Investigation Of Living Radical Photopolymerization Using A Situ NMR Technology

Living/controlled radical polymerization has been widely used in the synthesis of functional polymers with different topologic structures, such as block polymers, graft polymers, hyperbranched polymers and star-like polymers, due to its advantages of both radical polymerization and living polymerization. It has drawn widespread attention and significant advances have been made recently. Room temperature living radical polymerization, especially room temperature living radical polymerization under UV irradiation, has an important significance on the research and manufacture. In this article, a new NMR equipment transformed by introduction of UV light into the magnet of NMR, was used to investigate the process of living radical polymerization under UV irridiation. The in situ NMR was developed to investigate the UV stability of sulfides under variety of conditions. According to the results,2-(dodecylthiocarbonothioylthio)-2-methylpropanoic acid (DDMAT) was chosen as the chain transfer agent, to investigate the influence of concentration of sulfides, light intensity, wavelength, and solvents on the polymerization of methyl acrylate (MA), including both at the initial stage and high monomer conversion of polymerization reaction. The thesis can be mainly divided into the following three parts:1. A new NMR equipment transformed by introduction of UV light into the magnet of NMR, was used to investigate the process of living radical polymerization under UV irradiation. The photopolymerization of methyl acrylate has been studied under UV irradiation at room temperature in the presence of DDMAT by in situ1H NMR spectroscopy. The results suggest that no polymers could be formed until the completely consumption of DDMAT. The effect of solvents on the polymerization behavior was studied via in suit NMR, and it is demonstrated that well-controlled polymerization of MA has been successfully achieved with chloroform or benzene as the solvent. However, the polymerization was uncontrolled in DMSO due to the formation of extra radicals under UV irradiation, and the polymerization could become controlled by decreasing light intensity to slow down the rate of generation of radicals. The sulfides reacted with monomers until the completely consumption of oxygen when a few oxygen was existed, and the polymerization could be well controlled with the molecular weights higher than theoretical values. After cutting off the UV irradiation, the polymerization continued to occur for at least ten hours in the presence of photoinitiator, however, it was completely stopped in2h when no photoinitiator was used. No induction period was observed when the polymerization was irradiated again before the completely termination of radicals.2. DDMAT,2-phenylpropan-2-yl benzodithioate (CDB),2-cyanopropan-2-yl benzodithioate (CPDB), and benzyl1H-imidazole-1-carbodithioate (BICDT) were synthesized and the influence of concentration of sulfides, light intensity, solvents, and the existence of monomer on their UV stability was investigated via in situ NMR. The decomposition mechanism and decomposition products were also investigated under UV irradiation condition. A few conclusions can be drawn from all this. First, the sulfides are more stable at high concentration; second, the higher the light intensity is, the rapider the sulfides decompose; third, benzene has less weak effect on the UV-induced decomposition of sulfides than DMSO, so it is more suitable to act as the solvent in sulfides-moderated living radical polymerization under UV condition; finally, CPDB is the most stable under UV irradiation condition among these sulfides, which can be used as a chain transfer agent in UV-initiated RAFT polymerization. These results provide guidance for the choice of RAFT agent used in the living radical polymerization under UV irradiation.3. Polymerization of methyl acrylate under UV irradiation in the presence of DDMAT was investigated by in situ1H NMR spectroscopy. Effects of light intensity, wavelength, and concentration of DDMAT on the polymerization behaviors were studied in detail. Compared with light intensity and wavelength, the concentration of DDMAT plays a more important role for the polymerization. The photopolymerization has the "living" features even at high monomer conversion with a high concentration of DDMAT. However, with a low concentration of DDMAT, the polymerization proceeded in an uncontrolled manner and produced polymers with high molecular weights and broad polydispersities. Photochemical behavior of DDMAT was studied in detail via in suit NMR and LC-MS, and the results showed that the photolysis of DDMAT was reversible at high concentration, whereas contrarily, DDMAT decomposed irreversibly at low concentration. A possible mechanism was proposed for the reversible photolysis of DDMAT at high concentration, which may involve both reversible termination and reversible addition-fragmentation chain transfer approaches.