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Specifically designed homogeneous Ziegler-Natta metallocene catalysts: Strategy, synthesis and polymerization catalysis
Abstract
Strategies for designing specified Group IV ansa-metallocene olefin polymerization catalysts have been provided. In efforts to suppress the formation of undesired meso isomers of ansa-bis(indenyl)metallocene complexes, the previously enantiotopic faces of indenyl ligands were converted into diastereotopic faces by placing a chirality on the bridging atom, thus favoring the formation of an optically active, C$\sb2$-symmetric single diastereomer by means of diastereofacial selection. After achieving the correlation between the theoretical and experimental racemic/meso ratios in the synthesis of many known ansa-metallocenes by means of molecular mechanics (MM) calculations, MM studies were performed to predict racemic/meso ratios in the synthesis of specifically designed unknown ansa-metallocenes. A variety of 25 new organic ligands and organometallic complexes have been synthesized in this study. A thermal diastereomerization of indenylsilane ligands has been discovered and studied by the dynamic NMR technique. By carefully tuning the electronics and steric hindrance of ligand frameworks, five series of catalytic systems have been generated; they are a silacyclobutane-bridged bis(indenyl)zirconocene, a tetraphenylgermacyclopentadiene-bridged bis(indenyl)zirconocene derived from a stereoselective synthesis with aid of a MM calculation, a heptane-soluble homogeneous zirconocene catalyst with bis(menthoxy)silylene as the bridge, C$\sb{2\nu}$- and C$\sb2$-symmetric ansa-bis(fluorenyl)zirconocene catalysts, and silylene-bridged fluorenyl-containing ansa-zirconocene complexes with C$\sb1$ and C$\sb{s}$ symmetry. Polymerization catalysis has been carried out, leading to isospecific, syndio-specific or non-stereospecific polymerizations of $\alpha$-olefins with the five types of catalysts synthesized, using different activation methods. Three significant major objectives have been accomplished in the dissertation study: (1) generation of the first heptane-soluble homogeneous Ziegler-Natta catalyst with extremely high activity; (2) stereoselective synthesis of a temperature-invariant propylene polymerization catalyst over a broad polymerization temperature range; (3) the synthesis of a C$\sb{2\nu}$-symmetric catalyst that produces atactic polypropylene with a molecular weight of one-half million and with high activity.
Subject Area
Organic chemistry|Polymers
Recommended Citation
Chen, You-Xian, "Specifically designed homogeneous Ziegler-Natta metallocene catalysts: Strategy, synthesis and polymerization catalysis" (1995). Doctoral Dissertations Available from Proquest. AAI9524685.
https://scholarworks.umass.edu/dissertations/AAI9524685