cis-1,4-specific carbocationic polymerization and copolymerization of 1,3-dienes initiated by (S,S)-bis(oxazolinylphenyl)amine chromium complexes

Two new chiral (S,S)-bis(oxazolinylphenyl)amine chromium dichloride complexes have been synthesized and structurally characterized. In combination with 2 equiv. of borate and an excess of AlR₃, such Cr complexes serve as effective cationic initiators in the stereoregular carbocationic polymerization of 1,3-dienes such as isoprene (IP) and myrcene (MY), affording cyclized cis-1,4-PIPs/PMys (cis-1,4-selectivity up to 96%) with cyclic sequence contents ranging from 26% to 87%. Moreover, these Cr initiator systems also exhibit an unprecedented control over sequence distribution of comonomers in the carbocationic copolymerization of IP and MY, preparing novel copolymers with different microstructures from mainly cyclized cis-1,4-specific statistical copolymers to cyclic olefin copolymers. The nature of Cr complex, borate, AlR₃, temperature, molar ratio of comonomers has considerable effect on the (co)polymer's yield, stereoselectivity, cyclization, and comonomer sequence distribution. A plausible mechanism is suggested, which gives a new strategy for biomimetic synthesis of natural rubber.