Cyclic Acetals As Initiator Systems For Photopolymerization

Photoinitiator (PI) is an important component of photocuring systems. The photoinitiator system commercially used in UV-curing material and dental restorative composite are benzophenone/amine and camphorquinone/aromatic amine, amine co-initiator is necessary component in the photoinitiator system. However, the use of amine co-initiators often leads to yellowing, and that amine is toxic and mutagenic. Although the emergence of polymeric amine and polymerizable amine has improved the biocompatibility, seeking a more biocompatibile coinitiator is still a great challenge. Cyclic acetals with abstractable hydrogen atom had been used as coinitiator for bimolecular photoinitiating system, as a substitute for the conventional amine co-initiator. In this thesis, a series of cyclic acetals were investigated in regard to their abilities to function as photoinitiator or co-initiators, the main contents and conclusions are described as follows.A natural component, sesamin (SA) was used with the aim of replacing the conventional amine for dental composite. Camphorquinone (CQ)/SA were used to initiate the photopolymerization of 2-2-Bis [4-(2-hydroxy-3-methacryloxyprop-1-oxy) phenyl] propane (Bis-GMA)/triethylene glycol dimethacrylate (TEGDMA) (70/30 wt%). The kinetics was recorded by real-time Fourier transformation infrared spectroscopy (FTIR). The mechanical properties were measured by the dynamic mechanical analyzer (DMA), cell toxicity was investigated by MTT assay and CQ/ethyl 4-N, N-dimethylaminobenzoate (EDMAB) mixture was used as control in the same photocuring condition. The results indicated that the addition of SA as co-initiator greatly improved the rate of polymerization (Rp) and final double bond conversion (DC) when compared with the system initiated by CQ alone. Comparing with EDMAB, the final DC of CQ/SA system (71%) was slightly lower than that of CQ/EDMAB (76%); SA brought approximately the same storage modulus around 37℃, but slightly higher glass transition temperature (Tg). SA produced lower yellowing effect and good in vitro biocompatibility. The water sorption and solubility for two mixtures were very close and within the range of the standards of ISO 4049. These results suggested that SA is an effective alternative to conventional amine for a co-initiator. And the natural compound characteristics of SA made it more promising than amine in the dental resin formulations.Sesamin is a nature compound from sesame seed, which contains two cyclic acetal structures, as a co-initiator to replace the traditional amine co-initiator to reduce the toxicity and yellowing. In this research, sesamin as a co-initiator was used UV polymerization initiator system, HDDA has been used for the photopolymerization monomer, photopolymerization kinetics was recorded by real-time infrared, and investigated the effect of sesamin concentration, light intensity and kinds of monomers on the photopolymerization kinetics.N,N-di(methacryly-ethoxycarbonyl-ethyl)-N-(1,3-benzodioxole-5-methylene)(DMEBM) was synthesized and used to replace both triethylene glycol dimethylacrylate(TEGDMA) as a dilute and the non-polymerizable amine which is added as a co-initiator in dental resin mixtures.2,2-Bis[4-(2-hydroxy-3-methacryloxypropoxy) phenyl]-propane (Bis-GMA) and camphorquinone(CQ) were used as monomer and photoinitiator in these model dental resin systems, in comparison, ethyl 4-dimethylaminobenzoate (EDMAB) was used as co-initiator. DMEBM was synthesized via Michael-Addition reaction and characterized by using 1H NMR spectroscopy. Mixture of Bis-GMA/DMEBM/CQ was found to reach the double bond conversion of 67.5%, slightly higher than that of Bis-GMA/TEGDMA /CQ/EDMAB (66.8%) and Bis-GMA/TEGDMA/CQ/DMEBM (64.8%). In addition, the glass transition temperature of Bis-GMA/TEGDMA/CQ/EDMAB (93.4℃) were higher than that of Bis-GMA/TEGDMA/CQ/DMEBM (89.3℃) and Bis-GMA/DMEBM/CQ (80.4℃). The water sorption and solubility of Bis-GMA/TEGDMA/CQ/DMEBM were higher than that of Bis-GMA/TEGDMA /CQ/EDMAB, and Bis-GMA/DMEBM/CQ. However, the values were still within the range of the ISO 4049 standards. DMEBM could be used as a potential co-initiator and diluent for dental composite.A series of cyclic acetals was synthesized and investigated in regard to their abilities to function as photoinitiator or co-initiators in free radical photopolymerization induced by benzophenoe (BP), isopropylthioxanthone (ITX) and camphorquinone (CQ). On the UV irradiation, cyclic acetals could initiate polymerization of monomer, but the relative photopolymerization efficiencies were very low. When BP was added as photoinitiator, Polymerization occurred with the higher rate with 1,3-benzodioxolane (BDO) as co-initiator, cyclic acetals were inefficient co-initiator for isopropylthioxanthone (ITX). It was found that the polymerization efficiency of the co-initiators did not follow the efficiency of photoinduced formation of the initiating radicals. The reasons for this lack of correlation were discussed in terms of how the reactivity of the radicals formed affected the various stages of the polymerization. Further studies with a cleavage type photoinitiator in the presence of the cyclic acetals provided supporting evidence for the role of the chain-transfer reaction in the polymerization of the CQ/cyclic acetals systems.5-Benzoyl-1,3-benzodioxolane (BBDO), a hydrogen abstraction type photoinitiator for free radical polymerization, was synthesized and characterized. Its capability to act as an initiator for the polymerization of methyl methacrylate (MMA), styrene (St) and multifunctional monomers in the presence of air was examined. Relative photopolymerization efficiencies of BBDO, benzophenone (BP), and their combination with an amine synergist such as ethyl-4-dimethylaminobenzoate (EDAB) are compared. UV-Vis, FT-IR,13C NMR and polymerization studies revealed that the postulated mechanism is based on the intermolecular reaction. In contrast to BP-based photoinitiators, BBDO does not require an additional hydrogen donor for initiation, it can reduce the use of amine co-initiator.Benzophenone-1,3-dioxane (BP-DO), a novel photoinitiator for free radical polymerization, is synthesized and characterized. Its capability to act as an initiator for the polymerization of methyl methacrylate (MMA), styrene (St) and multifunctional monomers is examined. The relative photopolymerization efficiencies of BP-DO, benzophenone (BP), and BP/ethyl-4-dimethylaminobenzoate (EDAB) are compared. The amounts of leachable photoinitiator are determined through the extraction experiments. In contrast to BP-based photoinitiators, BP-DO does not require an additional hydrogen donor for the initiation; it could be a substitute for the conventional initiator systems BP and BP/EDAB, and eliminate the use of amine co-initiator.Benzophenone-di-1,3-Dioxane (BP-DDO), a novel photoinitiator for free radical polymerization, was synthesized and characterized. UV-vis absorption spectroscopy was used to investigate its photochemical behavior during the photophysical process. When this photointiator was used to efficiently initiate polymerization of acrylates and methacrylates, there was an optimum cure rate with the increase in BP-DDO concentration. Both the polymerization rate and final conversion increased with the increase in light intensity. The kinetics study of photopolymerization of TMPTA showed that BP-DDO was a more effective photoinitiator than benzophenone and benzophenone/ ethyl-4-dimethylaminobenzoate (EDAB).But the photopolymerization efficiency is the same with BP-DO.