Carbocationic Polymerization of Isoprene Co-initiated by B(C6F5)3: An Alternative Route toward Natural Rubber Polymer Analogues?

The cationic polymerization of isoprene using the 1-(4-methoxyphenyl)ethanol ( 1 )/B(C 6 F 5 ) 3 initiating system in solution (dichloromethane or α,α,α-trifluorotoluene) and in aqueous media (suspension, dispersion, or emulsion) is reported. In organic solvents the reaction proceeded by controlled initiation via 1, followed by irreversible termination, thus affording polymers with a number-average molar mass M n ≤ 5000 g mol −1 and a molar mass distribution or MMD ( M w / M n ≤ 2.5) and rather high content of intact double bonds (≥70%) in the polymer backbone. In particular, using α,α,α-trifluorotoluene as polymerization solvent in place of dichloromethane promoted the synthesis of polyisoprene chains with M w / M n ∼ 1.4 and larger content of intact double bonds (up to 88%). In the absence of 1, polyisoprenes with fairly high molar mass ( M n up to 18 000 g mol −1 ) and M w / M n < 2.4 were synthesized through adventitious water/B(C 6 F 5 ) 3 -initiated cationic polymerization of isoprene. In aqueous media, the cationic polymerization of isoprene with 1 /B(C 6 F 5 ) 3 proceeded without any side reactions (cyclization, branching). However, aqueous conditions afforded only moderate yield (up to 60% monomer conversion) and polyisoprenes with low M n (≤1200 g mol −1 ) and M w / M n ≤ 1.7 were obtained. By comparing different characterization data (NMR, mass spectrometry), it was demonstrated that under appropriate conditions (in organic solvents at −30 °C or in aqueous media) polyisoprenes carrying the initiator fragment from 1 at the α-end and an olefinic terminal group can be produced almost exclusively in a trans -1,4 configuration (92−96.5%).