Polymerization of Isoprene Promoted by Aminophosphine(ory)-Fused Bipyridine Cobalt Complexes: Precise Control of Molecular Weight and <i>cis</i>-1,4-<i>alt</i>-3,4 Sequence

Abstract

Ligands N-(dialkyl or arylphosphino)-(2,2'-bipyridin)-6-amine (L1, aryl = Ph; L2, alkyl = ^tBu; 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 = ^tBu; 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 η⁴- cis-1,4 and η²-3,4 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 °C. The ¹³C 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.