Catalytic and polymer composite applications of graphite oxide and the controlled synthesis of donor-acceptor conjugated polymers
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While much of the recent work regarding carbon materials has centered on monolayer graphene, we have focused on the relatively inexpensive, and readily accessible graphite oxide (GO) and thermally-reduced graphite oxide (TRG) to develop new catalysts and polymer composites. Indeed, the reactivity and physical properties inherent to GO has led to improved GO-based catalysis and composite materials with more robust mechanical properties. GO in combination with zeolite NaY dehydrated a variety of alcohols to their corresponding olefinic products under mild conditions. Although GO at 15 wt% loading was an active dehydration catalyst, it was found that the GO loading could be reduced by 50% when used cooperatively with NaY (7.5 wt%). Ultimately, it was determined that the acidity of GO was able to convert the NaY into an acidic form without the added step of ammonium exchange or high-temperature calcination. For the former application, polyethylene/thermally reduced graphite oxide (TRG) composites have been synthesized under mild conditions (1 atm ethylene and 40 °C) using (n-BuCp)₂ZrCl₂ activated with methylaluminoxane (MAO) as the polymerization catalyst. Although the thermal properties of the aforementioned composites were relatively unchanged when compared to polyethylene, significant enhancements in the mechanical properties were observed (e.g., up to 57% increase in the tensile strength and 170% increase the Young’s modulus for composites containing 5.2 wt% TRG). Donor-acceptor conjugated polymers, where the backbone consists of an alternating electron rich and electron poor π-conjugated repeat unit, have some of the highest reported power conversion efficiencies to date. Conventional synthetic methods for synthesizing donor-acceptor conjugated polymers have relied upon Pd-catalyzed Stille- or Suzuki-type polycondensations and typically afford ill-defined polymers with variable molecular weights and broad polydispersities (Đs). We were interested in synthesizing well-defined donor-acceptor conjugated polymers and have developed a method to polymerize a donor-acceptor monomer based on thiophene and 5,6-difluorobenzotriazole in a controlled fashion using Ni-catalyzed Kumada catalyst-transfer polycondensation (KCTP). By altering the initial catalyst-to-monomer ratio, the M [subscript n] of the resulting polymers (PFBTzHT) ranged from 6.2 kDa – 22.9 kDa, while maintaining narrow Đs (≤ 1.4). Moreover, under the optimized polymerization conditions, the M [subscript n] was found to increase linearly with monomer conversion. Ultimately, the quasi-living nature of the polymerization allowed for the synthesis of well-defined diblock copolymers of PFBTzHT and poly(3-hexylthiophene) (PFTBTz-b-P3HT).