Abstract
A comprehensive and systematic study of overall termination rate coefficients, k(t), in low-conversion radical (homo)polymerization of methyl methacrylate and styrene is presented. Values of k(t) were determined by gravimetric analysis of steady-state experiments, employing 2,2'-azoisobutyronitrile as initiator. The values delivered by this simple method were found to be in qualitative and quantitative agreement with those from more modern and sophisticated techniques for measuring k(t). Accordingly, correlations for bulk, low-conversion k(t) as a function of temperature are given for each monomer. The effects of initiator concentration, c(I), and temperature on bulk k(t) were studied in a controlled way for both monomers. Additionally, ethyl benzene was used as solvent in order to investigate rigorously the effect of monomer concentration, c(M), on styrene k(t). The trends found by these systematic studies were considered in the light of what is known about the chain-length dependence of termination. Styrene's behavior was always found to be qualitatively in accord with expectation, although the variations of k(t) with c(I) and c(M) were not as strong as should be the case. However its activation energy, 15 kJ . mol(-1), is shown to be almost perfectly in agreement with theory. Methyl methacrylate, on the other hand, is recalcitrant in that its overall k(t) does not make manifest the chain-length dependent termination that has been directly measured by other techniques. Possible reasons for these discrepancies are discussed, as are reasons for the difference in values between k(t) for the two monomers. On the latter topic it is concluded likely that the chain-length dependence of termination at short chain lengths is primarily responsible for styrene having k(t) that is higher by a factor of about 3, with there also being a contribution that arises from styrene's slower propagation.