Detailed Investigations into the Propagation and Termination Kinetics of Bulk Homo- and Copolymerization of (Meth)Acrylates

Abstract

Three aspects of free-radical polymerization have been addressed: (i) the propagation rate in acrylate/methacrylate copolymerizations, (ii) the chain-length dependence of the termination rate coefficient in bulk polymerizations, and (iii) the analysis of the type of radicals produced in acrylate homopolymerizations and their influence on the reaction rate.Propagation kinetics in free-radical bulk (co)polymerization have been investigated by applying the pulsed laser polymerization size-exclusion chromatography (PLP-SEC) technique to several binary acrylate/methacrylate systems. Copolymerization propagation rate coefficients and copolymer composition for the systems MA/MMA, DA/DMA, MA/DMA, and DA/MMA have been studied at 1000 bar and at temperatures between 22 and 40°C. The systems were chosen such as to consist of acrylates and methacrylates with alkyl ester chains being both small (methyl), both large (dodecyl) or one small (methyl) and the other large (dodecyl). The propagation rate data is analyzed in terms of the terminal and the IPUE models.For detailed investigations into the termination kinetics, single pulse initiation was combined with time-resolved electron spin resonance (ESR) detection of the decay in pulse-laser-induced radical concentration. Dodecyl methacrylate (DMA), cyclohexyl methacrylate (CHMA) and benzyl methacrylate (BzMA) were investigated via the novel single pulse-pulsed laser polymerization-electron spin resonance (SP-PLP-ESR) technique. An important advantage of SP-PLP-ESR over the conventional single pulse-pulsed laser polymerization-near-infrared spectroscopy (SP-PLP-NIR) technique is that the termination rate coefficient for two radicals of the same chain length may be determined by single differentiation of measured radical concentration vs. time traces, whereas monomer concentration vs. time traces, as obtained via SP-PLP-NIR, have to be differentiated twice to yield the termination rate coefficient.The ESR technique was used for direct determination of the structure of propagating radicals in free-radical homopolymerization of n-butyl acrylate (BA) and dodecyl acrylate (DA) under PLP conditions (laser frequency 20 Hz). The obtained large amount of mid-chain radicals explains the difficulties met in applying the PLP-SEC technique toward analysis of propagation rate coefficients of acrylates at temperatures of 30°C and above.