CAREER: Photomodulation of Polymer Molecular Weight Distribution

About This Grant

With this CAREER Award from the Macromolecular, Supramolecular and Nanochemistry Program in the Chemistry Division, Professor Melanie Chiu at Stony Brook University in New York is developing a new method for controlling polymer dispersity using light to mediate the chemical reactivity of polymer chain-ends. Polymers, long chains of small molecular units called monomers, are essential and ever-present in our everyday lives, from industrially-manufactured plastics, to naturally occurring cellulose in plants and plant-derived materials, like cotton. The material properties of such polymers, including stiffness, melting temperature, electrical conductivity, and degradation profile are often affected by the polymer’s dispersity, or the uniformity of the molecular chain lengths within a given sample. Controlling polymer dispersity—whether the chains are of similar or dissimilar lengths—is a promising means of tailoring a polymer’s physical properties without changing its molecular structure. This research is developing a one-pot system in which the polymerization initiator is turned on or off with light. This photo-switch results in two states: one that yields low dispersity polymers and another one that yields high dispersity polymers. Once these polymers are made, the research project features studies on how different reaction parameters affect optimize the dispersities that can be accessed. The second part of the research project focuses on systematically studying how polymer dispersity affects polymer properties. While these efforts focus on specific types of polymers, polyvinyl ethers, the last research activity expands the concept of using light to control polymer chain-end reactivity to form other types of polymers. Results associated with this award have the potential to significantly impact various fields of science and engineering where molecular control over polymer properties is of interest. Professor Chiu fosters integration of research and education by developing curriculum materials in collaboration with the New York State Master Chemistry Teacher's program to enhance the education of high school chemistry students throughout New York State. The research is focused on establishing a deterministic method for photoswitchable regulation of polymer dispersity and its applications to block copolymer synthesis by photochemically manipulating the rate of chain propagation versus deactivation of the chain-end in controlled cationic vinyl ether polymerizations. The project features the development of photochromic Bronsted acid initiators that yield polymers with low dispersity in one photoisomeric state, and high dispersity in the other photoisomeric state. In the first objective, the mechanism by which the carboxy-functionalized dithienylethenes enable photocontrol over polymer dispersity in cationic vinyl ether polymerizations is studied, and a comprehensive kinetic model is constructed to guide further optimization of the system. The second objective focuses on the application of the carboxy-functionalized dithienylethene initiators to the synthesis of block copolymers and investigation of their materials properties. The final objective expands the concept of externally manipulating relative rates of chain propagation and chain-end deactivation to achieve control over dispersity in other types of controlled polymerization reactions. The chemistry in this work expands the repertoire of polymerization reaction parameters that can be externally modulated and enables a better understanding of how dispersity influences polymer properties. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.