"Oscillating" metallocene catalysts: what stops the oscillation?

Area of Science:

• Polymer Chemistry
• Organometallic Chemistry
• Materials Science

Background:

• Metallocene catalysts are crucial for stereoselective olefin polymerization.
• Understanding catalyst behavior, particularly "oscillating" metallocenes, is key to controlling polymer microstructure.
• Previous models struggled to explain stereocontrol in polypropylene produced by certain metallocene catalysts.

Purpose of the Study:

• To analyze the microstructure of polypropylene produced by two "oscillating" metallocene catalysts with differing steric hindrance.
• To elucidate the mechanistic origins of stereocontrol in these polymerization processes.
• To investigate the role of catalyst conformation and its interaction with counterions.

Main Methods:

• 150 MHz (13)C NMR microstructural analysis of polypropylene samples.
• Comparison of catalysts: [(2-(3,5-di-tert-butyl-4-methoxyphenyl)indenyl)(2)ZrP](+) and [(2-phenylindenyl)(2)ZrP](+).
• Computer modeling of ion pairs to test hypotheses on stereorigidity.

Main Results:

• The highly hindered catalyst produced isotactic-stereoblock polypropylene, indicating oscillation between enantiomorphous rac-like conformations.
• The less hindered catalyst yielded predominantly stereoirregular polypropylene due to faster ligand rotation.
• A specific fraction from the less hindered catalyst suggested a conformationally locked rac-like species, influenced by counterion proximity.

Conclusions:

• The "oscillation" mechanism needs refinement to account for observed polymer configurations.
• Steric hindrance and ligand dynamics significantly impact polypropylene stereoregularity.
• Counterion proximity can induce stereorigidity in metallocene catalysts, influencing polymer microstructure.