Electron-Rich Metal Cations Enable Synthesis of High Molecular Weight, Linear Functional Polyethylenes

Article abstract of DOI:10.1021/jacs.8b04712

Group 10 metal catalysts have shown much promise for the copolymerization of nonpolar with polar alkenes to directly generate functional materials, but access to high copolymer molecular weights nevertheless remains a key challenge toward practical applications in this field. In the context of identifying new strategies for molecular weight control, we report a series of highly polarized P(V)-P(III) chelating ligands that manifest unique space filling and electrostatic effects within the coordination sphere of single component Pd polymerization catalysts and exert important influences on (co)polymer molecular weights. Single component, cationic phosphonic diamide-phosphine (PDAP) Pd catalysts are competent to generate linear, functional polyethylenes with M_w up to ca. 2 × 10⁵ g mol^-1, significantly higher than prototypical catalysts in this field, and with polar content up to ca. 9 mol %. Functional groups are positioned by these catalysts almost exclusively along the main chain, not at chain ends or ends of branches, which mimics the microstructures of commercial linear low-density polyethylenes. Spectroscopic, X-ray crystallographic, and computational data indicate PDAP coordination to Pd manifests cationic yet electron-rich active species, which may correlate to their complementary catalytic properties versus privileged catalysts such as electrophilic α-diimine (Brookhart-type) or neutral phosphine-sulfonato (Drent-type) complexes. Though steric blocking within the catalyst coordination sphere has long been a reliable strategy for catalyst molecular weight control, data from this study suggest electronic control should be considered as a complementary concept less prone to suppression of comonomer enchainment that can occur with highly sterically congested catalysts.