Very large cooperative effects in heterobimetallic titanium-chromium catalysts for ethylene polymerization/copolymerization

The heterobimetallic complexes, (Ε⁵-indenyl)[1- Me₂Si(^tBuN)TiCl₂]-3-C_nH _2n-[N,N-bis(2-(ethylthio)ethyl)amine]CrCl₃ (n = 0, Ti-C₀-Cr^SNS; n = 2, Ti-C₂-Cr^SNS; n = 6, Ti-C₆-Cr^SNS), (Ε⁵-indenyl)[1- Me₂Si(^tBuN)TiCl₂]-3-C₂H ₄-[N,N-bis((o-OMe-C₆H₄)₂P)amine] CrCl₃ (Ti-C₂-Cr^PNP), and (Ε ⁵-indenyl)[1-Me₂Si(^tBuN)TiCl ₂]-3-C₂H₄-[N,N-bis((diethylamine)ethyl)-amine] CrCl₃ (Ti-C₂-Cr^NNN), are synthesized, fully characterized, and employed as olefin polymerization catalysts. With ethylene as the feed and MAO as cocatalyst/activator, SNS-based complexes Ti-C ₀-Cr^SNS, Ti-C₂-Cr^SNS, and Ti-C ₆-Cr^SNS afford linear low-density polyethylenes (LLDPEs) with exclusive n-butyl branches (6.8-25.8 branches/1000 C), while under identical polymerization conditions Ti-C₂-Cr^PNP and Ti-C₂-Cr^NNN produce polyethylenes with heterogeneous branching (C₂, C₄, and C_{≥ 6}) or negligible branching, respectively. Under identical ethylene polymerization conditions, Ti-C₀-Cr^SNS produces polyethylenes with higher activity (4.5× and 6.1×, respectively), M_n (1.3× and 1.8×, respectively), and branch density (1.4× and 3.8×, respectively), than Ti-C₂-Cr^SNS and Ti-C ₆-Cr^SNS. Versus a CGC^EtTi + SNSCr tandem catalyst, Ti-C₀-Cr^SNS yields polyethylene with somewhat lower activity, but with 22.6× higher M_n and 4.0× greater branching density under identical conditions. In ethylene +1-pentene competition experiments, Ti-C₀-Cr^SNS yields 5.5% n-propyl branches and 94.5% n-butyl branches at [1-pentene] = 0.1 M, and the estimated effective local concentration of 1-hexene is ∼8.6 M. In contrast, the tandem CGC^EtTi + SNSCr system yields 91.0% n-propyl branches under identical reaction conditions. The homopolymerization and 1-pentene competition results argue that close Ti···Cr spatial proximity together with weak C-H···Ti and C-H···S interactions significantly influence relative 1-hexene enchainment and chain transfer rates, supported by DFT computation, and that such effects are conversion insensitive but cocatalyst and solvent sensitive.