Kinetic and Mechanistic Studies of the Polymerization of Isobutylene Catalyzed by EtAlCl2/Bis(2-chloroethyl) Ether Complex in Hexanes

The kinetics and mechanism of the polymerization of isobutylene (IB) initiated by tert-butyl chloride and catalyzed by ethylaluminum dichloride (EADC)•bis(2-chloroethyl) ether (CEE) complex was studied in hexanes at 0 °C. The polymerization is first order in [IB] and in dry hexanes the slopes of the first order plots are independent of the [CEE]/[EADC] ratio in the range 1–1.5. From the slopes the steady state concentration of propagating cations [Mn+] = 1.8 × 10–11 M was calculated, which suggests an equilibrium between dormant and active species. The olefin distribution is independent of IB concentration and conversion. At low conversion, the number average molecular weight (Mn) is proportional to the starting IB concentration. In hexanes saturated with water, the polymerization rate is 5 times higher at [CEE]/[EADC] = 1 compared to that in dry hexanes. However, at [CEE]/[EADC] = 1.5, the rates in dry or wet hexanes are identical. To account for the low concentration of active centers determined from the first order plots, an equilibrium between oxonium (dormant) and carbenium (active) ions is proposed. The existence of oxonium ions was confirmed by 1H NMR studies and by polymerizations initiated from preformed oxonium ions. While identical rates and olefin distributions were obtained using EADC solutions in hexanes or toluene to form the EADC·CEE complex at short reaction times, at long reaction times, the exo-olefin content decreased when EADC solution in toluene was used. This was attributed to a slow tert-butylation of toluene yielding 95% para isomer and the concomitant formation of AlCl3.