Synthesis And Properties Research Of α-Dicarbonyl-Based Photoinitiators For LED Photolymerization

As the light source used for traditional photopolymerization technology,high-pressure mercury lamps have some disadvantages such as high energy consumption,slow startup,large heat generation and environmental pollution.Moreover,the leakage of mercury can cause great harm to the human body and the environment.As the international community attaches great importance to environmental protection,many countries restrict or prohibit the production and sale of mercury products in the form of legislation.Therefore,it is the general trend that mercury light sources are replaced by more energy-saving and environmentallyfriendly LED light sources.Commercial UV photoinitiators are difficult to match with LED light sources and cannot be used in the field of LED photopolymerization.Therefore,the development of photoinitiators suitable for LED photopolymerization is an urgent problem to be solved.The reported photoinitiators for LED light sources solve the problem of matching between photoinitiators and LED light sources to a certain extent,but in order to have strong absorption at the emission wavelength of the LED light source,most of these photoinitiators introduce a large conjugated structure into their molecules,which leads to some shortcomings such as poor compatibility between photoinitiators and photocurable resins,the dark color of the polymerization system and not being able to be used for deep layer photopolymerization.At present,the main way to solve these problems is to endow photoinitiators with photobleaching capability.However,currently photoinitiators with photobleaching capability reported still has the deficiencies of slow photobleaching speed,incomplete photobleaching and high synthesis cost.Therefore,it has important practical significance to design novel photoinitiators with simple structures and preparation procedures,long absorption wavelengths,and an excellent photobleaching ability and biocompatibility for LED photopolymerization.In addition,the water-based photopolymerization technology using water as the diluent has developed rapidly in recent years due to its advantages of low toxicity,low odor and low cost.At present,most of the water-soluble photoinitiators were prepared by modifying oil-based photoinitiator,such as the introduction of ether chains,carboxyl groups,hydroxyl groups,carboxylate and quaternary ammonium salts.Although the introduction of these groups can increase the water solubility of the photoinitiators,the remodification of the photoinitiators will undoubtedly result in increasing the difficulty and cost of synthesis,which seriously limits the practical application of aqueous photoinitiators.Based on the background,this subject has designed and synthesized a series of ethyl cinnamoyl formate photoinitiators(ECFs)with excellent photobleaching properties and biocompatibility,and ethyl cinnamoyl formate photoinitiators containing hydrogen donors(ECFNs).The photoinitiating properties,photo-bleaching properties,photo-initiating and photo-bleaching mechanisms of these photoinitiators under visible LED were investigated.The relationship between the structures and the initiation performances of the photoinitiators was elucidated by theoretical calculation.Based on the high initiating activity of the molecules with biscarbonyl structure,we have selected three α-carbonyl ketone food additives(DEs)that possess good water solubility and have been widely used in the fields of food and medicine,as water-oil amphiphilic LED photoinitiators.The water solubility,the ability to initiate deep-layer photopolymerization,and cytocompatibility of DEs were investigated.The main research contents and conclusions are as follows:(1)The synthesized ECFs have strong absorption in the visible region,and their maximum absorption wavelengths are in the range of 300-360 nm.ECFs are Norrish II photoinitiators.Under visible LED irradiation,in the presence of ethyl 4-dimethylaminobenzoate(EDB),ECFs undergo an intermolecular hydrogen abstraction reaction to produce the amine alkyl radicals,which can initiate the polymerization of acrylate monomers.Theoretical calculations show that the introduction of electron-donating groups on the benzene ring can effectively reduce the HOMO-LUMO transition energy of ECFs molecules and increase their molar extinction coefficients at 405 nm and 455 nm,thereby improving their initiation performance.ECFs can effectively initiate the polymerization of acrylate monomers under the irradiation of both 405 nm and 455 nm LED.The initiation performances of both O-ECF and S-ECF were better than that of commercial photoinitiator 2-isopropylthioxanthone(ITX).ECFs have good capability of photobleaching and initiating deep curing.The photobleaching of ECFs is mainly caused by the destruction of the conjugated structure through the polymerization of carbon-carbon double bonds.The S-ECF/EDB system can initiate the deep-layer polymerization of tri-(propylene glycol)diacrylate(TPGDA)under the irradiation of 455 nm,and the polymerization depth reached 7 cm after 20 min of irradiation.S-ECF has excellent cytocompatibility because the carbon-carbon double bonds of S-ECF molecule can be polymerized into macromolecules or linked to the polymer backbone.(2)The maximum absorption wavelengths of ECFNs with hydrogen donor are concentrated at around 260-290 nm.The theoretical calculations show that the HOMO-LUMO electron cloud distribution of ECFNs molecules has a characteristic of typical D-A structure.Compared with those of ECFs molecules,the HOMO-LUMO orbital transition energies of ECFNs molecules with D-A structure are significantly reduced,and the absorptions of ECFNs in the visible region are enhanced.Owing to the introduction of hydrogen donors,ECFNs can generate amine alkyl radicals that effectively initiate photopolymerization of acrylate monomers under the irradiation of either 405 nm or 455 nm LED without any added tertiary amines as hydrogen donors,which simplifies the formulation and avoids odor and toxicity caused by small molecular tertiary amines.Therefore,ECFNs has certain practical value.In addition,ECFNs also exhibit excellent photobleaching properties during photopolymerization and has potential applications in the field of colorless polymerization.(3)The maximum absorption wavelengths of water-oil amphiphilic photoinitiators DEs are concentrated in the range of 260-290 nm,and their molar extinction coefficients in the range of 400-455 nm are very weak.DEs have two different initiation mechanisms under LED irradiation.DEs undergo a cleavage mechanism in the absence of any hydrogen donors,while DEs undergo a hydrogen abstraction mechanism in the presence of a hydrogen donor.DEs molecules can effectively initiate the polymerization of acrylate monomers under the irradiation of both 405 nm and 455 nm LED.The initiating performances of DEs under 405 nm LED are similar to that of commercial photoinitiator ITX,but much better than that of ITX under 455 nm LED.The water solubility of DEs is better than that of commercial photoinitiator Irgacure 2959,and the water solubility of BDE reached 35 wt %.The final double bond conversion of aqueous monomer PEG(400)DA initiated by DEs were more than 80% under the irradiation of either 405 nm or 455 nm LED.DEs have an excellent ability to initiate deep-layer polymerization.The curing depth of TPGDA initiated by BDE/EDB reached 7 cm after 455 nm LED irradiation for 120 s,while that after 405 nm LED irradiation for 75 s also reached 7 cm and was much higher than that(1.5 cm)of TPGDA initiated by ITX/EDB.The polymer prepared by BDE has better cytocompatibility compared with that prepared by S-ECF.DEs have great application potential in the field of water-based LED photopolymerization.