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.