The effect of increasing viscosity on the course of polymerization reactions has been studied for some time. The cause of the acceleration of the polymerization rate often observed has been shown to be due to the decrease in the rate at which polymeric radicals react with each Subsequently, polymerization rates decrease to a much greater extent than can be attributed to the decrease in monomer concentration. The develop ment of the thermal nonstationary state method of measuring apparent velocity coefficients for the growth and termination reactions has made it possible to analyze these effects quantitatively and to study the influence of environment upon them. This has been done for vinyl acetates and for the first 35% polymerization of methyl methacrylate.' The present paper deals with the whole range of the polymerization of methyl methacrylate. In addition, some observations have been made when a crosslinking agent (ethylene glycol dimethacrylate) is added and also when precipitation of the polymer occurs in the presence of a solvent for the monomer, but not for the polymer. EXPERIMENTAL MaterialsCommercially supplied methyl methacrylate was purified by several washings with 10% aqueous sodium hydroxide, followed by many washings with distilled water. Subsequent to drying with anhydrous sodium sulfate, the material was fractionally distilled under 81 mm. of nitrogen; the fraction distilling between 39.8 and 40°C. was collected, re-distilled, transferred to a high-vacuum line, and immediately outgassed. After prepolymerization by irradiation with U.V. light, the monomer was distilled under high-vacuum conditions into a graduated receiver and finally into a second graduated receiver joined to the reaction vessel.

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This work formed part of a general investigation into trapping radicals in polymeric gels with a view to electron spin resonance studies.' The entrapping of radicals has been evoked to explain the kinetic behavior of polymerizations in viscous media and in some heterogeneous vinyl polymerizations. *

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