Novel Photoarylation-based Controlled Polymerization of Conjugated Polymers and Its Application for Transparent Electrode

Abstract

In order to afford the production of conjugated polymers with uniform and desirable optical and electrical properties on a large scale, much effort have been devoted to light-induced polymerization. Various well-defined conducting polymers are potentially achievable by devising efficient photo-polymerization strategies. Solid-state photo-polymerization of conjugated conducting polymer films not only allows fast thin film fabrication but also is applicable to roll-to-roll process for cost-effective mass production. In spite of these and other invaluable benefits of photo-mediated polymerization, few research papers have reported outstanding optoelectronic values through the photopolymerization. In this dissertation, a new one-pot photo-mediated polymerization for conjugated polymers with a high molecular weight and narrow polydispersity was devised and its polymerization mechanism was mechanistically investigated. The overall polymerization is composed of two sequential photochemical reactions; 1) photo-arylation between ProDOT molecules and 2,5-diiodo-ProDOT derivatives by 365nm LED irradiation to afford conjugated oligomers, 2) chain extension of diiodinated ProDOT oligomers by addition of iodine source followed by q-switched 532nm pulsed laser to make large molecular weight conjugated polymers. The adoption of the pulsed laser having the longer wavelength not only facilitates photochemical couplings of the diiodinated oligomeric precursors by matching their absorption wavelength but also prevents the photodegradation of the extended chains by the pulsed illumination. All the photochemical reactions were manipulated by carbon-iodine bonds of growing chain ends that reversibly produce corresponding radical species in the presence of a hydrogen-free solvent, CBrCl3 (bromotrichloromethane), preventing radical consumption by hydrogen-containing solvent molecules. This polymerization process was also successfully employed in the copolymerization of ProDOT monomers with different side chains where the copolymer composition is determined by the feed ratio of the monomers. The characterization of the polymerized products was performed through UV-vis absorption, 1H-NMR, mass spectroscopy, and GPC to analyze the photo-mediated polymerization mechanism for conjugated polymers. The developed photoarylation-based polymerization for conjugated polymers in solution was adopted in devising solid-state photopolymerization of ProDOT and EDOT derivatives to fabricate conducting and transparent thin polymer films. The composition of the copolymerized thin films from the thiophene derivatives with different side chains was analyzed by XPS and FT-IR. The photochemical coupling between 2,5-diiodo ProDOT or EDOT and a series of heterocyclic molecules in solid states rendered solvent-resistant polymer thin films on glass substrates. A subsequent chemical doping of the polymer thin films with protonic acids provided electric conductivity and optical transparency. Great electrical conductivity (~2,200 S/cm) and high transparency (>80%) were achieved from ProDOT and EDOT derivatives by photo-arylation based UV polymerization. Experimental parameters affecting the polymerization efficiency such as reaction temperature, light source intensity, and side chains of precursors were systematically investigated to enhance the optical and electrical properties of the conducting polymer thin films. In the absence of solvent, mobility of the monomers and growing polymer chains turned out to be critically important to realize large molecular weight polymers and consequential high conductivity. The best optical and electrical properties of the resulting polymer thin films was achieved from the copolymers of 2,5-diiodo EDOT incorporating a diethylene glycol side chain (DIEDOT-DEG) and EDOT. While the flexible side chain of the former imparts good mobility to the growing polymer chains, the latter without having any side chain contributes to high electrical conductivity by allowing close molecular packing owing to the minimized insulating and steric volume inevitably imposed by side chains.