Heterodinuclear zinc and magnesium catalysts for epoxide/CO2 ring opening copolymerizations. Issue 17 (26th March 2019)

• Record Type: Journal Article
• Authors: Trott, Gemma; Garden, Jennifer A.; Williams, Charlotte K.
• Is Part Of: Chemical science. Volume 10:Issue 17(2019)
• Page Start: 4618
• Page End: 4627
• Publication Date: 2019-03-26
• Subjects: Chemistry -- Periodicals; 540.5
• DOI: 10.1039/c9sc00385a
• Languages: English
• ISSNs: 2041-6520

Abstract

Abstract : Eight heterodinuclear catalysts comprising Zn(ii ) and Mg(ii ) metal centres show high activity and selectivity for CO2 /epoxide ring-opening copolymerization. Detailed kinetic analyses are reported and a chain-shuttling mechanistic hypothesis proposed. Abstract : The ring-opening copolymerization of carbon dioxide and epoxides is a useful means to make aliphatic polycarbonates and to add-value to CO2 . Recently, the first heterodinuclear Zn(ii )/Mg(ii ) catalyst showed greater activity than either homodinuclear analogue ( J. Am. Chem. Soc. 2015, 137, 15078–15081). Building from this preliminary finding, here, eight new Zn(ii )/Mg(ii ) heterodinuclear catalysts featuring carboxylate co-ligands are prepared and characterized. The best catalysts show very high activities for copolymerization using cyclohexene oxide (TOF = 8880 h −1, 20 bar CO2, 120 °C, 0.01 mol% catalyst loading) or cyclopentene oxide. All the catalysts are highly active in the low pressure regime and specifically at 1 bar pressure CO2 . The polymerization kinetics are analysed using in situ spectroscopy and aliquot techniques: the rate law is overall second order with a first order dependence in both catalyst and epoxide concentrations and a zero order in carbon dioxide pressure. The pseudo first order rate coefficient values are compared for the catalyst series and differences are primarily attributed to effects on initiation rates. The data are consistent with a chain shuttling mechanistic hypothesis with heterodinuclear complexes showing particular rate enhancements by optimizing distinct roles in the catalytic cycles. The mechanistic hypothesis should underpin future heterodinuclear catalyst design for use both in other (co)polymerization and carbon dioxide utilization reactions.