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 (Mn = (3.4–6.9) × 104;
polydispersity of Đ = 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.