Controlled Radical Polymerization for Novel Soft Materials

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The research program's broad objective is to synthesize, characterize and process novel block, random and gradient copolymers for tailored stimuli-responsive polymers to organic photovoltaic/light emitting diode (OPV/OLED) materials. We have applied state-of-the-art controlled radical polymerization (CRP) techniques to achieve the requisite microstructure vital for such applications. CRP approaches the control of microstructure found in truly "living" polymerization methods without the exhaustive purification, functional group protection and air-free conditions required of the latter. Further, CRP can link different blocks and/or monomers which otherwise could not achieved by ionic polymerization and offers the possibility of controlled polymerization in dispersed aqueous media. This versatility allows an unprecedented myriad of possible combinations to achieve desired nano-structured materials. In particular, we use a CRP technique termed nitroxide mediated polymerization (NMP), which has some favourable characteristics compared to other CRP methods. For example, it does not require metallic-containing catalysts or sulfur-based chain transfer agents that may remain as impurities which may affect the performance of stimuli-responsive biomedical materials and organic electronic materials. A traditional drawback to NMP was its restriction to the polymerization of styrenic monomers – this has been largely lifted by the use of second-generation alkoxyamine initiators such as BlocBuilder, which has enabled the homopolymerization of acrylates and the polymerization of methacrylate-rich compositions by using a small amount of co-monomer (typically styrenic ~ 5-10 mol%). We have examined controlling co-monomers that have added functionality such as the carbazole-functional controlling co-monomer (eg. VBK) to impart electron-donating/fluorescent properties in stimuli-responsive poly(methacrylates). We applied NMR and ESR to identify species and understand radical copolymerization kinetics. Notably, we found radical reactivity ratios were pivotal in determining how little co-monomer was necessary to maintain a controlled methacrylate polymerization with BlocBuilder initiators. Extending our scope to OPV/OLED applications, we have begun examining electron-acceptor groups in the controlling co-monomer, such as oxadiazoles. We have recently found that not all styrenic co-monomers are effective as controllers for methacrylates, even if the homopolymerizations were controlled. A short-term objective is thus to determine the effect of co-monomer donor/acceptor functionality on methacrylate-rich NMP via a pseudo-Hammett parameter analysis, which will enable how to incorporate such functionality into the desired poly(methacrylate).The longer-term goal for pursuing the electron-donor/acceptor side chain polymers is to take advantage of their easier processability and to self-assemble them into well-ordered domains not necessarily found in the typically stiff, lamellar main-chain polymers associated with OPV/OLEDs. Our initial studies will examine the combination of donor-acceptor block copolymers that could be applied as a water-borne latex, followed by selective removal of the poly(methacrylate) domain (eg. via photolithography). Further, domain amplification and epitaxial growth that has been applied in other block copolymer systems will be attempted as few studies have addressed directed self-assembly of side-chain donor/acceptor copolymers, despite reports of the critical role of morphology on device performance. The program focus on stimuli-responsive and OPV/OLED materials via NMP is a unique niche and will provide HQP (2 PhDs) with synthetic and processing skills that are portable to many vital sectors of the Canadian economy.