Reports: DNI1049734-DNI10: Development of Stereoregular Electroactive Block Copolymers for the Supramolecular Assembly of Organic Photovoltaics

Barry Thompson, PhD , University of Southern California

During the second year of the award period, the focus continued to be on developing synthetic routes toward stereoregular pendant-functionalized electroactive polymers. As originally proposed, all efforts continued to be directed toward radical polymerization as the most attractive of potential options. At the end of the first year we had gained success in the free radical polymerization of naphthyl-acrylates, achieving molecular weights (Mn) as high as 13,000 g/mol with typically broad polydispersities of ~5 using standard AIBN initiation in a variety of solvents, including dichlorobenzene. It was found that addition of the Lewis Acid Y(OTf)3 (5 mol %) significantly increased the molecular weight of the polymer samples (Mn – 57,000 g/mol) and narrowed the PDI to ~1.5 under the same reaction conditions. However, no effect on the tacticity of the polymer was observed by NMR. Increasing the loading of Lewis acid (up to 50 mol %) was observed to inhibit polymerization. We also investigated the effect of solvent polarity and other Lewis acids, but no significant influence on the tacticity of these naphthyl-acrylate polymers was ever observed.

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.

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