| LED photopolymerization technology has attracted much attention due to the advantages of LED lamp,such as low energy consumption,no ozone generated during curing process,instantaneous switching on(off),low calorific value and long service life.However,the emission wavelengths of commonly used LED light sources having narrow-band emission are more than 360 nm.As a result,most commercial photoinitiators cannot be used in LED photopolymerization currently,which seriously limits the application of this technology.Therefore,the development of photoinitiators used for LED photopolymerization has become a research hotspot.At present,the research and development of photoinitiators used for LED photopolymerization are mostly focused on multi-component photoinitiating systems,and only a few studies involve cleavable photoinitiators.The one-component cleavable photoinitiators have the advantages of convenient application and fast initiation rate.Therefore,the development of new cleavable photoinitiators used for LED photopolymerization is urgently needed in the field of photopolymerization.Naphthalimide has good absorption at the emission wavelength of commonly used LED light source,while the C-S bonds in phenylthiobenzophenone can be broken under the irradiation of mercury lamp,resulting in the generation of free radicals which initiate photopolymerization,and the thiyl radicals have high initiation efficiency.Based on this,this paper aims to design and develop a series of novel cleavable naphthalimide derivatives containing sulfur ether bonds through combining the characteristics of naphthalimide and C-S bond for LED photopolymerization.The influences of the structures of photoinitiators on their photochemical property,photolysis and initiation mechanism,and the ability to initiate photoplymerization were discussed in depth.The intrinsic relationship between structures and properties was obtained in order to provide some valuable inspiration for the design of photoinitiators and enrich the theoretical basis of photopolymerization technology.The research has important theoretical significance and application value,and the main research contents and conclusions are as follows:(1)Six naphthalimide aryl sulfide derivatives photoinitiators(NASs)were designed and synthesized.Their light absorption properties,photolysis mechanism,sensitization mechanism to cationic photoinitiators,the ability to initiate photoplymerization and application properties were studied.The results show that NASs have excellent absorption properties at the emission wavelengths of LED light sources.The introduction of strong electron-withdrawing nitro group into benzene ring connected with sulfur atom reduced the molar extinction coefficient of the photoinitiator.Under the irradiation of 405 nm LED light source,the two C-S bonds in NASs broke to generate free radicals thereby initiating the polymerization of acrylate monomers.The electron-donating groups are beneficial to improve the initiation activity of the photoinitiators.In addition,NASs can sensitize cationic photoinitiators to initiate cationic polymerization of the epoxy monomer and hybrid system under the irradiation of 405 nm LED light source.More importantly,NASs have high stability in monomers under sunlight,which makes the preparation,storage,and application of photocurable formulations convenient because avoiding sunlight is no longer necessary.(2)In order to further improve the initiation rate of NASs,three naphthalimide aryl benzyl sulfide derivatives(NABSs)were designed and synthesized by introducing benzyl sulfur bond which is weaker than aryl sulfur bond.Their light absorption properties,photolysis mechanism,sensitization mechanism to cationic photoinitiators,the ability to initiate photoplymerization and application properties were researched.The results indicate that NABSs have excellent absorption properties at emission wavelengths of LED light sources,and their molar extinction coefficients are greater than 104 M-1 cm-1.The structure of sulfur-substituted naphthalimide is the main factor affecting the absorption properties of these molecules.The weak benzyl sulfur bond in NABSs mainly broke to produce free radicals thereby initiating acrylate monomers polymerization under 405 nm irradiation.The introduction of aryl benzyl sulfide structure increased the fluorescence quantum yields(?f)of the molecules,resulting in the decrease of the initiation efficiency of the molecules.However,when the strong electron-donating methoxy group was introduced into the molecules at the same time,the easiness to break C-S bond was further enhanced thereby reducing the influence and improving the initiation efficiency of the molecules.In addition,NABSs can sensitize cationic photoinitiators to initiate cationic polymerization and hybrid system polymerization under the irradiation of 405 nm LED light source.More importantly,the polymers prepared with NABSs photoinitiator have photoluminescent properties,which may provide a simple,rapid and environmentally friendly new method for preparing photoluminescent polymers.(3)Based on the oxygen inhibition effect for the photopolymeriztion systems initiated by naphthalimide aryl sulfide and aryl benzyl sulfide derivatives photoinitiators,six naphthalimide aryl sulfide and aryl benzyl sulfide derivatives with hydrogen donor structure(N-1?N-6)were designed and synthesized.Their light absorption properties,photolysis mechanism,the mechanism of intermolecular hydrogen abstraction reaction,the sensitization mechanism to cationic photoinitiators and the ability to initiate photopolymerization were explored.Strong electron-donating group dimethylamine group enhanced the light absorption of photoinitiators,but caused that the C-S bond was difficult to break.By studying the relationship between structure and properties of N-1?N-6,it is found that the degree of difficulty in C-S bond breaking of photoinitiators may be related to the variation of electron cloud distribution of sulfur atom from HOMO to LUMO.The N-5 and N-6 had greater initiating activity for free radical polymerization of acrylate monomers than N-1?N-4.Moreover,N-5 can undergo intermolecular absorb hydrogen,thereby mitigating the oxygen inhibition to a certain extent.In addition,N-2,N-4 and N-5 can sensitize cationic photoinitiators to initiate cationic polymerization of epoxy monomers. |
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