Heterodinuclear catalysts for CO2/epoxide copolymerisations

• Main Author: Trott, Gemma
• Other Authors: Williams, Charlotte K.
• Published: University of Oxford 2018
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.748952

This thesis describes the preparation of new heterodinuclear catalysts for the selective copolymerisation of epoxides with CO₂ or anhydrides. The catalysts are based on a symmetrical diphenol-tetraamine macrocycle to prepare heterocomplexes coordinated to zinc and magnesium. The coordination sphere is completed by co-ligands derived from carboxylates. Optimisation of the synthesis is carried out to enable a straightforward route to new catalysts through the metathesis of a dibromide complex using a range of potassium salts. The characterisation of these complexes is explored using multinuclear NMR spectroscopy, mass spectrometry, infrared spectroscopy and elemental analysis. The heterodinuclear complexes are highly active for the copolymerisation of cyclohexene oxide and CO₂, under atmospheric pressures [TOF = 645 h^-1, 120 °C, 0.1 mol%, 1 bar]. At higher temperatures (120 °C), excellent selectivity for poly(cyclohexene carbonate) formation (> 99 %) is maintained. The catalysts are highly active under low catalyst loadings, with excellent activities [TOF = 8830 h-1, 120 °C, 0.01 mol%, 20 bar]. Telechelic polymers are produced upon addition of chain transfer agents, and the system is stable under aerobic conditions. A linear increase in molar mass with conversion shows the polymerisation is controlled. The copolymerisation of CO₂ with a range of epoxides is explored, allowing formation of poly(propylene carbonate), albeit with low selectivity [25 %, 60 °C, 0.01 mol%, 20 bar], and high molar mass poly(cyclopentene carbonate) in excellent (> 99 %) selectivity [TOF = 76 h^-1, 80 °C, 0.01 mol%, 20 bar]. The catalysts are also tested in the copolymerisation of phthalic anhydride with cyclohexene oxide to yield polyesters in excellent (> 99 %) selectivity. These catalysts are also highly active and well controlled [TOF = 289 h^-1, 100 °C, 0.13 mol%]. A range of anhydrides were tested, including succinic anhydride, glutaric anhydride and diphenic anhydride, which generally show high selectivity, although a variation in catalytic activity is observed. Kinetic studies are carried out to probe the rate equation for the copolymerisations of CO₂ and phthalic anhydride, each with cyclohexene oxide. For the copolymerisation of phthalic anhydride and cyclohexene oxide, the data suggests a zero order dependence on anhydride and first order dependence on catalyst and epoxide. The rate equation determined for the copolymerisation of CO₂ and cyclohexene oxide suggests a first order dependence on catalyst and epoxide, with a zero order dependence on CO₂ pressure, and is in line with previous studies on similar systems. Computational analysis, applying density functional theory, shows that the previously hypothesised homodinuclear 'chain-shuttling' mechanism could be plausible for the analogous heterodinuclear systems.