Effect of the ester side-chain on the propagation kinetics of alkyl methacrylates - An entropic or enthalpic effect?

The technique of pulsed-laser polymerization (PLP) was used to measure homopropagation rate coefficients for styrene (STY), methyl methacrylate (MMA), ethyl methacrylate (EMA), n-butyl methacrylate (BMA), and benzyl methacrylate (BzMA). Special attention was given to the molecular weight analysis of PLP distributions by size exclusion chromatography (SEC) with online low-angle laserlight scattering (LALLS), differential viscometry (DV), and differential refractive index (DRI) detectors connected in series. The PLP distributions were analyzed using two different approaches: first, by transformations to the universal calibration curve via Mark-Houwink-Sakurada (MHS) constants and, second, by direct analysis of the molecular weight distributions using absolute detection methods. The accuracy and precision of the two approaches is discussed with reference to the measured Arrhenius parameters. The variation in the propagation rate coefficients with ester side chain size is attributed primarily to the preexponential factor. This is consistent with theoretical predictions but in opposition to a recent PLP study of this series. An explanation for this apparent contradiction is suggested.