Synthesis Of CO₂ Copolymer:Design Of Highly Active SalenCo(Ⅲ)X Catalyst And Its Copolymerization Mechanism

The selective transformation of carbon dioxide (CO2) into biodegradable polycarbonates by the alternating copolymerization with epoxides has attracted much attention during the last decades due to economic and environmental benefits arising from the utilization of renewable source and the growing concern on the greenhouse effect. For CO2/aliphatic epoxides copolymerization, there exists much interesting information, such as polymer/cyclic product selectivity, ether and carbonate linkages, regiochemistry of epoxide ring-opening, and stereochemistry of carbonate unit sequence in a polymer, which bears a memory of the reaction pathway leading to its formation. Therefore, the catalytic synthesis of polycarbonates with high carbonate linkages and high molecular weight is one of the important issues, while approaching to the accurate control of polymer stereochemistry and its properties remains challenge. The present dissertation focuses on exploring the mechanistic aspects of the CO2/epoxides copolymerization with SalenCo(Ⅲ)X catalyst systems, as well as further designing highly active catalyst with excellent thermal stability with an emphasis of polycarbonate property or its regio- and stereochemistry.Considering that the mechanism of the binary or bifunctional catalyst systems based on cobalt-Salen complexes is not clear, a Co(Ⅲ)-salen 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 developed for the alternating copolymerization of CO2 and propylene oxide (PO). This catalyst exhibits highly active and selective poly(propylene carbonate) (PPC) formation even at high temperatures (up to 100℃), high [epoxide]/[catalyst] ratios, and/or low CO2 pressures. Electrospray ionization mass spectrometry (ESI-MS) and Fourier transform infrared spectroscopy (FTIR) studies, in combination with some verified 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. These studies provide a new mechanistic understanding of our previously studied binary catalyst systems based on Co(Ⅲ)-Salen complexes in which alternating chain-growth and dissociation of propagating carboxylate species derived from the nucleophilic axial ligand and the nucleophilic cocatalyst take turns at both sides of the Co(Ⅲ)-Salen center. This arrangement significantly increases the reaction rate and also helps to stabilize the active Co(III) species against decomposition to inactive Co(Ⅱ) even at low CO2 pressures and/or relatively high temperatures.According to the intramolecular cooperative catalysis mechanism, a series of new bifunctional Co(Ⅲ)-salen complexes have been synthesized. The complex anchored a quaternary ammonium salt on the three position of one aromatic ring by three methylene units exhibited excellent activity and polymer selectivity even at a high temperature for the copolymerization of CO2 with PO or with cyclohexene oxide (CHO). The catalytic activity is highly sensitive to the reaction temperature. The highest TOF up to 6105 h-1 was obtained at 120℃for the copolymerization of CO2 and CHO, without sacrificing polymer selectivity. This functionalized Co(Ⅲ)-salen complex could operate very efficiently for the terpolymerization of CHO and aliphatic epoxides with CO2 to provide selectively polycarbonates with a narrow polydispersity at various temperatures. The resulting terpolymers have only one thermolysis peak and one adjustable glass-transition temperature (Tg) dependable on cyclohexene carbonate unit content. Also, with the use of a TBD-appended SalenCo(Ⅲ) catalyst, a novel triblock polymer (PO-alt-CO2)x-b-(CHO-alt-CO2)y-b-(PO-alt-CO2)z was synthesized for the first time. The resulting triblock polymer has only a broad Tg, but two thermolysis peaks. Furthermore, several novel chiral cobalt-based complexes containing a derived chiral-BINOL and TBD were developed for asymmetric, regio- and stereoselective alternating copolymerization of CO2 and racemic PO. The (S,S,S)-Co(Ⅲ) complex with sterically hindered substituent group resulted in a near perfectly regioregular PPC, with>99% head-to-tail connect and a Krel of 24.4 for the enchainment of (R)-PO over (S)-PO, which all are the highest record.