Synthesis And Properties Of Coumarin-based Photoinitiators

Photopolymerization technology is widely used in coatings,microelectronics,3D printing and other fields in recent years because of its advantages of fast reaction rate,mild reaction conditions,low energy consumption and easy control and regulation.With the rapid development of light-emitting diode(LED)technology,LED lamps have gradually become one of the most commonly used irradiation sources in photopolymerization.Compared with traditional high-pressure mercury lamps,LED lamps emit photons of long wavelengths,mainly locating at near-ultraviolet and visible light regions with narrow emission range.Such characters have caused difficulty to match the conventional UV photoinitiators with LED irradiation sources.In order to develop efficient LED photoinitiators,thioxanthone,carbazole and other chromophores with excellent long-wavelength photoactivity have been widely used in the rational molecular design of LED photoinitiators.In this thesis,a series of coumarin-based photoinitiators were designed and prepared based on coumarin derivatives as the core structures.The absorption wavelength of photoinitiators was adjusted via introducing different electron-donating/absorbing substituents,and the effects of initiating groups on the initiation and migration properties were systematically investigated.In the first part of the thesis,a series of unimolecular type II photoinitiators were straightforwardly prepared via two-step reactions using coumarin as the chromophore and sesamol as the initiating group.The main absorption band of the photoinitiators locates at260-450 nm,and the introduction of electron-donating substituents can cause a significant red shift in absorption wavelength.Under the irradiation of 365 nm and 405 nm LED light irradiation,the photoinitiators can generate reactive radicals to initiate the polymerization of acrylates via hydrogen abstraction,and the photoinitiator containing the methoxy moiety exhibits excellent photoinitiating performance.The combination of the novel photoinitiators with iodonium salts can realize free radicals promoted cationic polymerization and effectively induce ring-opening polymerization of epoxy monomers,and the epoxy conversion of EPOX reach to more than 50%.In addition,the thermal decomposition temperatures of the photoinitiators are all above 170℃,possessing good thermal stability.In the second part,a series of coumarin-based oxime ester photoinitiators with different terminal aliphatic ring moieties were prepared using 7-diethylamino coumarin as the chromophore and oxime ester as the initiating group.The maximum absorption wavelengths of the photoinitiators are all located at 433 nm.The terminal aliphatic ring with different structures had marginal impact on the absorption and emission behaviors.Under 405 nm LED irradiation,the photoinitiators undergo photodecarboxylation,alone with photobleaching,and the generated alicyclic radicals enable efficient polymerization of acrylate monomers.The coumarin-based alicyclic oxime esters,exhibiting high initiation rate and double bond conversion in the thiol-ene system,can be used in DLP 3D printing to construct 3D structures with high accuracy,and the finest line width is approximately 0.72 mmIn the third part,a low-mobility polyhedral oligomeric silsesquioxane(POSS)-based coumarin oxime ester photoinitiator was prepared via thiol-ene click reaction between the coumarin oxime ester with terminal double bond and the functionalized POSS with thiol group.Under 365 nm LED light irradiation,the POSS-based photoinitiator containing highly reactive thiol moieties can effectively initiate the polymerization of acrylate monomers,and the initiation rate and double bond conversion rate are superior to those of corresponding small molecules.The bulk effect of POSS and the polymerizable property of thiol moieties lead to much lower residual-initiator migration than that of the reference photoinitiator,and the migration rate is reduced from 19.5% of the reference photoinitiator to 0.2%,exhibiting great potential in food packaging applications.