Geometrically Constrained Cofacial Bi-Titanium Olefin Polymerization Catalysts: Tuning and Enhancing Comonomer Incorporation Density

A series of shape-persistent bis-[C(sp3)-chelating] Ti2 (plus Zr2 and Hf2) complexes with a rigid linker component (xanthene or dibenzofuran) are presented. These structurally diverse assemblies display limited yet different conformational flexibility, and crucially, such geometric constraints confer access to a range of intermetallic separations and orientations to potentially enhance catalytic activity and cooperative effects. For ethylene polymerizations, the Ti2 catalysts (in conjunction with trityl borate) exhibit greater efficiencies and produced polymers with higher Mw than mononuclear controls, which is significant considering the more crowded environment for cofacial bimetallic sites. Proficient 1-hexene incorporations were observed for ethylene-(α-olefin) copolymerization reactions. The F-substituted m-aryl/dibenzofuran-linked catalyst (5), which is revealed by NMR analysis to be conformationally dissimilar to its F-absent congener, produced copolymers with higher Mw and elevated 1-hexene incorporation levels (up to 44%), when compared with its mono-Ti control (19%). These results suggest that catalyst frameworks with suitably adjustable conformations and Ti···Ti distances can facilitate bimetallic enchainment interactions with α-olefin substrates and their insertion.