Synthesis Of Polymerizable Disulfide Compounds And Their Regulation Of Volume Shrinkage And Performance Of LED Photopolymerization

Light emitting diode(LED)photopolymerization technology has the advantages of long service life,low energy loss,narrow emission wavelength range and low heat,which results in the fact that it is gradually replacing the traditional ultraviolet polymerization technology using mercury lamps as light sources.However,the free radical LED photopolymerization system inevitably has a problem of serious volume shrinkage because of its chain polymerization mechanism,which will greatly limit its wide application in the fields of high-performance and high-precision materials.Therefore,how to reduce the volume shrinkage is also an urgent problem facing to LED photopolymerization technology.A variety of methods for reducing volume shrinkage have been reported,but these methods still have some shortcomings,such as the increasing viscosity of the system,difficult coating,scarce material sources,high cost and complicated process.Our research group recently has proposed a simple and feasible new approach to reduce polymerization volume shrinkage by utilizing the dynamic reversible properties of "breakrecovery" of disulfide bonds under UV light irradiation.However,the disulfide compounds designed and synthesized in the previous work cannot be used in LED light curing systems due to their shorter maximum light absorption wavelengths.By contrast,aromatic disulfides have the potential to be used in LED photopolymerization systems because of the conjugation effect of aromatic rings that is beneficial to the red shift of molecular UV absorption wavelength and the reduction of S-S bond energy.In addition,we found that the aromatic disulfide itself has a certain photoinitiating ability,which means it has the possibility to be used as a LED photoinitiator at the same time.Based on the above background,this project aims to synthesize a series of polymerizable aromatic disulfides through molecular design.These disulfide compounds can not only reduce the volume shrinkage of the LED photopolymerization system to a certain extent,but also well initiate the photopolymerization of acrylate monomers,which will greatly simplify the formulation and reduce the harm to the environment and human body caused by the addition of commercial initiators.Moreover,the structure-activity relationship of polymerizable aromatic disulfides with their photochemical properties and photoinitiative properties,as well as the effects of different structures of aromatic disulfides on volume shrinkage,heat resistance,frictional resistance,and hardness were investigated.The intrinsic relationship between structure and performance has been obtained.It is expected to provide an important theoretical basis and technical support for the application of the aromatic disulfides in LED photopolymerization.The development of this subject has important theoretical significance and potential application value for the development of LED photopolymerization technology and the expansion of application scope of disulfide compounds.The main research contents and conclusions of this project are as follows:(1)Four polymerizable dithiosalicylic acid derivatives,AEBS,MAEBS,APBS and MAPBS,were synthesized by two-step esterification based on dithiosalicylic acid.The photochemical properties,photodegradation mechanism and initiating ability of the dithiosalicylic acid derivatives were investigated.The results show that these derivatives have a certain absorption at 365 nm,and the disulfide bond can be broken under the irradiation of 365 nm LED to generate aryl sulfide radicals.The arylthio radicals can well initiate the photopolymerization of the Bisphenol A glycerolate dimethacrylate(Bis-GMA)Triethylene glycol dimethacrylate(TEGDMA)system being used for medical dental materials.After 365 nm LED irradiation for 300 s,the maximum conversion of double bonds reaches 64.9%.At the same time,we studied the volume shrinkage,heat resistance,hardness and friction resistance of the cured films prepared by MAPBS as a photoinitiator.The results show that with the increase of MAPBS content,the thermal stability of the cured film is slightly reduced,the hardness is slightly improved,and the ability to withstand friction is reduced.MAPBS can effectively reduce the volume shrinkage of the cured film to 5.07% that is lower than that(8.10%)of the cured film prepared by the commercial photoinitiator.(2)Four polymerizable dithioaniline derivatives,OABS,OMABS,PABS and PMABS,were designed and synthesized based on dithioaniline.The photochemical properties,photodegradation mechanism and initiating ability of four polymerizable dithioaniline derivatives were also explored.The results show that the UV-Vis absorption wavelengths of these derivatives reach above 405 nm and even have weak absorption at around455 nm,and their disulfide bond is broken under 385 nm LED irradiation to generate aryl sulfide radicals.The arylthio radicals can well initiate the photopolymerization of the Bis-GMA/TEGDMA system.After 385 nm LED illumination for 200 s,the maximum double bond conversion of the system reaches 83.2%.The self-polymerization of OMABS well is carried out under the irradiation of 385 nm light source,and the maximum conversion reaches 62.9%.The Raman Mapping test results of the selfpolymerizing film of OMABS show that some the aryl sulfide radicals generated by the cleavage of the S-S bond of OMABS under the irradiation are involved in the initiation,and the others are restored to the disulfide bond.Meanwhile,we investigated the volume shrinkage,heat resistance,hardness and friction resistance properties of the cured films prepared with OMABS as an initiator.The results show that with the increase of OMABS content,the thermal stability of the cured film is slightly reduced,the hardness is slightly improved,and the friction resistance is reduced,but the overall performances are better than those of the polymerizable dithiosalicylic acid derivative systems.OMABS can effectively reduce the volume shrinkage to 4.89%.(3)Six disulfides containing Diphenyl disulfide(PDS),p-Tolyl disulfide(MPDS),2,2’-Difluorodiphenyl disulfide(FPDS),5,5’-Dithio bis-(2-nitrobenzoic acid)(DTNB),2,2’-Dithiodipyridine(PDDS)and 2,2’-Dithienyl Disulfide(TPDS),respectively,were selected for researching the effects of electron-donating groups,electron withdrawing group,electronrich five-membered aromatic heterocycles and electron-deficient sixmembered aromatic heterocycles on photochemical properties,photodegradation mechanism and initiation ability of aromatic disulfides.The maximum absorption wavelengths of these aromatic disulfides are all less than 300 nm.The introduction of the electron-donating group methyl group and aromatic heterocycle cause red-shift of the maximum absorption wavelengths and increase of their molar extinction coefficients.Electron groups such as fluorine atom and nitro group lead to blue shift of maximum absorption wavelength.Under the irradiation of 385 nm and 405 nm LED,PDS,MPDS,FPDS,DTNB and PDDS can break down the disulfide bond to produce aryl sulfur radical and effectively initiate the photopolymerization of tripropylene glycol diacrylate(TPGDA)and trimethylol propane triacrylate(TMPTA).TPDS could not produce stable aryl sulfur radicals,so it is incapable of initiating the polymerization of TPGDA and TMPTA.