α-Methylstyrene (α-MS) and isobutyl vinyl ether (IBVE) were copolymerized by using the H2O/EtAlCl2initiator system and CH2Cl2and CH3Cl solvent in the temperature range from -30 to -90°C. As compared to homopolymerization of α-MS, both yields and molecular weights are reduced upon addition of small amounts of IBVE to the feed. The reactivity ratios were calculated by the method of Kelen and Tödös as well as the Fineman and Ross method, and the combined effect of change of solvent and temperature on reactivity ratios was determined. Effects of feed composition and temperature on the copolymer yield, composition, and number-average molecular weightMnwere studied in detail.Mnshowed a novel exponential dependence on the IBVE concentration in the feed. The overall activation energies of molecular weight were determined from the Arrhenius plots for both homo-and copolymerization systems. Based on these and the yield data, a speculation is made regarding reaction mechanism for molecular weight control. NMR and DTA data are reported, which establish the random nature of the copolymers.