Toward the development of sustainable polymers – zinc and iron complexes for poly(carbonate), poly(ether) and poly(ester) synthesis

In recent decades, significant effort has been put toward the development of sustainable polymers to compliment or replace the materials currently in use. Two major classes of interest are poly(carbonate)s and poly(ester)s. The allure of these stems from the materials that are used to synthesize them. Copolymerization of carbon dioxide and epoxides is one method of synthesizing poly(carbonate) and carbon dioxide levels in our atmosphere continue to rise – a result of anthropogenic activity – therefore, utilizing the small molecule for various transformations is convenient and necessary. The most common epoxides used in poly(carbonate) synthesis are produced from petroleum but there are naturally sourced compounds as well. Similarly, poly(ester) is produced mainly in the form of poly(ethylene terephthalate) but recently, naturally sourced reagents (i.e. lactide) have begun commercial production. The synthesis of the polymers is often facilitated through metal-based catalysts that encourage controlled reactivity between the substrates. In this work, a zinc amino-bis(phenolate) complex was investigated for catalytic activity toward carbon dioxide and cyclohexene oxide copolymerization in the presence of benzyl alcohol co-catalyst producing poly(ether-co-carbonate) with high molecular weights containing both ether and carbonate linkages. The catalyst was active at 1 bar carbon dioxide – a trait shared by some of the most active catalysts in this area. The complex also exhibited activity toward copolymerization of carbon dioxide and limonene oxide – a naturally sourced epoxide. Poly(limonene carbonate) typically exhibits physical properties more similar to commercially produced bisphenol-A poly(carbonate)s. Iron complexes were also examined for carbon dioxide and epoxide coupling as part of a collaboration between the groups of Drs. Kerton and Kozak with my own contributions studying three Fe(III) amino-bis(phenolate) complexes. Toward cyclohexene oxide and carbon dioxide coupling in the presence of bis(triphenylphosphine)iminium chloride co-catalyst, only cyclic cyclohexene carbonate was observed in low yields. However, the activity of two of the complexes toward lactide ring-opening polymerization was high. Furthermore, co- and terpolymerization reactions involving lactide, phthalic anhydride and epoxides produced poly(ester-co-ether)s with narrow dispersities. A zinc alkylperoxo complex was synthesized and structurally characterized. This could be applied toward the epoxidation of α-ß unsaturated ketones and subsequent copolymerization with carbon dioxide in one-pot in low yields. This represents the first instance of poly(carbonate) synthesis from epoxide-devoid starting materials in one-pot.

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