RADICAL TRAPPING AND TERMINATION IN FREE-RADICAL POLYMERIZATION OF MMA

Post-effect measurements of radical concentrations for bulk free-radical polymerization of methyl methacrylate (MIMA) at 25 °C with 5.0 wt % 2,2′-azobis(2-methylpropionitrile) (AIBN) initiated by an ultraviolet light (UV) were made by using an on-line ESR spectrometer. Number-average termination rate constants were therefrom directly measured by using radical decay rates. It was found that these termination rate constants decrease dramatically in the post-effect period at high conversions. These observations are in disagreement with the concept of termination by propagation-diffusion that was expected to be the dominant mode of termination at these high conversion levels. It was also found that a fraction of the radicals are trapped during the course of polymerization. In other words, there exist two radical populations in the reacting mass: free radicals and trapped radicals. The former is in the liquid state (as indicated by 13-line ESR spectra) while the latter are in solid state (9-line spectra). These two radical populations have very different reactivities in both propagation and termination reactions. Our data show that there is no minimum value of the termination rate constant for the long-lived trapped radicals. The trapped radical fractions measured directly herein suggest that heterogeneity in the polymerization medium can have a significant effect and must be properly accounted for when an analysis of the kinetics of polymerization is made. © 1990, American Chemical Society. All rights reserved.