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

Abstract

Several analogues of the sterically expanded constrained geometry catalyst Me₂Si(η¹-C₂₉H₃₆)(η¹-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(η⁵-C₂₁H₂₂)(η¹-N-tBu)ZrCl₂ (3); Me₂Si(η⁵-C₂₃H₂₆)(η¹-N-tBu)ZrCl₂ (4); and Me₂Si(η⁵-C₂₅H₃₀)(η¹-N-tBu)ZrCl₂ (5); Me₂Si(η⁵-C₂₁H₂₂)(η¹-N-tBu)ZrMe₂ (6); and Ph₂Si(η⁵-C₂₉H₃₆)(η¹-N-tBu)ZrCl₂ (7). Complexes 4, 5, 6, and 7 were characterized by X-ray crystallography and displayed η⁵ 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.