Design,Synthesis And Performance Study Of Naphthalimide Sulfide Photoinitiators Containing Hydrogen Donor

LED light curing technology has gradually become the development direction of light curing technology because of its advantages such as high energy conversion and utilization rate,no ozone generation and long service life.Therefore,the research of photoinitiators that are suitable for LED photopolymerization has also become a hot point in recent years.At present,the development of these photoinitiators is focused on the multi-component Norrish Ⅱ type photoinitiator systems.However,there are still problems of small molecule volatilization and migration in their application process,which leads to the environmental pollution and health hazard.The one-component Norrish Ⅰ photoinitiators have advantages of simple composition,convenience and fast initiation rate Therefore,the development of one-component Norrish Ⅰ photoinitiators for LED light curing has important theoretical significance and application value.Our previous work has confirmed that the one-component Norrish Ⅰ photoinitiators based on naphthalimide aryl benzyl sulfides have good initiating ability under 405 nm LED irradiation.However,they have serious disadvantage of oxygen inhibition.Based on this,this work introduced some hydrogen donor structures into naphthalimide-based aryl benzyl sulfide photoinitiator molecules in order that the photoinitiators can simultaneously undergo the cleavage reaction and hydrogen abstraction reaction under the illumination of the LED light source to reduce the influence of oxygen inhibition and improve their ability of initiating photopolymerization under aerobic conditions.In this paper,four kinds of hydrogen donor-containing naphthalimide-based aryl benzyl sulfide photoinitiators were synthesized.Their photophysical properties were studied by UV-vis spectroscopy and transient fluorescence spectroscopy.Their photolysis mechanism,oxygen inhibition mechanism and sensitization mechanism to cationic photopolymerization were thoroughly explored through steady-state photodegradation test,nuclear magnetic resonance spectroscopy,electron paramagnetic spectroscopy,C-S bond energy theory calculation and Gibbs free energy calculation.We investigated their photopolymerization kinetics in acrylate monomers and epoxy resin monomers using real-time infrared spectroscopy.And obtained the internal relationship between the structure and performance of the photoinitiators.Moreover,we have carried out the preliminary exploration of the synthesis method of waterborne hydrogen donor-containing naphthalimide-based aryl benzyl sulfide photoinitiators to provide theoretical basis and experimental data for the development of waterborne photoinitiators for LED photopolymerization.These researches have important theoretical significance and application value.The main research contents and conclusions are as follows:(1)The as-synthesized ether chain hydrogen donors-containing naphthalimide-based aryl benzyl sulfide photoinitiators NABS-HD-1 and NABS-HD-2 had nice absorption properties at the emission wavelength of LED light sources commonly used in the industry.And their molar extinction coefficients were basically higher than 1×104 M-1 cm-1.They were excited to the excited singlet state from the ground state under LED illumination,and then most of them generated aryl sulfide,methoxybenzyl and alkyl free radicals by direct cleavage of C-S bond(position 2)of benzyl sulfide group and hydrogen abstraction reaction,while a small number of photoinitiator molecules generated aryl and p-methoxybenzyl sulfide free radicals with initiating ability through the cleavage of the C-S bond(position 1)of aryl sulfide group to initiate photopolymerization.They can also produce cationic active species by the electron transfer reaction between they and diphenyl iodine hexafluorophosphate(Iod)under excited singlet state to initiate cationic polymerization of epoxy monomers.These two photoinitiators were able to effectively initiate the free radical polymerization of acrylate monomers and greatly reduce the oxygen inhibition.Among them,NAB S-HD-2 exhibited a more obvious effect of reducing oxygen inhibition.In addition,NABS-HD-2 with carbon-carbon double bonds also had good migration stability,which leads to a great potential application in food packaging industry.(2)We explored influences of the feeding amount of palladium catalyst as well as the addition amount of the solvent and its type on yields of waterborne hydrogen donor-containing naphthalimide-based aryl benzyl sulfide photoinitiators NABS-HD-OH and NABS-HD-COOH,and obtained their optimum synthesis process.The optimum synthesis condition of NABS-HD-OH is as follows.The molar ratio of compoundⅡ/Ⅰ was 1:1;the addition amount of bis(dibenzylideneacetone)palladium Pd(dba)2 was 0.6 mmol;the feeding amount of 1,1’-bis(diphenylphosphino)ferrocene(DPPF)was 0.3 mmol;the addition amount of K3PO4 was 4.8 mmol;8 mL of toluene and 4 mL of acetone were mixed.The synthesis reaction was carried at 110 ℃ for 12 h under nitrogen protection.For the photoinitiator NABS-HD-COOH,the polar dimethyl sulfoxide(DMSO)was selected as the solvent in its synthesis reaction.The synthesis reaction was carried at 140℃ for 12 h under nitrogen protection at the above feeding raw materials ratio to obtain highest yield.(3)The photopolymerization kinetics study showed that NABS-HD-OH and NABS-HD-COOH cannot effectively initiate photopolymerization of hexamethylene diacrylate(HDDA)under 405 nm LED light source because of the formation of intramolecular hydrogen bonds.In addition,compared with NABS-HD-1 and NABS-HD-2,the introduction of water-soluble groups made the water solubility of NABS-HD-OH and NABS-HD-COOH be improved slightly,but their water solubility was poor than that of photoinitiator 2959 due to the fact that the ratio of water-soluble groups in their molecules was low.