DETERMINATION OF FREE-RADICAL CHAIN-TRANSFER RATE COEFFICIENTS BY PULSED-LASER POLYMERIZATION

Analysis of the molecular weight distributions (MWDs) of polymer produced by pulsed-laser techniques-extensively used as a robust tool for the determination of free-radical propagation rate coefficients (k(p))-is shown to be an effective method for measuring chain-transfer rate coefficients (k(tr)). In contrast to the determination of k(p), experimental conditions to study chain transfer must be chosen to minimize the effect of radical-radical termination on the MWD. Employing the slope of the semilog number distribution rather than number-average MW provides a robust, reproducible method for the estimation of k(tr)/k(p). Methodology for the new technique is developed through an extensive study of chain transfer to n-dodecyl mercaptan (DDM) in methyl methacrylate (MMA), for which K-tr/k(p) is shown to be independent of temperature in the range of 20-80 degrees C. Results for chain transfer to DDM in styrene, ethyl methacrylate (EMA), and n-butyl methacrylate (BMA) are also presented; the value of k(tr)/k(p) does not vary with methacrylate type but is 20 times higher for styrene than for the methacrylates.