Termination of trapped radicals at elevated temperatures during copolymerization of MMA/EGDMA

Bulk free-radical copolymerization of methyl methacrylate (MMA)/ethylene glycol dimethacrylate (EGDMA) with 0, 25, 50, 75, 100 wt% EGDMA initiated by 2.0 wt% 2,2′-azobis[2-methylpropionitrile] (AIBN) at 70°C was carried out for 96 h (120 h for pure MMA). The final stable radical concentrations obtained were 1.3 × 10−5, 7.8 × 10−5, 9.2 × 10−4, 1.5 × 10−3, 2.5 × 10−3 mol/l, respectively. Post-effect measurements of bimolecular termination of radicals at terminal conversions (X = 0.60–0.95) and at following elevated temperatures, 105, 120, 150 and 180°C were made using an on-line e.s.r. spectrometer. Termination rate constants were estimated using the measured radical decay rates. It was found that the termination rate constants in the post-effect period for the samples having high EGDMA weight fractions and at the lower temperature levels decreased with time. This can be explained by assuming that there exist two radical populations attached to crosslinked gel, active radicals and inactive radicals. An increase in temperature causes a transformation of inactive radicals to active ones. The bimolecular termination rate constants for these active radicals were measured. It was found that the rate constants for active radicals decrease with increasing EGDMA level. The bimolecular termination constant for inactive radicals was taken to be zero. The activation energies for these termination constants decrease with increasing EGDMA weight fraction and are of the order of 10 kcal/mol. The inactive radical concentrations were also estimated. A discussion of the polymerization mechanisms at very high conversions is given based on an analysis of the experimental data. It can be concluded that for linear PMMA at very high conversions during the glassy-state transition, the propagation rate constant, the initiation efficiency and the termination rate constant fall almost simultaneously and dramatically. These conclusions may also be true for crosslinked PMMA.