Photoinduced Controlled Radical Polymerization Of Methacrylates In The Presence Of Various Photoredox Catalysts

It is still an important and challenging task to develop a universal controlled free radical polymerization at room temperature.Photopolymerization system, especially visible light induced polymerization,is one of the important tools to solve this problem.In this dissertation, we developed numbers of green, efficient catalytic system, in which we investigated the effects of photoredox catalyst, initiator,and monomer on the visible light induced polymerization behavior.Additionally, well-controlled radical polymerization of methacrylates in the presence of various photoredox catalysts were demonstrated. Follows are main results:1. Visible-light-induced controlled polymerization with fac-Ir(ppy)3 as a photoredox catalystFirstly, with fac-Ir(ppy)3 as photoredox catalyst and alkyl bromine as initiator, the homopolymerization of poly(ethylene glycol) methyl ether methacrylate (PEGMA) and 2-(dimethylamino)ethyl methacrylate (DMAE-MA) in anisole were run under irradiation of an LED lamp (4500μW·cm-2@420 nm). A well-controlled polymerization of PEGMA was demonstrated and the clean block copolymer of PMMA-b-PPEGMA was prepared with PDI less than 1.3; however, the DMAEMA polymerization was poorly controlled. With the PMMA as macromolecular initiator, the block copolymer PMMA-b-PDMAEMA could be prepared with PDI around 2.0.On this basis, visible light CRP of propargyl methacrylate (PgMA) was investigated using ethyl-a-bromophenylacetate (EBPA) as initiator and fac-Ir(ppy)3 as a photoredox catalyst. By controlling the polymerization time, a linear increase of Mn with monomer conversion and narrow polydispersity index was achieved. Specifically, with [PgMA]:[Initiator]:[fac-Ir(ppy)3]=210:1:0.0285, the products showed Mn=8300 g·mol-1 and PDI=1.24 at 17%monomer conversion and Mn=17100 g·mol-1 and PDI=1.52 at 50% monomer conversion. When the ratio of MMA/PgMA was 5:1, the copolymerization product showed Mn=25300 g·mol-1 and PDI=1.75 at 66% monomer conversion.The FTIR, 1H-NMR and 13C-NMR spectra of the PPgMA showed no sp2-sp2 carbon-carbon bond, indicating that acetenyl groups are intact. In conclusion,this technology which was much better than that of thermally activated Cu-ATRP of PgMA offers a versatile route for the preparation of (co)polymers with pendant alkynyl groups.As an extension of our preliminary work, a new strategy was developed to prepare both α,ω-dithiol and α,ω-divinyl linear telechelic polythiolether oligomers by visible light induced thiol-ene chemistry in the presence of a fac-Ir(ppy)3 photoredox catalyst. Polythiolether oligomers of well-defined end groups and controlled molecular weights had been successfully synthesized at varying monomer molar ratios of 1,4-benzenedimethanethiol (BDMT) to diethylene glycol divinyl ether (DEGVE). The successful polyaddition of polythiolethers bearing a,co-dithiol and a,co-divinyl groups demonstrated that as-prepared polythiolethers possessed high end-group fidelity. For example,with the α,ω-dithiol- (Mn=1900 g·mol-1; PDI=1.25) and a,co-divinyl-terminated(Mn=2000 g·mol-1; PDI=1.29) polythiolethers as macromonomers, the molecular weight of resulting polythiolether was up to 7700 g·mol-1 with PDI as 1.67. The reactivity of the terminal thiol group is further confirmed by the addition reaction with N-(1-pyrenyl)maleimide.2. Visible-light-induced controlled polymerization with Cu(dap)2Cl as a photoredox catalystIt cannot be ignored that the ruthenium and iridium-based photocatalysts are expensive and scarce in nature. Therefore, there is a continuous effort to introduce base metal photoredox catalyst. With Cu(dap)2Cl/N,N-dimethyl-aniline (DMA) or Cu(dap)2Cl/tris[2-(dimethylamino)ethyl] amine (Me6TREN)as photoredox catalyst, EBPA as initiator, the optimization for the controlled radical photopolymerization had been achieved. With optimized amounts of DMA and Me6TREN, well-controlled polymerizations of PEGMA and MMA had been demonstrated, respectively. Specifically, with [PEGMA]:[EBPA]: [Cu(dap)2Cl]: [Me6TREN]=31:1:0.015 :(0.15-0.45), PPEGMA with PDT as low as 1.15 were obtained. To further verify the living nature of this system, block copolymers of PPEGMA-b-PMMA with high molecular weights and narrow molecular weight distributions (Mn,GPc=59200 g·mol-1,PDI=1.28 and Mn,GPc=93700 g·mol-1, PDI=1.44, respectively) were prepared using PPEGMA-Br (Mn,GPC=11600 g·mol-1; PDI=1.13) as a macroinitiator.3. Photoinduced controlled radical polymerization of methacrylates with benzaldehyde derivatives as organic catalystsThe use of metal catalysts not only increases the cost but also involves the risk of toxicity. Therefore, there is a tremendous interest in utilizing organic photocatalysts in controlled radical polymerization system for the synthesis of polymers. As a result, photo-induced controlled radical polymerization of methacrylates with perfluoro-1-iodohexane as an initiator and benzaldehyde derivatives, including p-anisaldehyde, p-cyanobenzalde-hyde and 2,4-dimethoxy benzaldehyde, as organic photocatalysts were demonstrated by using 23 W compact fluorescent lamps as the light source in the presence of a potential reductant DMA. Good linear evolution of molecular weight with monomer conversion was observed under optimized condition. Additionally, successful chain extension reactions were obtained by both one-pot process and with the as-prepared polymers as macroiniators.Specifically, one-pot synthesis of PPEGMA (conversion=85%) with a high molecular weight and narrow molecular weight distribution (Mn,GPC=44000 g·mol-1,PDI=1.54) was obtained by using PPEGMA-I (Mn,GPC=10200 g·mol-1,PDI=1.33) as a macroinitiator and p-anisaldehyde as an organocatalyst.Although the current technology exhibited a somewhat low controllability for preparing block copolymers, compared with the common ATRP and RAFT polymerization process, it offered a promising alternative as a metal-free organo-catalyzed photo-induced variant of ATRP.