PLP−ESR Monitoring of Midchain Radicals in n-Butyl Acrylate Polymerization

Abstract

The percentage of two types of free radicals occurring within n-butyl acrylate pulsed laser polymerization (PLP) has been measured via ESR spectroscopy. Polymerization in solution of toluene, at monomer concentrations between 0.50 and 2.53 mol L -1 and temperatures between -50 and 70°C, was induced by 351 nm excimer laser pulses applied at a repetition rate of 20 Hz. The ESR spectra obtained at low degrees of monomer conversion may be adequately represented by superimposing a six-component four-line spectrum assigned to secondary propagating radicals (SPRs) and a nine-line spectrum assigned to tertiary midchain radicals produced by backbiting (MCR1s). The percentage contribution to total radical concentration of the latter species, which is produced from an SPR by a 1,5-H shift backbiting reaction, is negligible at -50°C and is close to 80% at 70°C. The transition of an SPR to a MCR1 species invalidates the linear correlation of time t after applying a laser pulse with the size of the growing radical. The observed high concentration of MCR1 species explains the known severe limitations met in acrylate PLP-SEC experiments directed toward measuring the propagation rate of acrylate radicals with chain-end functionality.

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

“…At 0 C, already around 50% and at 60 C around 80% of all radicals belong to the MCR type. 43 Interestingly, virtually no dependence of the MCR content on monomer concentration was seen, which would not directly be anticipated regarding eq. 7 (see later).…”

“…2 Because of the much lower propagation rate, the MCR species accumulates due to the interconversion of MCRs into SPRs being much slower than the formation of MCRs, which can be readily seen in the ESR measurements. 43,44 As a result, the overall polymerization rate is retarded, which is the second notable impact of MCRs on acrylate polymerization (for details on the retardation effect see ''Effective Propagation Rate Coefficients'' section).…”

“…Willemse et al 43 achieved such conditions by using an ESR setup that is coupled with UV-laser initiation 51,59 to polymerize under PLP conditions at temperatures as low as À50…”

Impact of Competitive Processes on Controlled Radical Polymerization

“…1,2,6,19 This has been cited as the cause for the inability to accurately measure the propagation rate coefficient, k p of acrylic monomers by pulsed laser polymerization except for at low temperatures, where backbiting is significantly reduced, 4, [20][21][22] and, additionally, the reason why the measurement of k p in the absence of backbiting results in a value that is unable to describe polymerization kinetics in conditions where backbiting is significant. 2 Because most of the branches are formed by backbiting, in the homopolymerization of acrylate monomers the formation of branches can be described by Scheme 1.…”

Chain‐Length‐Dependent Termination in Radical Polymerization of Acrylates

“…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.…”