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

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posted on 2017-07-06, 00:00 authored by Jiazhen Chen, Yanshan Gao, Shuoyan Xiong, Massimiliano Delferro, Tracy L. Lohr, Tobin J. Marks
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.