Polymerization Of Isoprene Promoted By Aminophosphine(Ory)-Fused Bipyridine Cobalt Complexes: Precise Control Of Molecular Weight And Cis-1,4-Alt-3,4 Sequence

Ligands N-(dialkyl or arylphosphino)-(2,2'-bipyridin)-6-amine (L1, aryl = Ph; L2, alkyl = Bu-t; L3, alkyl = adamantyl (Ad)) as well as the corresponding oxidized N-(2,2'-bipyridin-6-yl)-P,P-dialkyl or aryl phosphinic amide (L4, aryl = Ph; L5, alkyl = Bu-t; L6, alkyl = Ad) congeners were designed and coordinated to cobalt dichloride. The structures of formed complexes were characterized by IR and elemental analyses, as well as characterizations of the X-ray diffractions for complexes Co4 and Co6, which revealed the cobalt center is expectedly pentacoordinated in a distorted trigonal bipyramidal configuration with a prolonged Co-O(=P) bond. In combination with MMAO, complex Co2 was highly active in cis-1,4-alt-3,4 enchained polymerization. The hemilabile nature of O=P is possible for the alternating eta(4)-cis-1,4 and eta(2)-3,4 coordination, and insertion at the metal-carbon bond ensued. In combination with AlEt2Cl, each of complexes Co4, Co5, and Co6 was capable of converting isoprene to polyisoprene in a control mode with observed polymerization rate constants (k(obs) = 0.1531 L mol(-1) min(-1) (Co4), 0.1382 L mol(-1) min(-1) (Co5), and 0.0902 L mol(-1) min(-1) (Co6)). The activation energy of the polymerization by Co4 falls in the range of 27-31 kJ/mol by determining k(obs) values at 0, 30, and 50 degrees C. The C-13 NMR analyses of the obtained polyisoprene revealed that complexes Co4, Co5, and Co6 have a cis-1,4 selectivity of 86.6-93.4% with a 3,4 selectivity of 6.6-13.4%. This catalyst system can also be applied to block copolymerization of isoprene and myrcene in a living cis-1,4 fashion; therefore, a new biosourced monomer-based elastomer has been achieved.