Very Large Cooperative Effects in Heterobimetailic Titanium-Chromium Catalysts for Ethylene Polymerization/Copolymerization

摘要

The heterobimetailic complexes, (η~5-indenyl)[1-Me_2Si(~tBuN)TiCl_2]-3-C_nH_(2n)-[N,N-bis(2-(ethylthio)ethyl)-amine]CrCl_3 (n = 0, Ti-C_0-Cr~(SNS) n = 2, Ti-C_2-Cr~(SNS); n = 6, Ti-C_6-Cr~(SNS), (η~5-indenyl)[1-Me_2Si(~tBuN)TiCl_2]-3-C_2H_4-[N,N-bis-((o-OMe-C_6H_4)_2P)amine]CrCl_3 (Ti-C_2-Cr~(PNP)), and (η~5-indenyl)[1-Me_2Si(~tBuN)TiCl_2]-3-C_2H_4-[N,N-bis((diethylamine)-ethyl)-amine]CrCl_3 (Ti-C_2-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-C_2-Cr~(SNS), and Ti-C_6-Cr~(NSN) afford linear low-density polyethylenes (LLDPEs) with exclusive n-butyl branches (6.8-25.8 branches/1000 C), while under identical polymerization conditions Ti-C_2-Cr~(PNP) and Ti-C_2-Cr~(NNN) produce polyethylenes with heterogeneous branching (C_2, C_4, and C_(≥6)) or negligible branching, respectively. Under identical ethylene polymerization conditions, Ti-C_0-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-C_2-Cr~(SNS) and Ti-C_6-Cr~(SNS). Versus a CGC~(Et)Ti + SNSCr tandem catalyst, Ti-C_0-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-C_0-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~(Et)Ti + 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.