Kinetic Study of Cationic Polymerization of α-Methylstyrene Initiated by BuOTICl3In Dlchloromethane Solution

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Abstract
The cationic polymerization of α-methylstyrene Initiated by n-BuOTiCl3 has been studied at -70° C in dlchloromethane solution by using a calorlmetric technique. Polymerizations were performed under high vacuum either in dry conditions for which low monomer conversions were observed (4-12%), or In the presence of added cocatalyst (H2O or HCl). In these last cases, yields were quantitative, and it was shown that polymerization rate was proportional to added water concentration and first order with respect to catalyst and to monomer. A kinetic scheme is proposed, based on a monomer-Independent initiation step and on a unimolecular termination process. At -70° C, the initiation rate is higher than termination rate during the whole course of the polymerization, and the concentration of active centers increases continuously. The following rate constants were found at -70°C: ki. = 17 ± 6, k = 2.2 ± 1.1 ± 104,ktrm = 30 ± 15 liter/mole-Sec, and kt =0 54 ± 0 05 sec−1. At -50 and -30° C, the concentration of active centers goes through a maximum during the polymerization and incomplete monomer conversions were observed, showing that all the catalyst is consumed. The different rate constants were tentatively estimated at these temperatures by using a simulation method, and this led to a negative value of ca. -7 kcal/mole for the apparent activation energy for propagation, and to a value of ∼ 5 kcal/ mole for Ei. The observation of a negative (Ep)app might be explained either by a shift of the dissociation equilibrium of the growing ends or by a solvation process of these growing ends by monomer prior to the propagation step.