Stereoregularity, Regioselectivity, and Dormancy in Polymerizations Catalyzed by C-1-Symmetric Fluorenyl-Based Metallocenes. A Theoretical Study Based on Density Functional Theory

摘要

C-s-Symmetric propylene polymerization catalysts 1 with a bridged cyclopentadienyl and fluorenyl architecture are known to produce syndiotactic polymers. On the other hand, related C-1-symmetric catalysts, such as 2, that are obtained from 1 by the introduction of a bulky substituent (tert-butyl) on the cyclopentadienyl ring afford isotactic polymers. In this study we employ DFT calculations in order to analyze several aspects of olefin polymerizations catalyzed by the fluorenyl-based C-1-symmetric zirconocene 2. Modeling of the propagation in naked cationic systems, disregarding the noncoordinating counterion, yields information on the factors that affect streoselectivity (and ultimately stereoregularity), regioselectivity, and reactivity of the "crowded" site of the zirconocene relative to the "open" one. Several hvpotheses are investigated, with the aim to rationalize the experimental observation that 2 affords isotactic polymers whereas I gives rise to syndiotactic polymers. We provide in addition an analysis of the stability of dormant species 5 produced from 2,1 propylene mis-insertions. For this task, the need to include explicitly the counterion in the modeling seems to be inevitable. Comparative studies of the energetics of beta-H elimination to the metal or beta-H transfer to the monomer, relative to insertion into a Zr-secondary C bond, indicate that dormant species 5 are prone to beta-H elimination.