Title: Highly Efficient Aluminum-Catalyzed Ring-Opening Polymerization of Cyclic Carbonates, Lactones, and Lactides, Including a Unique Crystallographic Snapshot of an Intermediate
Authors: Jurgen Koller and Robert Bergman
Affiliation: Department of Chemistry, University of California, Berkeley
Snapshot of Catalysis: The crystal structures of the pre-catalyst (left), and the catalyst after addition of 1 equivalent of monomer. Subsequent additions result in polymers. (Carbon = Grey, Nitrogen = Purple, Silicon = Brown, Oxygen =Red, Aluminum = Pink, Hydrogen = White).
Our standard of living is the way it is because of polymers. They are an integral part of nearly every machine, utensil, or household item that we use everyday. One of the most common polymers in use is polyethylene, which if found in most plastics. The development of polymerization catalysts over the last 60 years has allowed industry to control the properties of the plastics that they produce, and has led to the widespread use of polyethylene. We all know that polyethylene is not the greatest thing for the environment, and so there has been recent interest in the synthesis of biodegradable polymers.
The researchers on this project have developed an aluminum (Al) complex which acts as a Lewis acid and catalyzes the polymerization of cyclic carbonates, lactones, and lactide monomers. There are a few attractive features of this catalyst system; it uses Al, which is cheap and environmentally benign, the monomers are widely available and relatively inexpensive, and the resulting polymers contain ester (or carbonate) functionalities, which are easily cleaved by microorganisms, making them biodegradable. The precatalyst is an Al(III) atom complexed by a diaminobenzene, demethylamide, and dimethylamine. The authors note that the coordination geometry is such that there is an exposed face of the Al which may be occupied by an oxygen atom on the monomer. Nucleophilic attack of the coordinated dimethylamide breaks the monomer ring, resulting in the formation of an Al-O bond (and can be seen in the picture above). The process continues, except the bound alkoxide performs the nucleophilic attack. One Al complex is able to repeat this process 10 times every second, resulting in the formation of some pretty large polymers (59,000 g/mol).
This is not the only catalyst able to produce biodegradable polymers, there are even some that can incorporate CO2 into polymers. The next challenge facing those that want to replace old polymers with biodegradable ones is to find ways to tune their properties so that they act the same as various polyethylenes.