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During the first year of the award period, the focus has been on the development of a synthetic route toward stereoregular pendant-functionalized electroactive polymers. As originally proposed, all efforts have been directed towards radical polymerization, as the most attractive of potential options, based on the accessibility of radical polymerization and the relatively broad range of functional group tolerance offered. Literature precedent for the stereocontrolled (isospecific) radical polymerization of simple polar vinyl monomers, such as acrylates and acrylamides served as the starting point. It has been observed by others in the past, that addition of small molar percentages of large Lewis Acids, such as yttrium triflate (Y(OTf)3) or scandium triflate (SC(OTf)3) has resulted in radically grown polymers with isotactic dyad contents of more than 90%.

For the majority of the first year of the award period efforts were made to extend literature precedent from simple vinyl monomers to functionalized vinyl monomers with a pi-conjugated pendant group. We have chosen naphthalene as an attractive model for the optimization of the polymerization conditions. Both naphthyl acrylates and acrylamides have been investigated.  Using AIBN as a simple radical initiator and the naphthyl monomers, polymerizations were attempted using literature conditions with the known Lewis Acid additives. It rapidly became apparent that the choice of reaction solvent was a point in need of critical examination. Typical stereocontrolled radical polymerizations reported in the literature had utilized polar solvents, such as methanol, which were effective for dissolving the Lewis Acid salts. With monomers containing pi-conjugated groups, monomer solubility was significantly less in these polar solvents. A variety of other solvents and solvent mixtures have thus been investigated, but until now, the search for an optimal reaction condition using pi-conjugated monomers and Lewis Acid additives has yet to be found.

As a parallel and potential alternative route, we have recently begun to explore the idea of generating a stereoregular polymer backbone as a scaffold for the attachement of the pi-conjugated electroactive pendants. For this, we are pursuing the synthesis of alkyne-functionalized acrylamides, which are far more soluble in methanol and other polar solvents than those monomers with large pi-conjugated pendants. The focus is thus on the attachement of azide-functional pi-conjugated groups to the polymer backbone via the well-known click reaction. Our goal is to generate a family of isotactic and atactic polyacrylamide homopolymers with various different pi-conjugated groups (beginning with naphthalene), and then to establish the influence of tacticty on the optical and electronic properties of the polymers. Most importantly, we will be using space charge limited current mobility as a figure of merit.

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.