Chain-Length-Dependent Termination in Radical Polymerization of Acrylates
The technique of SP PLP EPR, which is single-pulse pulsed-laser polymerization (SP PLP) in conjunction with electron paramagnetic resonance (EPR) spectroscopy, is used to carry out a detailed investigation of secondary (chain-end) radical termination of acrylates. Measurements are performed on methyl acrylate, n-butyl acrylate and dodecyl acrylate in bulk and in toluene solution at –40 °C. The reason for the low temperature is to avoid formation of mid-chain radicals, a complicating factor that has imparted ambiguity to the results of previous studies of this nature. Consistent with these previous studies, composite-model behavior for chain-length-dependent termination rate coefficients, kt i,i, is found in this work. However, lower and more reasonable values of α s, the exponent for variation of kt i,i at short chain lengths, are found in the present study. Most likely this is because of the absence of mid-chain radicals, thereby validating the methodology of this work. Family-type termination behavior is observed, with the following average parameter values adequately describing all results, regardless of acrylate or the presence of toluene: α s = 0.79, α l = 0.21 (long chains) and ic ≈ 30 (crossover chain length). All indications are that these values carry over to termination of acrylate chain-end radicals at higher, more practical temperatures. Further, these values largely make sense in terms of what is understood about the physical meaning of the parameters. Variation of the rate coefficient for termination between monomeric radicals, kt 1,1, is found to be well described by the simple Smoluchowski and Stokes-Einstein equations. This allows easy prediction of kt 1,1 for different alkyl acrylates, solvent and temperature. Through all this the unrivalled power of SP PLP EPR for measuring and understanding (chain-length-dependent) termination rate coefficients shines through.