Molecular weight distributions in free-radical polymerizations .2. Low-conversion bulk polymerization

Results are reported for the low-conversion bulk polymerizations of methyl methacrylate (up to 15% conversion) and styrene (up to 8%) at 50°C with AIBN as the initiator. The rates were measured gravimetrically, and the time evolution of the full molecular weight distributions (MWDs) were measured by GPC. For both monomers, the rates obeyed [AIBN]0.5±0.05 ,independent of conversion. MWD data were analyzed as In P(M), where P(M) is the pseudoinstantaneous number distribution; approximate analytic solutions for the MWD suggest that plots of InP(M) vs M should show extensive linear regions if chain stoppage is by transfer, disproportionation, and or chain-length-dependent termination. The cumulative number distribution is approximately the GPC distribution divided by the square of the molecular weight; pseudoinstantaneous number distributions were obtained by subtracting the appropriately normalized cumulative distributions at successive conversions. All MWD data show In P(M) is linear in M for higher M, with the slope of this region being independent of conversion but increasing significantly with increasing (AIBN). The combined MWD and rate data for styrene are consistent with chain stoppage being both by transfer to monomer and by termination (by combination) between long and short chains, long-long termination being too slow to be important; parameter values reproducing both the rate and the MWD involve both chain-length-dependent and chain-length-independent events, with the critical distance of interaction for termination being significantly greater than that of a monomer unit (as distinct from what is seen at intermediate conversion). For MMA, where termination is dominated by disproportionation, the rate and MWD data are best fitted by assuming that center-of-mass (chain-length-dependent) termination is at most only weakly rate-determining. Attempts to use the low-M shape of plots of In P(M) to indicate whether termination occurs by combination or disproportionation were inconclusive