Metal and Counteranion Nuclearity Effects in Organoscandium-Catalyzed Isoprene Polymerization and Copolymerization

The binuclear organoscandium half-sandwich complexes (Me₃SiCH₂)₂(THF)Sc[C₅Me₄-Si(CH₃)₂-(CH₂)_n-Si(CH₃)₂-C₅Me₄]Sc(CH₂SiMe₃)₂(THF) (n = 0, Sc-C₀-Sc; n = 2, Sc-C₂-Sc) and monometallic C₅Me₄SiMe₃Sc(CH₂SiMe₃)₂(THF) (Sc1) were prepared and fully characterized by conventional spectroscopic, analytical, and diffraction techniques. These complexes are active catalysts for isoprene polymerization and ethylene/isoprene copolymerization upon activation by the co-catalysts trityl perfluoroarylborate (Ph₃C⁺)B(C₆F₅)₄^- (B₁) and trityl bisperfluoroarylborate (Ph₃C⁺)₂[1,4-(C₆F₅)₃BC₆F₄B(C₆F₅)₃]^2- (B₂). Marked catalyst and co-catalyst nuclearity effects on product polymer microstructure are achieved in isoprene polymerization. Thus, the percentage of cis-1,4- units in the polyisoprene products increases from 24% (Sc1) to 32% (Sc-C₂-Sc) to 48% (Sc-C₀-Sc) as the catalyst nuclearity increases and the Sc···Sc distance contracts. The binuclear catalysts regulate the isometric unit distributions and favor 3,4-3,4-3,4 blocks. Furthermore, the percentage of polyisoprene trans-1,4- units increases ∼5 times when binuclear co-catalyst (B₂) is used, in comparison to B₁. In ethylene/isoprene copolymerizations, the binuclear catalysts produce polymers with higher molecular weights (M_n = (3.4-6.9) × 10⁴ polydispersity of = 1.4-2.0) and with comparable isoprene enchainment selectivity versus Sc1 under identical reaction conditions. However, isoprene incorporation is curiously reduced by ∼50% when B₂ is used versus B₁. These results highlight the importance of both ion pairing and imposed nuclearity in these polymerizations, and these results indicate that both catalyst and co-catalyst nuclearities can be used to access specific polyisoprene polymer/copolymer microstructures.