Alpha and beta diimine cobalt complexes in isoprene polymerization: a comparative study

Two cobalt complexes, the alpha-diimine complex 1,2-bis[(2,6-diisopropylphenyl) imino] acenaphthene-cobalt( II) dibromide (A) and the beta-diimine complex 2,4-bis[(2,6-diisopropylphenyl) imino] pentane-cobalt(II) dibromide (B) were synthesized and fully characterized by elemental analysis, electrospray MS and single crystal X-ray diffraction. These complexes were then assessed as pre-catalysts of the polymerization of isoprene in chlorobenzene using diethylaluminiumchloride (DEAC) as co-catalyst at 35 degrees C. While the aryl substituents were identical in each case, the beta-diimine complex B was over 100-fold more active (R-p = 1.17 x 10(-3) M s(-1)) than alpha-diimine complex A (R-p = 1.07 x 10(-5) M s(-1)). A produced polyisoprene with molecular weight (M-n = 2.1 x 10(5) g mol(-1)) double that achieved with B. The polymers produced by both A and B were predominantly cis-1,4 enchained (80%) with 3,4 regioerrors (20%). For B, this 1,4 : 3,4 selectivity ratio was not influenced by aluminium alkyl reagent (ethylaluminiumsesquichloride (EASC), or DEAC) whereas for A, where EASC co-catalyst was used, a 1,4-trans-rich (61%) polyisoprene was obtained. Moreover, increasing Al mole ratio decreased the activity for B but increased it for A.