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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 (

Butadiene polymerization, in both a highly cis- and trans-specific manner, was achieved by using a Cp*Nd(BH4)2(thf)2–Bu2Mg system as an initiator. The cis-/trans- ratio can be tuned by the amount of trialkylaluminum-depleted modified methylaluminoxane (dMMAO). The cis-regularity of the polymer was much higher than those obtained by Nd(BH4)3(thf)3. The molecular weight of cis-regular polymer was increased according to polymer yield, showing that there was no termination or chain transfer reaction during the polymerization. Synthesis of stereodiblock polybutadiene, which showed a high melting temperature (Tm) compared with stereodiblock polyisoprene, was also performed by the addition dMMAO during the polymerization.