Iron and aluminum phosphido complexes for the sterocontrolled synthesis of biodegradable polymer brushes and stars

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Green chemistry is a set of chemical ideals that drive many chemists towards sustainable and environmentally friendly operations. These concepts include: (i) atom economy, where ideally 100% of the atoms you put into a reaction vessel end up in the product, (ii) reduced toxicity, where toxic reagents are exchanged for bioassimilable alternatives, (iii) renewable feedstocks, replacing depleting petroleum-based products with sustainable resources, and (iv) catalysis, where specialized transformations can be performed without the wasteful use of stoichiometric reagents. In particular, the use of petroleum-based non-renewable resources in the production of plastics and polymers and the environmental impact of the associated waste necessitates research into alternatives. Poly(lactic acid), or PLA, is a commercially relevant, non-toxic and biodegradable thermoplastic which is readily produced through the ring-opening polymerization of a chemical called lactide. This lactide monomer is also renewable, as the lactate from which it is ultimately produced is derived from the fermentation of agricultural by-products. Research into PLA has placed it at the forefront of biodegradable polymer research, however applications remain limited to the clothing and packaging industries. Expanding the use of PLA requires chemists to improve and tune the properties of the polymer. Just as our left and right hands cannot be overlain, many molecules also have a characteristic handedness. By controlling how these hands, or stereoisomers, fit together in a polymer chain, we can improve polymer properties: a random arrangement gives a sticky glass, an alternating arrangement gives a white solid, while the desired stereocomplex form of PLA offers enhanced thermal properties. This project designs new catalysts for the stereocontrolled polymerization of lactide. These catalysts will be based on inexpensive metals including iron and vanadium supported by novel phosphorus ligands. Stereocontrol will also be incorporated into larger polymer architectures, including shapes resembling brushes and stars to produce new value-added products. Production of these new materials will expand the applications of environmentally friendly biodegradable polymers to the benefit of society.