Simultaneous control of molecular weight and 1,4-cis selectivity in the polymerization of 1,3-Butadiene with N,N'-Bis(3,5-di-tert-butylsalicylidene)-1,2-diphenylethylenediaminato cobalt (I) in combination with diethylaluminum chloride

High 1,4-cis poly(butadiene) (PBd) is a useful material as a synthetic rubber, and a large amount of 1,4-cis PBds are manufactured by Ziegler catalysts. Such catalysts are effective to produce a 1,4-cis PBd in a relatively high level (1,4-cis content > 90 %), but the molecular weight control is not well. The controlled polymerizations of Bd with homogeneous transition metal cpmplexes have been reported in the last decade. The high 1,4-cis PBd can be synthesized with C₅H₄(CH₂CH₂OMe)TiCl₃/MAO catalyst and (C₅Me₄ⁱPr)₂Sm(THF)/MAO catalyst. We reported that N,N’-Bis(3,5-di-tert-butylsalicylidene)-1,2-diphenylethylene-diaminato cobalt (I) in combination with MAO showed the high activity for the polymerization of Bd in CH₂Cl₂ at -30 °C to give polymers with narrow molecular weight distribution (M_w/M_n = 1.29-1.43). However, the effect of alkylaluminums as a cocatalyst on the polymerization of Bd was not investigated. Thus, we investigated the polymerization of Bd with the (salen)Co(II) complex (1) in combination with various alkylaluminiums were investigated.
Among the alkylaluminums examined as a cocatalyst, the 1/Et₂AlCl catalyst showed the high activity for the polymerization of Bd to give high molecular weight polymers in high yields. The activity for the polymerization was high even when the polymerization of Bd was carried out at the lower Al/Ti mole ratio, which is contrast to that a large amount of MAO was required to show high activity for the polymerization of Bd. We demonstrated that the 1/Et₂AlCl catalyst could be promoted 1,4-cis specific polymerizations (1,4-cis content > 98 %) with a living nature in the polymerization of Bd.