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Reports: DNI752731-DNI7: Flower-Mimetic Hydrogels: Materials that Grow Towards Light

Jeremiah A. Johnson, PhD, Massachusetts Institute of Technology

Photo-responsive materials have numerous potential applications in energy conversion, light harvesting, biomimetics, microelectronics, and waste-remediation. To capture the features of plant phototropism, we proposed and developed a new mode of synthetic material photo-response termed photo-growth. The photo-growth process involves direct incorporation of new monomers into the elastic chains of a polymer network, which results in an increased MW between crosslinks, and increased swelling (i.e., growth) in irradiated regions. The specific mechanism we employed to achieve photo-growth was a photo-controlled living radical polymerization of N-isopropylacrylamide (NiPAAm) from a bis-norbornene trithiocarbonate (TTC) that served as both the polymerization initiator and a chain transfer agent.  In this reaction, light induces temporary cleavage of the TTC to generate free radicals; in the presence of monomer, these radicals propagate to yield new polymer via a reversible addition fragmentation chain transfer (RAFT) process.  When the light source is turned off the radicals recombine to regenerate the TTC moiety at the center of a newly grown polymer chain.  This process is reversible, which allows for on/off photo-growth behavior, and the formation of block copolymers. 

Based on mechanistic insights, we showed that increased polymerization control could be achieved by using low light intensity (through the suppression of radical formation).  Given that sunlight possesses low intensity UV light, we were able to successfully develop polymerization conditions for living polymer growth in natural sunlight. Thanks to the mild, controlled nature of this polymerization, we were able to produce polymers with norbornene end-functionality that could then be further functionalized using inverse electron-demand Diels-Alder cycloaddition reactions with tetrazine derivatives. In particular, exposure of these polymers to tris-tetrazine molecules led to gel formation through an A2+B3 end-linking reaction. When the resulting gels were swollen in NiPAAm/MeCN solutions and exposed to sunlight their swelling ratios increased up to three-fold and the molecular weight between crosslinks expanded by four-fold. These results demonstrated, for the first time, the controlled photo-induced insertion of monomers into the chains of polymer networks to achieve light-induced network growth.  To summarize, we have achieved the following milestones: (1) the development of photo-controlled polymerization for growth of telechelic polymers in solution, (2) the subsequent crosslinking of these polymers to form gels, and (3) the demonstration of light-induced gel growth in the presence of sunlight.

Being the first research grant secured by the PI, ACS PRF support helped kick-start the area of controlled photo-polymerization within the group. The two researchers involved in this project at the outset, both post-doctoral scholars, began their careers at DOW chemical within the past year. Their work on this project undoubtedly helped strengthen their job applications to make them excellent candidates for the very competitive DOW hiring process.  The two graduate students currently involved in this work both expect to have significant portions of their thesis comprise follow-up studies related to photo-growth, which would not have been possible without initial PRF support.  Photo-controlled polymerization is currently a popular topic in polymer science, and our published manuscript represents a key advance in the field.  To support this notion, the editors of Angewandte Chemie named our paper a Hot Paper in 2013.  Furthermore, our paper received full highlights in Nature Chemistry and Materials 360. This significant external attention led to numerous exciting new collaborations and avenues for this research.