Aluminum Alkyl Induced Isomerization of Group IV meso Metallocene Complexes

Area of Science:

• Polymer Chemistry
• Organometallic Chemistry
• Catalysis

Background:

• Metallocene precatalysts for propylene polymerization produce both isotactic (iPP) and atactic (aPP) polypropylene isomers.
• Atactic polypropylene (aPP) has limited applications due to poor physical properties.
• Separating meso and racemic isomers is challenging, hindering industrial application of metallocene catalysts.

Purpose of the Study:

• To develop an efficient isomerization protocol to convert meso metallocene complexes into racemic counterparts.
• To enhance the yield of isotactic polypropylene (iPP) in metallocene-catalyzed polymerization.
• To investigate the structural requirements for isomerization and elucidate the reaction mechanism.

Main Methods:

• Development of a novel isomerization protocol for meso metallocene complexes.
• Application of the protocol to hafnocene and zirconocene complexes with various activation methods.
• Ligand framework modification and density functional theory (DFT) calculations to identify key structural features.
• Characterization using liquid injection field desorption ionization mass spectrometry (LIFDI-MS) and nuclear magnetic resonance (NMR) spectroscopy (¹H and ²⁹Si).

Main Results:

• The isomerization protocol successfully converted meso metallocene complexes to racemic forms.
• Isotactic polypropylene (iPP) yield increased by 400% compared to non-isomerized controls.
• The protocol demonstrated broad applicability across different metallocene types and activation methods.
• Methoxy groups on indenyl moieties were identified as crucial for isomerization, confirmed by DFT calculations.

Conclusions:

• The developed isomerization protocol offers a significant advancement for metallocene-catalyzed propylene polymerization.
• This method effectively increases iPP yield and maintains polymer quality, addressing a key industrial limitation.
• The findings provide a mechanistic understanding of metallocene isomerization, paving the way for catalyst design.