Design And Synthesis Of Fluorophenyl Oxime Esters Photoinitiators For UV-LED Photopolymerization And Their Application In Photochromic Polymers

LED photopolymerization technology has attracted more and more attention because of the development of LED lamps,which have advantages of low energy consumption,low heat release,no bad smell,long service life and instant switching on(off).The emission spectra of mercury lamps are broad and mainly locate at ultraviolet region,while the emission spectra of LED lamps are narrow and concentrated at 385 nm,395 nm and 405 nm.Hence,the traditional photoinitiators applied in the UV photopolymerization cannot initiate photopolymerization rection under LED light,the development of new photoinitiators used under LED light has received more and more concern.Photochromism is the change of color of photochromic molecules under light and heat stimulus,and it can be used to fabricate smart optical materials used for data storage and data transformation.Photochromism need fragment and rotation of chemical bonds of photochromic molecules and reconstruction of conjugated structure.Hence,the photochromic reactions of photochromic molecules in solid material with finite activation volume are extremely restrained.The existing strategies are to decrease the glass transition temperature of the polymer or increase activation volume of the microenvironment at which the photochromic molecules are located.However,these two solutions have drawbacks of reducing the rigidity of the materials,complex design and hard synthesis,which tremendously limit the development of photochromic polymers.Also,the thermal polymerization which needs complex operation and costs much energy is main way to fabricate photochromic polymers.Therefore,it is still a challenge to design and synthesize novel photochromic molecules which need a little free volume to conduct photochromism,and develop a new,simple,efficient and environmentally friendly method to prepare photochromic materials.Taking all these backgrounds into consideration,the aim of this project are to design and synthesize a series of fluorophenyl oxime esters photoinitiators used under LED light basing on characteristics of 4,4'-difluorobenzil and oxime ester group,as well as to fabricate photochromic materials by fast,efficient and environmentally friendly LED photopolymerization technology.The photolysis and initiating mechanism as well as photochromic mechanisms of photoinitiators have been fully researched.The influences of structures of the photoinitiators on their performances and structure-function relationship have been investigated and the research results can provide some inspirations for the design of photoinitiators and photochromic molecules,enrich kinds of the photochromic molecules and extend application fields of the photoinitiators.The research is meaningful and valuable in theory and application.The main research contents and conclusions are as fellow:(1)Two fluorophenyl oxime esters photoinitiators(E-FBOXEs)have been designed and synthesized,and their absorption ability,photolysis mechanism,photoinitiating capability,photochromic mechanism,photochromic conditions of polymers initiated by E-FBOXEs and fatigue resistance of the polymers have been fully investigated.The results show that the twisted conformation of E-FBOXEs makes their maximum absorption peaks locate at nearly 260 nm and E-FBOXEs have poor absorptions at typical UV LED emission wavelengths.Under the exposure of 395 nm,E-FBOXEs undergo two steps of decomposition.The first step involves the cleavage of the N-O bonds of E-FBOXEs to form planar iminyl radical and acyloxy radical.The drive to form planar iminyl radical is large conjugation effect which make it more stable and the acyloxy radical does not undergo decarboxylation to generate radical with initiating ability;the second step is the further decomposition of the planar iminyl radical to generate active fluorine-substituted benzoyl radical which is the only radical to initiate photopolymerization.Polymerization of tripropylene glycol diacrylate(TPGDA)is able to be initiated by E-FBOXEs under the irradiation at 395 nm,and its final double bond conversion(nearly 80%)is close to that of TPGDA initiated by the commercial oxime ester photoinitiator at same experimental conditions,however,the polymerization initiated by E-FBOXEs has a long induction period because of the two-step decomposition.(2)The polymer films initiated by E-FBOXEs have photochromism,and the mechanism of photochromism is that the planar iminyl radical with stable conjugated structure generates after the first step of decomposition of colorless and nonplanar E-FBOXEs molecules under light irradiation,contributing to the switch of the polymer film from colorless to brown;then the movement of polymeric networks under heat provides enough free volume for the recombination of planar iminyl radical and acyloxy radical,resulting in the change of the film from brown to colorless.Using E-FBOXEs as photoinitiators and photochromic molecules,the method to fabricate photochromic polymeric materials by photopolymerization is simpler,more efficient and environmentally friendly.The development of E-FBOXEs provides new avenue for the preparation of photochromic polymers.In addition,E-FBOXEs can be used in other photochromic systems solely as a photochromic molecule.The photochromic conditions of polymers initiated by E-FBOXEs are gentle and have little influence on polymers themselves.The fatigue resistance of polymer initiated by E-FBOXEs is good,so E-FBOXEs have a great potential in photochromic polymers.(3)To shorten induction period of the polymerization initiated by E-FBOXEs,four kinds of fluorophenyl oxime esters photoinitiators with trifluoromethyl group(E-FBOXE-CF3s)were prepared by introducing trifluoromethyl group into ester groups of E-FBOXEs and changing the carbon chain length.The photolysis,photoinitiating and photochromic mechanisms of E-FBOXE-CF3s are indentical to these of E-FBOXEs,but the introduction of trifluoromethyl group and different carbon chain have a dramatic influence on the rate of the first step of decomposition and photochromic rate.For six photoinitiators,the order of their ability to initiate photopolymerization is E-FBOXE-CF3>E-FBOXE-CH2-CF3>E-FBOXE-Ph-CF3>E-FBOXE-Me>E-FBOXE-Ph>E-FBOXE-CH2CH2-CF3.(4)The relationship between structure and proformence of photoinitiators is obtained by experimental results and computational calculation.Firstly,the photoinitiators with HOMO locating on iminyl group and fluorine-substituted benzene moiety adjacent to the iminyl group and LUMO locating on the carbonyl group and fluorine-substituted benzene moiety next to the carbonyl group have better initiating performance;secondly,photoinitiators with smaller dihedral angle of the two fluorophenyl groups have better initiating performance;thirdly,when the field effect brought by trifluoromethyl group promopts the fragment of N-O bond of the photoinitiators,they have better initiating performance,and when the electron withdrawing group is closer to N-O bond,the intensity of field effect is stronger,the initiating ability of the photoinitiators is more improved.Among three structural-function factors,the electron withdrawing group,trifluoromethyl group,has greater effect on the photoinitiating performance of fluorophenyl oxime esters photoinitiators.