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

The heterobimetallic complexes, (η5-indenyl)­[1-Me2Si­(tBuN)­TiCl2]-3-CnH2n-[N,N-bis­(2-(ethylthio)­ethyl)­amine]­CrCl3 (n = 0, Ti-C0-CrSNS; n = 2, Ti-C2-CrSNS; n = 6, Ti-C6-CrSNS), (η5-indenyl)­[1-Me2Si­(tBuN)­TiCl2]-3-C2H4-[N,N-bis­((o-OMe-C6H4)2P)­amine]­CrCl3 (Ti-C2-CrPNP), and (η5-indenyl)­[1-Me2Si­(tBuN)­TiCl2]-3-C2H4-[N,N-bis­((diethylamine)­ethyl)-amine]­CrCl3 (Ti-C2-CrNNN), are synthesized, fully characterized, and employed as olefin polymerization catalysts. With ethylene as the feed and MAO as cocatalyst/activator, SNS-based complexes Ti-C0-CrSNS, Ti-C2-CrSNS, and Ti-C6-CrSNS 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-CrPNP and Ti-C2-CrNNN produce polyethylenes with heterogeneous branching (C2, C4, and C≥6) or negligible branching, respectively. Under identical ethylene polymerization conditions, Ti-C0-CrSNS produces polyethylenes with higher activity (4.5× and 6.1×, respectively), Mn (1.3× and 1.8×, respectively), and branch density (1.4× and 3.8×, respectively), than Ti-C2-CrSNS and Ti-C6-CrSNS. Versus a CGCEtTi + SNSCr tandem catalyst, Ti-C0-CrSNS yields polyethylene with somewhat lower activity, but with 22.6× higher Mn and 4.0× greater branching density under identical conditions. In ethylene +1-pentene competition experiments, Ti-C0-CrSNS 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 CGCEtTi + 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.

本研究合成并全面表征了一系列异双金属配合物，包括(η⁵-茚基)[1-二甲基硅基(叔丁基亚氨基)二氯化钛]-3-CnH2n-[N,N-双(2-(乙硫基)乙基)胺]三氯化铬（其中n=0时记为Ti-C0-CrSNS；n=2时记为Ti-C2-CrSNS；n=6时记为Ti-C6-CrSNS）、(η⁵-茚基)[1-二甲基硅基(叔丁基亚氨基)二氯化钛]-3-C2H4-[N,N-双((双(邻甲氧基苯基)膦)胺]三氯化铬（记为Ti-C2-CrPNP）以及(η⁵-茚基)[1-二甲基硅基(叔丁基亚氨基)二氯化钛]-3-C2H4-[N,N-双((二乙胺基乙基)胺]三氯化铬（记为Ti-C2-CrNNN）；上述配合物均经充分表征，并被用作烯烃聚合催化剂。以乙烯为聚合单体、甲基铝氧烷（MAO）为助催化剂/活化剂时，基于SNS配体的配合物Ti-C0-CrSNS、Ti-C2-CrSNS与Ti-C6-CrSNS可制备仅含正丁基支链的线型低密度聚乙烯（linear low-density polyethylene, LLDPE），支化度为6.8~25.8支链/1000C；而在相同聚合条件下，Ti-C2-CrPNP与Ti-C2-CrNNN分别得到具有非均一分布支链（包括C2、C4及C≥6支链的聚乙烯，或几乎无支链的聚乙烯。在相同乙烯聚合条件下，相较于Ti-C2-CrSNS与Ti-C6-CrSNS，Ti-C0-CrSNS制备的聚乙烯具有更高的催化活性（分别为其4.5倍与6.1倍）、数均分子量（Mn，分别为其1.3倍与1.8倍）以及支化密度（分别为其1.4倍与3.8倍）。相较于CGCEtTi + SNSCr双功能串联催化剂，Ti-C0-CrSNS在相同条件下制备的聚乙烯催化活性略低，但数均分子量提升22.6倍，支化密度提升4.0倍。在乙烯与1-戊烯竞争聚合实验中，当1-戊烯浓度为0.1 M时，Ti-C0-CrSNS可得到5.5%的正丙基支链与94.5%的正丁基支链，经估算体系中1-己烯的有效局部浓度约为8.6 M。与之相反，在相同反应条件下，CGCEtTi + SNSCr双功能串联体系仅得到91.0%的正丙基支链。均聚与1-戊烯竞争聚合的实验结果表明，Ti与Cr之间的近距离空间邻近性，以及弱的C-H···Ti与C-H···S相互作用，会显著影响1-己烯的插入与链转移速率，该结论得到密度泛函理论（DFT）计算验证；且此类效应与转化率无关，但受助催化剂与溶剂影响。