Effect of Structure on Cationic Initiation Efficiency of a Carbocatonic/ATRP Dual Initiator

The dual initiator, 3,3,5,5,7-pentamethyl-7-chlorooctyl 2-bromo-2-methylpropionate (IB3BMP), possessing initiating sites for both living carbocationic polymerization (LCP) and atom transfer radical polymerization (ATRP), was synthesized to overcome the low initiation efficiency of previously reported dual initiators such as 3,3,5-trimethyl-5-chlorohexyl 2-bromo-2-methylpropionate (IB2BMP). Low initiation efficiency of the latter and similar initiators has been attributed to complexation between the Lewis acid polymerization catalyst and the ester carbonyl group of the initiator. IB3BMP was synthesized by reacting IB2BMP with 2-methallyltrimethylsilane at low temperature, followed by hydrochlorination. IB3BMP, which differs from IB2BMP by the inclusion of one additional isobutylene (IB) repeating unit, showed quantitative initiation efficiency (Ieff ∼ 1) in TiCl4-co-initiated LCP of IB under various reaction conditions, including low temperature (−70 °C) and low monomer/initiator ratios (48–164). Ieff and polydispersity index of the resulting polyisobutylenes were identical to those obtained with the commonly used, monofunctional cationic initiators 2-chloro-2,4,4-trimethylpentane (TMPCl) and 2-chloro-2,4,4,6,6-pentamethylheptane (PMHCl). The superiority of IB3BMP compared to IB2BMP in carbocationic initiation was attributed to increased separation between the tert-chloride initiating site and the TiCl4:carbonyl complex at the ATRP initiating site. A PIB macroinitiator prepared from IB3BMP was used to initiate ATRP of methyl acrylate. Efficiency of radical initiation was quantitative, the targeted poly(methyl acrylate) block length was obtained, and the resulting block copolymer possessed narrow polydispersity.