With this award from the Macromolecular, Supramolecular and Nanochemistry Program, Professor James Canary of New York University is developing renewable and biodegradable plastics that could find use in the medical field, such as in implants and the development of new means to deliver medicines or drugs to more precise locations in the body. One of the most important of the biodegradable plastics in use today is polylactic acid, or PLA. This plastic is used in many applications to make containers, wrappings and other common materials, but it also plays a key role in the manufacture of medical implants and drug delivery systems. One of the features of PLA that makes it particularly useful is the relative orientation of the groups that are strung along the backbone of the molecule. When these relative orientations are varied, the stiffness of the resulting plastic can be changed, as can its biodegradability. The goal of this project is to develop specialized catalysts that can precisely control the orientation of groups in the PLA structure, thus controlling its properties. This work is having a broader impact on the development of new materials with a variety of uses in the medical field. It is having a further broad impact on the environment, both through the production of new biodegradable plastics and through the fact that the starting material used to make PLA is typically obtained from recycled material. The work is also having an impact on the training of the next generation of scientists through the participation of students at all levels, even high school, in the research. The group is further expanding the impact of their research by helping to prepare educational modules on their work for a local museum, the New York Hall of Science.
This project focuses on the development of catalysts with the capability to be switched electrochemically and, consequently, invert enantio- or diastereoselectivity in organic reactions. This special property, essentially unknown in other systems, is being tested in the polymerization reaction of d,l-lactide (normally obtained from recycled materials) to form polylactide (PLA), a renewable and biodegradable polymer. The relative orientation of the methyl groups along the polymer background, property known as tacticity, can be varied if appropriate catalysts are used. The group is exploring a set of copper complexes that exhibit dramatic reconfiguration of the organic ligand upon oxidation or reduction of a central copper atom. One interesting system, synthesized from L-methionine, results in the inversion of the helicity of the complex upon chemical or electrochemical oxidation or reduction. The overall shapes of the diastereomeric copper (I) and copper (II) complexes are nearly mirror image right- and left-handed propeller structures. In recent work, catalytic diphenylurea groups were attached to the periphery of the invertible redox switch, creating a new catalyst for asymmetric reactions. The right-handed catalyst (the Cu(II) oxidation state) gave mainly product of (S)-absolute configuration, while the left-handed Cu(I) catalyst produced mainly (R) product. Control experiments indicate that the helicity of the catalyst governs the stereochemical outcome of the reaction. The unique feature of this ambidextrous catalyst approach is the opportunity to switch enantioselectivity in real time while a reaction is underway. This dynamic feature of the catalyst is being applied to the synthesis of PLA, thus allowing unprecedented control over the stereochemistry of the product and the resulting properties of this important polymer.
