Mechanistic Aspects of the Copolymerization of CO₂ with Epoxides Using a Thermally Stable Single-Site Cobalt(III) Catalyst

Abstract

The mechanism of the copolymerization of CO(2) and epoxides to afford the corresponding polycarbonates catalyzed by a highly active and thermally stable cobalt(III) complex with 1,5,7-triabicyclo[4,4,0] dec-5-ene (designated as TBD, a sterically hindered organic base) anchored on the ligand framework has been studied by means of electrospray ionization mass spectrometry (ESI-MS) and Fourier transform infrared spectroscopy (FTIR). The single-site, cobalt-based catalyst exhibited excellent activity and selectivity for polymer formation during CO(2)/propylene oxide (PO) copolymerization even at temperatures up to 100 degrees C and high [epoxide]/[catalyst] ratios, and/or low CO(2) pressures. The anchored TBD on the ligand framework plays an important role in maintaining thermal stability and high activity of the catalyst. ESI-MS and FTIR studies, in combination with some control experiments, confirmed the formation of the carboxylate intermediate with regard to the anchored TBD on the catalyst ligand framework. This analysis demonstrated that the formed carboxylate intermediate helped to stabilize the active Co(III) species against decomposition to inactive Co(II) by reversibly intramolecular Co-O bond formation and dissociation. Previous studies of binary catalyst systems based on Co(III)-Salen complexes did not address the role of these nucleophilic cocatalysts in stabilizing active Co(III) species during the copolymerization. The present study provides a new mechanistic understanding of these binary catalyst systems in which alternating chain-growth and dissociation of propagating carboxylate species derived from the nucleophilic axial anion and the nucleophilic cocatalyst take turns at both sides of the Co(III)-Salen center. This significantly increases the reaction rate and also helps to stabilize the active SalenCo(III) against decomposition to inactive SalenCo(II) even at low CO(2) pressures and/or relatively high temperatures.

Alternating copolymerization of CO2 with propylene oxide and terpolymerization with aliphatic epoxides by bifunctional cobalt Salen complex

“…The catalyst activity was over four times greater than that of the binary catalyst system of complex 2 in conjunction with N-methylmorpholine under the same conditions. In addition, the previously reported Co Salen complex as catalyst was inactive for the copolymerization of CO 2 /PO at low CO 2 pressures, 11,13,22,23 perhaps because the Lewis base anchored on the catalyst ligand framework helps to trap the dissociated anion, resulting in higher catalytic activity and stabilizing ability.…”

“…Simple Salen Co III X complexes alone have proven to be inactive under low CO 2 pressure or at elevated temperature, where they are reduced to Co II derivatives. 11,16,17,22,23,30 The key factor in the catalyst design was the use of Lewis base units as phenolate para substituents on complex 1 to stabilize the active Salen Co III against decomposition to inactive Salen Co II .…”

“…These end-groups are commonly observed in this polymerization reaction. 12,20,22 A lower molecular number series of peaks was assigned to two series of peaks. .…”

“…21 Compared with the binary catalyst systems composed of a Co Schiff base complex and a Lewis base or organic salt, the bifunctional Salen complexes displayed excellent activity and polycarbonate selectivity because the anchored units of aromatic rings had an important role in maintaining the thermal stability and high activity of the catalyst. 22 On the other hand, Lu et al 16 reported that simple changes in the axial group X of the Salen Co III X complex could drastically affect the selectivity of poly (propylene carbonate) (PPC) and cyclic carbonate in PO/CO 2 copolymerization. Recently, it was found that the axial group 2,4-dinitrophenoxy has an important role in CO 2 /epoxides coupling reactions.…”

Lewis Basic Ionic Liquids‐Catalyzed Conversion of Carbon Dioxide to Cyclic Carbonates

“…Interestingly, the selectivity of the reaction increased with reaction time, being probably attributed to the in situ formation of polycarbonates and depolymerization into PC during the reaction at elevated temperature and low CO 2 pressure, as reported in the literature. [25] It is well-known that the stability and reusability of a catalyst system are the two key factors that identify whether it finds potentially practical application in industry. To test catalyst reusability, the reaction was carried out in the presence of a catalytic amount of [HDBU]Cl under the optimal conditions.…”