Overcoming aggregation in indium salen catalysts for isoselective lactide polymerization

Area of Science:

• Organometallic Chemistry
• Polymer Chemistry
• Catalysis

Background:

• Controlling catalyst aggregation is crucial for optimizing activity and selectivity in polymerization reactions.
• Indium catalysts have shown promise in ring-opening polymerization (ROP) but their aggregation states can influence performance.
• Understanding the relationship between catalyst structure, aggregation, and polymerization behavior is key to designing efficient catalytic systems.

Purpose of the Study:

• To develop a methodology for controlling aggregation in highly active and isoselective indium catalysts.
• To synthesize and characterize dinuclear and mononuclear indium complexes for ROP of racemic lactide.
• To investigate the impact of nuclearity on catalyst initiation, activity, and polymer tacticity.

Main Methods:

• Synthesis and full characterization of dinuclear indium ethoxide complexes with salen ligands [(ONNO_R)InOEt]₂.
• Synthesis of mononuclear indium analogues (ONNO_R)InOCH₂Pyr using chelating 2-pyridinemethoxide functionality to control aggregation.
• Confirmation of metal complex nuclearity using Pulsed Field Gradient Spin Echo (PGSE) NMR spectroscopy.
• Detailed kinetic studies to analyze polymerization behavior and initiation periods.
• Investigation of polymer isotacticity using direct integration and peak deconvolution methodologies.

Main Results:

• Dinuclear indium catalysts with varying salen ligands (R = Br, Me, admantyl, cumyl, t-Bu) were successfully synthesized.
• Mononuclear analogues were prepared by controlling aggregation via chelating ligands.
• PGSE NMR spectroscopy confirmed the nuclearity of the synthesized indium complexes.
• Kinetic studies revealed a distinct initiation period for dinuclear catalysts, absent in mononuclear analogues.
• Polymerization behavior and resulting polymer isotacticity were found to be identical for analogous dinuclear and mononuclear catalysts, indicating a mononuclear propagating species.

Conclusions:

• The methodology effectively controls aggregation in indium catalysts, leading to highly active and isoselective systems for lactide ROP.
• Both dinuclear and mononuclear indium catalysts exhibit similar polymerization behavior and produce polymers with comparable isotacticity.
• The propagating species in the ring-opening polymerization of racemic lactide using these indium catalysts is mononuclear, regardless of the initial catalyst nuclearity.