Barry Thompson, PhD , University of Southern California
As an alternative approach, we also focused on naphthyl-acrylamides, hoping to capitalize on the stronger Lewis Base character of the acrylamides to improve the interaction with the Lewis Acid additives and influence the polymer tacticity. With the naphthyl-acrylamides atactic polymers with molecular weights (Mn) of 20,000 g/mol and PDI of 2.0 were synthesized using standard free radical polymerization with AIBN in dichlorobenzene. Once again, addition of Lewis Acids such as Y(OTf)3 had no effect on the tacticity of the polymer.
Multiple other routes were tried unsuccessfully including polymerization of an alkyne-functionalized acrlyamide monomer designed to lead to a precursor polymer that could be functionalized with a pi-conjugated pendant group via “click” post-polymerization “click” chemistry. The main obstacle in this case was once again the inability to demonstrate control of stereoregualrity during the radical polymerization using Lewis Acid additives.
During the second half of the final reporting period our focus has begun to shift away from radical polymerization as an effective route to target stereoregular pendant-functionalized polymers. We have begun to focus instead on the development of novel pi-conjugated monomers with interesting optical and electronic properties beyond the simple naphthalene-based monomers we used as model systems for much of the award period. The goal here is to demonstrate in general that pi-pendant polymers with a non-conjugated backbone can display electronic properties competitive with fully conjugated polymers. Our ultimate goal continues to be to demonstrate that control of stereoregularity in such polymers can improve charge transport and function in electronic devices. Our continuing work is focused on identifying effective polymerization strategies to generate highly isotactic and syndiotactic polymers based on monomers with pi-conugated pendants possessing attractive optical and electronic properties.
The research supported under this grant has great potential impact in the field of electroactive and semiconducting polymers, specifically for use in polymer-based solar cells. Focusing on the ultimate goal of a donor-acceptor diblock copolymer using the stereoregular pendant-functionalized architecture, a new class of semiconducting polymer is targeted, which will undergo programmed self-assembly into optimal donor-acceptor morphologies from a single solution-processable component. Such polymers are also designed to display improved mechanical and ambient stability. Importantly, the polymers are designed to self-assemble into bicontinuous morphologies of distinct donor and acceptor phases in which crystalline ordering of electronic pendant groups is enforced within each phase via the stereoregularity of the polymer backbone. It is thought that this morphology will be optimal for solar cells.
The pursuit of this project has had a great impact on the research group as a whole, specifically on the graduate student (Beate Burkhart) who has been the lead synthetic chemist on the project. The entire field of electroactive and semiconducting polymers has long been exclusively focused on conjugated polymers. Searching for a new set of materials properties has thus been limited to finding a new polymerizable combination of heteroaraomatic monomers and studying the new conjugated structure. With this project on stereregular, non-conjugated polymers we are seeking new ways to achieve desirable electronic properties and now have access to a much broader range of chemistries to target those properties. As such, this project has added a new dimension to my research program.