Radiation‐induced solid‐phase polymerization. II. Polymerization of vinyl acetate

Abstract

The radioinduced polymerization of vinyl acetate (VA) was studied in the liquid phase and two solid phases (glasslike and crystalline) over the temperature range of -196 to 0 deg C. A calorimetric technique was used to determine whether polymerization takes place in thc solid state or during subsequent heating; the presence of active centers during irradiation and their disappearance after irradiation was followed by the ESR method. Post- polymerization kinetics were also studied. The activation energy for VA polymerization was 3.7 kcal/mole for the liquid phase and zero for the solid phase. No post-effects occurred in the solid state for VA up to the melting point. Unlike the case with acrylonitrile, an increased radiation dose did not result in the occurrence of a limiting polymer yield. In crystalline, as well as in the glassy state the polymerization of VA took place only during irradiation, without activation energy, and ended with the switching off of the beam, in spite of the persistence of the shape and intensity of the ESR signal. Solid-state and liquid-phase investigations on the temperature dependence of polymerization rate for methyl methacrylate, formaldehyde, isobutene, and phenylacetylene showed that for the first two the temperature dependence is of themore » same nature as for VA and acrylonitrile. For isobutene, there was a maximum polymerization rate close to the melting point. A low activation energy was observed for phenylacetylene over the whole temperature range from -196 to +85 deg C for both solid and liquid phases. These data support the concept of specificity of solid-state polymerization during irradiation. Such polymerization may be favored by formation of short-lived excited states participating in the development of energy chains, by the motion of molecules upon electron impact, the defreezing of rotation, and excitation of molecular vibrations along tracks of primary and delta -electrons. (auth)« less