A cationic aluminium carbonylation catalyst works in tandem with a zinc alkoxide polymerization catalyst to provide an efficient route to an important biodegradable polymer.
Polypropylene has far too many applications to list, but it isn't biodegradable; a potential alternative with that property is poly-3-hydroxybutyrate (P3HB). However, there are problems with the current methods for the production of this polymer: A useful method needs to produce a high-molecular-weight polymer in good yield and ideally will be simple and atom-economical. Fermentation can produce such a polymer and can potentially be used to produce bespoke functionalized polymers, but is too energy intensive for most bulk applications. The direct co-polymerization of carbon monoxide and propylene oxide would be the most atom-economical, but produces a low-molecular-weight product. A polymerization of β-butyrolactone is an alternative that produces high-molecular-weight polymers but requires a high-purity starting material that is unfortunately toxic.
Enter Erin Dunn and Geoffrey Coates from Cornell University, who have achieved1 a one-pot two-catalyst approach to high-molecular-weight P3HB from polypropylene oxide and carbon monoxide. The first catalyst — a cationic aluminium complex — mediates the epoxide carbonylation to form the β-butyrolactone, and the second — a zinc alkoxide species — effects the polymerization in situ. On paper, this sounds like an obvious step, but in practice it requires that the two catalysts do not interfere with one another and are suitably reactive under the same conditions. Dunn and Coates were able to show that their pair of catalysts had essentially orthogonal reactivity — each one doing only its own job — and ultimately identified conditions that allowed production of a high-molecular-weight, low-polydispersity product with low catalyst loadings.
Dunn, E. W. & Coates, G. W. Carbonylative polymerization of propylene oxide: a multicatalytic approach to the synthesis of poly-3-hydroxybutyrate. J. Am. Chem. Soc. 10.1021/ja1049862 (2010).