Heterotrinuclear Ring Opening Copolymerization Catalysis: Structure–activity Relationships

Heteronuclear complexes are highly active catalysts in alternating ring opening copolymerizations (ROCOP) of cyclohexene oxide (CHO)/carbon dioxide or CHO/phthalic anhydride (PA). In this contribution, a series of new trinuclear complexes is investigated through a structure activity study that reveals the influences of ligand electronic properties and metal selection over the catalytic activity and linkage selectivity. The fastest catalyst shows high activity for CHO/CO₂ ROCOP at low pressure (1 bar CO₂ pressure) and features the ligand coordinated to two Zn­(II) centers and with inexpensive and abundant Na­(I) (TOF = 1084 h^–1, catalyst/CHO 1:4000, 1 bar, 100 °C). High CHO/PA copolymerization activity is achieved with the combination of two Mg­(II) and one Na­(I) center (TOF = 142 h^–1, catalyst/PA/CHO 1:200:4000, 100 °C); further the catalyst undergoes “switchable” epoxide/anhydride ROCOP and epoxide ring opening polymerization (ROP), allowing for controlled polyether block formation. Polymerization kinetic investigations reveal the influences of the ligand electronics, complex stability and metal–metal separation, and their influences over the retention of high activity throughout the catalysis. The findings should help in guiding future polymerization catalyst design, facilitate block polymer production, and inspire other CO₂ utilization processes.