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

• Published in: Inorganic chemistry 2018-04, Vol.57 (7), p.4088-4097
• Main Authors: Zhao, Junyi, Chen, Huafeng, Li, Wenxin, Jia, Xiaoyu, Zhang, Xuequan, Gong, Dirong
• Format: Article
• Language: English
• ISSN: 0020-1669; 1520-510X
• DOI: 10.1021/acs.inorgchem.8b00270

Ligands N-(dialkyl or arylphosphino)-(2,2′-bipyridin)-6-amine (L1, aryl = Ph; L2, alkyl = t Bu; 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 = t Bu; 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 C o 4 and C o 6, 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.