Controlled Catalytic Chain Transfer Polymerization of Isobutylene in the Presence of tert-Butanol as Exo-Enhancer

Catalytic chain transfer polymerization (CCTP) of isobutylene in the presence of alcohol as an exo-enhancer with tert-butyl chloride/ethylaluminum dichloride (EADC)·bis(2-chloroethyl) ether (CEE) has been investigated in hexanes at 0 °C. Increasing exo-olefin content was observed with increasing steric bulkiness of the alkyl group of the alcohol, i.e., tert-butyl > isopropyl > methyl. Here, we report that tert-butanol (t-BuOH) is an excellent exo-enhancer compared to other tert-alcohols such as tert-amyl alcohol (AmOH), 2-methyl-2-pentanol (MPOH), and 3-ethyl-3-pentanol (EPOH). The aromatic tert-alcohol cumyl alcohol was not an exo-enhancer but acted as an initiator. In the reaction of EADC.CEE and t-BuOH, t-butoxyaluminum dichloride (t-BuOAlCl2) was formed, which is the real exo-enhancer and is not stable at room temperature. Molecular weights were virtually unchanged in the presence t-BuOAlCl2 with [t-BuOAlCl2]:[EADC.CEE] < 0.5, and exo-olefin content increased ∼15% relative to polymerization in the absence of t-BuOAlCl2. This is presumably due to stabilization of the cation by t-BuOAlCl2 which slows isomerization of the PIB+. Stabilization of the cation was confirmed by 1H NMR and UV–vis spectroscopy at 0 °C by adding t-BuOAlCl2 to the diphenylmethyl cation, a representative stable cation. The rate constant of chain transfer (ktr) was determined to be 2 × 108 L mol–1 s–1 at 0 °C, which is not affected by t-BuOAlCl2. Addition of an exo-enhancer is especially important for polymerization at CSTR conditions at low steady state monomer concentrations. This is the first report identifying the role of alcohols in CCTP and opens new vistas in the synthesis of highly reactive polyisobutylene.