Sterically expanded CGC catalysts: substituent effects on ethylene and α-olefin polymerization.

Several analogues of the sterically expanded constrained geometry catalyst Me2Si(η(1)-C29H36)(η(1)-N-tBu)ZrCl2·OEt2 (2) were synthesized to assess the effect on branching and molecular weight for ethylene homopolymerization. Catalysts based on tetramethyltetrahydrobenzofluorene (TetH), ethylTetH, t-butylTetH, and octamethyloctahydrodibenzofluorenyl (OctH) bearing a diphenylsilyl bridge were prepared and characterized: Me2Si(η(5)-C21H22)(η(1)-N-tBu)ZrCl2 (3); Me2Si(η(5)-C23H26)(η(1)-N-tBu)ZrCl2 (4); and Me2Si(η(5)-C25H30)(η(1)-N-tBu)ZrCl2 (5); Me2Si(η(5)-C21H22)(η(1)-N-tBu)ZrMe2 (6); and Ph2Si(η(5)-C29H36)(η(1)-N-tBu)ZrCl2 (7). Complexes 4, 5, 6, and 7 were characterized by X-ray crystallography and displayed η(5) hapticity to the carbon ring in each case, in contrast to 2. In comparison to 2, complexes 3, 4, 5, and 7 (in combination with methylaluminoxane = MAO) showed diminished branching, higher molecular weight, and higher polydispersity indices for obtained ethylene homopolymers. While 4/MAO produced the greatest molecular weight polymers, no branching was observed. Reactivity ratios were determined for the copolymerization of ethylene and 1-decene with 2/MAO. A value of r(ethylene) = 14.9 and an exceedingly high value of r(1-decene) = 0.49 were found--in line with previous reports of this catalyst's unusual affinity for α-olefins.