatively, the radical function can be transferred intramolecularly to any position within the chain (also forming an MCR) or intermolecularly to another polymer strand. This article aims at providing a comprehensive overview of the up‐to‐date knowledge about the rates at which MCRs are formed, their secondary reactions as well as the consequences of their occurrence under variable reaction conditions. We explore the latest aspects of their detection (via electron spin resonance spectroscopy) as well as the characterization of the polymer structures to which they lead (via high resolution mass spectrometry). The presence of MCRs leads to the formation of branched polymers and the partial formation of polymer networks. They also limit the measurement of kinetic parameters (such as the SPR propagation rate coefficient) with conventional methods. However, their occurrence can also be used as a synthetic handle, for example, the high‐temperature preparation of macromonomers. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 7585–7605, 2008

Chain‐Length‐Dependent Termination in Radical Polymerization of Acrylates

“…[52] This awakened workers to the reality of this reaction in acrylate systems, and rapidly led to an appreciation of how this simple reaction affects the measurement of all fundamental rate coefficients. As a result one may now survey a landscape in which much is known about acrylate kinetics [1,13] where previously only mystery abounded. This paper adds further details to this landscape, notably a clearer picture of acrylate termination kinetics.…”

“…Most studies have concentrated on butyl acrylate (BA), [1][2][3][4][5][6][7][8][9][10][11][12][13] with this monomer being taken as typical of the entire family. Understanding of the polymerization kinetics of acrylate-type monomers is important not just for scientific reasons, but also because the industrial importance of these monomers makes it essential that their kinetic behavior can be well modeled.…”

Impact of Competitive Processes on Controlled Radical Polymerization

“…[1][2][3][4][5][6][7][8][9][10][11][12] Transfer to polymer in acrylic monomers has been shown to be mainly due to intramolecular transfer via a six membered ring transition state, resulting in a mid chain radical. [13][14][15][16][17] In general, for most polymerizations which are conducted at a temperature of <100°C , the most likely fate of the midchain radical is propagation, leading to the formation of quaternary carbons that are a fingerprint of this process and can be detected by 13 C NMR.…”

Computer‐Aided Optimization of Conditions for Fast and Controlled ICAR ATRP of n‐Butyl Acrylate

“…Of special interest are chain transfer to polymer and bC-scission reactions, [12][13][14][15] which influence polymer properties, such as the average chain length and branching content. These properties can be directly manipulated in view of the desired application of the final polymer product, e.g., by promoting or inhibiting the occurrence of side reactions.…”