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) Si(CH3)(2)-C5Me4]Sc(CH2SiMe3)(2)(THF) (n = 0, Sc-C-0-Sc; n = 2, Sc-C-2-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)(-) (B-1) and trityl bisperfluoroarylborate (Ph3C+)(2)[1,4-(C6F5)(3)BC(6)F(4)13(C6F5)(3)](2-) (B-2). 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-C-2-Sc) to 48% (Sc-Co-Sc) as the catalyst nuclearity increases and the Sc center dot center dot center dot 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 (B-2) is used, in comparison to B-1. In ethylene/isoprene copolymerizations, the binuclear catalysts produce polymers with higher molecular weights (M = (3.4-6.9) X 10(4); polydispersity of D = 1.4-2.0) and with comparable isoprene enchainment selectivity versus Scl under identical reaction conditions. However, isoprene incorporation is curiously reduced by 50% when B-2 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.