Stereospecific polymerization of conjugated dienes using neodymium alkylborohydride complexes

Abstract

Two neodymium alkylborohydride complexes that differ with respect to the number of hydrides on the boron center were synthesized and characterized. The μ₂-ligated complex exhibits Nd–B bond lengths that are approximately 10% longer than those in previously reported μ₃-ligated borohydride complexes. For the μ₃-alkylborohydride complex, the Nd–H–B stretching bands observed by IR spectroscopy suggested stronger borohydride coordination than that of [BH₄]⁻, which is probably due to the electron donation of the alkyl groups. These newly prepared complexes successfully promoted trans-1,4 specific polymerization of conjugate dienes upon activation by Bu₂Mg, whereas activation of these complexes by Bu₂Mg/MMAO gave cis-1,4 regular polymer. The trans-1,4 defect of the polyisoprene obtained from the Nd μ₂-alkylborohydride complex/Bu₂Mg/MMAO system (<2%) was much lower than that obtained from previously reported μ₃-BH₄-ligated neodymium complex (11%). Moreover, μ₂-alkylborohydride complex was a good initiator for the polymerization of MMA, probably because of its anionic nature.