Synthesis And Properties Of Free Radical Photoinitiators Containing Aromatic Heterocycles

Light curing is a new green technology.It has been widely used in coatings,3D printing,nanotechnology,medical filling materials and other technical fields because of its unique advantages of high polymerization efficiency,energy saving and environmental protection.Photoinitiator is an essential part of the light curing system,which plays a decisive role in the speed and degree of curing,and affects the properties of products.The development of light curing technology depends on the development of photoinitiator to a certain extent.In this paper,benzophenones and sulfur compounds were designed and synthesized as photoinitiators containing aromatic heterocycles.（1）The first part:synthesis and application of benzophenones.A series of aromatic heterocyclic benzophenones were synthesized from benzophenones containing hydroxyl group and acyl chlorides with different substituents.Firstly,the target compounds were characterized by ¹H NMR,¹³C NMR and HRMS,and the optimal reaction conditions were explored:at room temperature,the molar ratio of benzophenone derivatives and acyl chlorides was 1:1.2,the solvent was anhydrous CH₂Cl₂,the base was Et₃N（1 eq）,the highest yield could reach 94%after 1.0 h.Subsequently,the UV absorption of 3 was determined and 3 were used as a photoinitiator for UV curing experiment.It was found that compared with benzophenone（BP）,the maximum absorption wavelength of 3 was red shifted,the absorption range was broadened and the capacity was enhanced.3c,3e,3f had high initiation efficiency,excellent properties and commercial potential.（2）The second part:synthesis and application of thioethers and thioesters.Thioethers or thioesters with heteroatomic aromatic structure were synthesized by the reaction between mercaptans and2-bromoacetophenone or p-toluoyl chloride.The target compounds were characterized and the optimum conditions were determined:at room temperature,the molar ratio of mercaptans and 2-bromoacetophenone was 1:1,the solvent was a mixture of ethanol and water（the volume ratio of ethanol and water was 1:1）,the base was K₂CO₃（1.5 eq）,the reaction was completed after 1.5 h.The reaction conditions were mild and the solvent was environmentally friendly.Subsequently,the UV absorption of 6、7 was determined and 6、7 were used as a photoinitiator for UV curing experiment.The results showed that the molar extinction coefficient of 6 and 7 was improved compared with BP;The absorption range of 6a-6e was widened,6e had an absorption peak near 330 nm and the range could be extended to360 nm above;The maximum absorption wavelength of 7a-7c was slightly red shifted.6 could initiate polymerization;6a curing time was only 10 s,the curing film was smooth and transparent,the hardness could reach 5H,the gel content was high and the migration was low.