Propene Polymerization with C₁-Symmetric Fluorenyl-Metallocene Catalysts.

Area of Science:

• Polymer Chemistry
• Organometallic Chemistry
• Catalysis

Background:

• Metallocene catalysts are crucial for olefin polymerization, enabling control over polymer properties.
• The stereochemistry of propene polymerization is influenced by catalyst structure and reaction conditions.
• Understanding polymerization mechanisms is key to designing catalysts for specific polymer architectures.

Purpose of the Study:

• To investigate the effect of C1-symmetric metallocene catalysts with t-butyl substituents on propene polymerization.
• To elucidate the influence of steric hindrance on polymerization mechanisms and polymer microstructure.
• To establish correlations between catalyst structure, polymerization behavior, and resulting polypropene properties.

Main Methods:

• Synthesis and activation of three C1-symmetric metallocene catalysts (1, 2, and 3) with t-butyl substituents.
• Propene polymerization experiments at varying temperatures and monomer concentrations.
• Microstructure analysis of polypropene using 13C NMR spectroscopy.
• Chain end group analysis using 1H NMR spectroscopy.

Main Results:

• The steric bulk of t-butyl groups on metallocene catalysts promotes site epimerization, competing with the dominant chain migratory insertion mechanism.
• Catalyst 3, with two t-butyl groups in close proximity, mandates site epimerization, leading to lower activity compared to catalyst 2.
• Vinylidene end groups, formed via beta-H elimination or chain transfer, are the primary chain termination route.
• A negative correlation exists between vinylidene end group concentration and polypropene molar mass.

Conclusions:

• Steric hindrance in metallocene catalysts is a critical factor in controlling propene polymerization pathways.
• Site epimerization becomes a significant mechanism, influencing polymer microstructure and molecular weight.
• The concentration of vinylidene end groups serves as an indicator for polypropene molar mass.