Cationic polymerizations at elevated temperatures by novel initiating systems having weakly coordinating counteranions 2. Isobutylene/isoprene copolymerizations
As a sequel to our studies on isobutylene (IB) homopolymerizations, we have investigated the copolymerization of IB/isoprene (IP) mixtures containing up to ∼20 mole% IP in the feed by the use of the in situ (CH3)3Si[B(C6F5)4] initiating system in close-to-neat monomers in the temperature range from −35 to −8 (reflux)°C. The effects of temperature and IB/IP feed ratio on copolymer molecular weights were determined and compared with those produced by AlCl3. The molecular weights of butyl rubbers obtained by the novel initiating system under a variety of conditions are invariably and significantly higher than those made with AlCl3. High molecular weight gel-free rubbers containing up to ∼5 mol% unsaturation can be obtained at relatively high temperatures. Copolymer compositions can be controlled by the IB/IP ratio in the feed. Product molecular weights decrease with increasing IP content. To gain insight into the copolymerization mechanism, the activation enthalpy of molecular weights (ΔH‡=−5.9 kcal/mol, −24.7 kJ/mol) and the reactivity ratios (rIB= 1.8, rIP= 1.5) have been determined.
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