Rates of 2–21−azobisisobutyronitrile initiated polymerization of methyl methacrylate in benzene were determined at 77.2, 65.0, and 50.0°C. The variation of molecular weight of the polymer with temperature and conversion was also studied. At a fixed conversion of 2.0%, the molecular weight decreased from 2.05 × 105at 50°C to 1.4 × 105at 77.2°C. The ratio of the propagation rate coefficient to the square root of the termination rate coefficient was found to be 0.61, 0.397, and 0.374 at 77.2, 65.0, and 50.0°C, respectively, with an uncertainty of ±0.5°C in temperature. The effect of active carbon on the rates of polymerization at 77.2°C was measured. Rates of polymerization decreased in the presence of active carbon. For example, the initial rate of polymerization decreased from 7.8 × 10−4mole/(liter min) to 4.6 × 10−4mole/(liter min) when the carbon concentration was varied from 0 to 9.65 g/liter. The molecular weight of the polymer increased from an average of 1.4 × 105in the absence of carbon to 1.5 × 105when carbon was present.