Kinetic Investigation of the Living Cationic Polymerization of Isobutylene Using a t-Bu-m-DCC/TiCl4/2,4-DMP Initiating System

A kinetic and mechanistic study of the living cationic polymerization of isobutylene was conducted using 5-tert-butyl-1,3-bis(2-chloro-2-propyl)benzene (t-Bu-m-DCC)/TiCl4 initiating system with 2,4-dimethylpyridine (DMP) as an electron donor in 60/40 (v/v) hexane/methyl chloride cosolvents. The complex between TiCl4 and DMP was observed to be largely soluble in this medium in the temperature range −50 to −80 °C. Over the same temperature range using a [t-Bu-m-DCC]o:[TiCl4]o ratio of 1:20, polymerizations were free of termination and chain transfer reactions, up to monomer conversions in excess of 95%, and molecular weight distributions were narrow. It was observed that the concentration of DMP had to be maintained in excess of the concentration of protic impurities to obtain living polymerizations. At −80 °C, the kinetic order of the rate of polymerization was unity with respect to both monomer and initiator concentrations, approximately 2 with respect to [TiCl4]eff, defined as [TiCl4]o − [DMP], and −0.25 with respect to DMP. These results were similar to those obtained using pyridine as an electron donor, which forms an insoluble complex with TiCl4. Similar polymerization rates were obtained for various polymerizations in which the same [TiCl4]eff was obtained from different combinations of [TiCl4]o and [DMP], demonstrating that the fractional, negative kinetic order of the rate with respect to DMP reflects the reduction in [TiCl4]o caused by complexation. The apparent activation energy for the rate of polymerization was observed to be negative, attributed to an equilibrium between dormant and active growing chains. It was proposed that the observed second-order kinetic dependency on the TiCl4 concentration results from the fact that propagation takes place predominantly through chains possessing dimeric gegenions and that the latter must form by reaction of additional TiCl4 with monomeric gegenions, rather than direct ionization of chains by neutral, dimeric Ti2Cl8.