Synthesis And Properties Of Sulfonium Salt Photoinitiators Based On Coumarin Skeleton

Photopolymerization technology has been widely used in digital storage,coatings,additive manufacturing and medical materials due to its advantages of high efficiency,environmentfriendly,low energy consumption and mild reaction conditions.Photoinitiator(PI)is the central component of photopolymerization,which is excited by light to produce active species(radicals,plasmonic acids,etc.)that can initiate photopolymerization.PI determines the wavelength of the excitation light source,the types and rate of photopolymerization,and affects the final performance of the polymeric materials.Compared with the traditional high-pressure mercury lamp,light-emitting diode(LED)light source has the advantages of narrow emission spectrum,long service life,low energy consumption,no ozone generation,etc.,and has become one of the most commonly used light sources in photopolymerization.Matching the absorption wavelength of the photoinitiator with the emission wavelength of the excitation light source is necessary for it to initiate photopolymerisation.The LED light source emits wavelengths larger than 360 nm,which is difficult for commercial cationic photoinitiators to match effectively.How to construct efficient single-component LED cationic photoinitiators through rational molecular design is one of the challenging research directions in the field of photopolymerization.In this thesis,a series of one-component sulfonium salt photoinitiators were designed and synthesized based on the coumarin skeleton.By introducing substituents with different electronic effects at each site of the chromophore coumarin and changing the substituents of the initiation center sulfur ion,the wavelength of absorption and the initiation activity of the photoinitiators are regulated,with a view to developing a series of singlecomponent coumarin sulfonium salt photoinitiators with high initiation activity,long-wave absorption and easy availability,which provides a new idea for the design and development of efficient,long-wavelength single-component cationic photoinitiators.The research of this thesis can be divided into three parts as follows:In the first part of this thesis,a series of sulfonium salt cationic photoinitiators containing different aliphatic chains(-Me,-Bu,-Hep)were synthesized by a simple 3-step reaction using coumarin as the chromophore and introducing acyl sulfonium salts as initiating groups at the 3-position of coumarin.The photinitiators have UV-Vis absorption between 300 nm and 420 nm and can be matched with 365 nm and 405 nm LED light sources.The aliphatic chain length of the synthesized photoinitiator has a great influence on the photoinitiation activity.With the increase of the aliphatic chain length,the photoinitiation activity will increase first and then decrease.Compared with the commercial 201 s,the butyl substituted sulfonium salt showed higher photoinitiation activity.The epoxy conversion of epoxy monomer was up to 69 %,and the double bond conversion of acrylate was up to 69 %.The thermal polymerization temperature of these sulfonium salts with epoxy monomer is higher than 95 ℃ and with acrylate is higher than 195 ℃,which shows good thermal stability.Based on the study of the first system,it was found that linking the sulfonium salt directly to the coumarin ring might affect the absorption wavelength and initiation activity of the initiator.Based on this,in the second part of this paper,the sulfonium salt was connected to the3-position of coumarin through structural design,and different push / pull electron groups were introduced into the aromatic ring of coumarin.Four coumarin aryl-alkyl sulfonium salt photoinitiators(H-PI,OMe-PI,OH-PI and Ph-PI)were synthesized by a simple two-step reaction.The relationship between the structure of the photoinitiator and its initiation performance was systematically studied.The main absorption wavelengths of these photoinitiators are in the range of 350～450 nm,which can be matched with LED light sources at 365 nm and 405 nm.Among them,phenyl substituted sulfonium salt(Ph-PI)has the highest initiation efficiency,and the epoxy conversion of epoxy monomer can reach 81%,which is much higher than that of commercial cationic photoinitiators(ITX / Iod and 201s).The double bond conversion of acrylate can reach 71 %.In addition,the combination of these sulfonium salts with upconversion nanoparticles(UCNPs)can achieve near-infrared(NIR)induced hybrid polymerization.The NIR light source greatly improves the epoxy conversion(85 %)in the hybrid system,which is beneficial to improve the mechanical properties of the material.In the third part,aiming at the safety problems caused by the residual and migration of photoinitiators,a macromolecular sulfonium salt photoinitiator(P(Acrylate-PI))was synthesized by introducing an acrylate group at the 7-position of coumarin and copolymerizing it with methyl methacrylate(MMA).Under the irradiation of LED@ 365 nm,405 nm light source and 980 nm laser,it can initiate the polymerization of various cationic and free radical monomers.The migration stability of P(Acrylate-PI)in the polymer was tested by immersion test.It was found that the mobility of P(Acrylate-PI)was only about 1 % of that of small molecule photoinitiators,which proved that the diffusion of macromolecular photoinitiators in the polymer was limited.This kind of macromolecular sulfonium salt photoinitiator has potential application value in food packaging,medical materials and other fields.