ansa-Bis(fluorenyl)neodymium Catalysts for Cyclocopolymerization of Ethylene with Butadiene

Metallocene borohydride complexes {(Me₂Si(C₁₃H₈)₂)Nd(μ-BH₄)[(μ-BH₄)Li(THF)]}₂ (1) and (Me₂Si(2,7-tBu₂C₁₃H₆)₂)Nd(BH₄)(μ-BH₄)Li(ether)₃ (2) were prepared by reaction of the dilithium salts of silylene-bridged bis(fluorenyl) ligands with the borohydride precursor Nd(BH₄)₃(THF)₃. The solid state structures of dimeric 1 and monomeric 2 ate complexes were established by X-ray diffraction studies. We showed that these complexes used in combination with (nBu)(nOct)Mg are highly efficient for cyclocopolymerization of ethylene with butadiene leading to a new class of elastomers. Catalyst 1/(nBu)(nOct)Mg provided elastomers with a polyethylene skeleton incorporating unsaturated groups (trans double bond and pendant vinyl units) and 1,2-cyclohexane rings. These rings are formed via an intramolecular cyclization which occurs with a high trans selectivity. The investigation of catalyst 2/(nBu)(nOct)Mg has revealed that the tertio-butyl substitution in positions 2 and 7 of fluorenyl ligands influenced the microstructure of copolymers since in addition to 1,2-cyclohexane rings, 1,4-cyclohexane rings were formed. This original microstructure was characterized using 2D NMR ¹H−¹H and ¹H−¹³C with direct and long-range correlations. Mechanisms of stereoselective formation of trans-1,2-cyclohexane and trans-1,4-cyclohexane rings were fully investigated.