Design,Synthesis And Properties Of Photobleaching Photoinitiators

Photo-curing technology is an environmental friendly technology,which has been widely used in inks,coatings,electronic communication,photoresist,dental material and 3D printing.Since visible photo-curing technology and LED light source have the advantages of light source stability,environmental protection and safety,it is of great significance to develop new photoinitiator sensitive to visible light.As an important component of visible curing system,visible photoinitiator has great effect on the curing rate and degree of the photocurable materials.However,the color of photo-curing system does not fade after the polymerization finished,which may bring about a series of problems such as heterogeneous polymerization,easy aging and low curing depth of photo-cuing materials.These problems will affect visible photoinitiators applied in photocurable materials,especially 3D printing and dental restorative materials.In order to solve these problems,developing new photoinitiator system with high efficiency and unique properties has become a research hotspot.Based on this guidance,a series of visible photoinitiators with photobleaching properties are designed,synthesized and studied in this paper.The main contents are described as follows:In the first chapter,the recent reaserch progress of photoinitiators,the classification of photoinitiators are briefly introduced and visible photoinitiators for free radical polymerization are emphatically reviewed.Conbined with the current development trend of photoinitiators,the design ideas and main research contents of this paper are proposed.In the second chapter,three new cinnamoyl-based photobleachable visible initiators(CAs)are prepared by aldol condensation reaction of 2,4-bis(diethylamino)benzaldehyde and different esters(including ethyl acetate,diethyl malonate and ditertbutyl malonate),For comparison,the photoinitiator(TA)itself without is also prepared.Their structures are confirmed by nuclear magnetic resonance(NMR)and high resolution mass spectrometry(HR-MS).The possible mechanisms of the radicals generated from CAs and TA are proposed through electron paramagnetic resonance(EPR)and visible photolysis experiments analysis.They can effectively initiate the photopolymerization of 1,6-hexanediol diacrylate(HDDA)under LED@405 nm exposure.Among them,CA-2 shows the best visible photoinitiating efficiency and photobleachable characteristic and the obtained colorless polymer is up to 2.8 mm thickness.These photobleachable visible initiators show great potentials in deep curing materials.Due to the low initiating efficiency of photoinitiators,and the structure-property relationship is not fully understood in the second chapter,in the third chapter,a series of visible photoinitiators CA-IDs are designed and synthesized.Their D-π-A structures not only show wide absorptions in visible region,but also have high molar extinction coefficients.The results of electron paramagnetic resonance(EPR)and visible light photolysis show that CA-ID-3～7 could yieldα-aminoalkyl radicals and occur visible light photolysis.The study of photopolymerization kinetics indicates that CA-ID-3～7could successfully initiate the photopolymerization of HDDA and CA-ID-4 exhibits the highest photoiniating efficiency among these photoinitiators.The CA-ID-4/MDEA(N-methyldiethanolamine)has the same photoiniating efficiency as the commercial CQ(camphorquinone)/MDEA system.The results of structural optimization experiments illustrate that the introduction of ortho groups(alkyl or aminoalkyl)to double bond on the benzene ring in cinnamyl-indanedione molecules is an effective way to develop photoinitiators with high light absorption in visible light region for radical polymerization.Combined with the analysis of electron paramagnetic resonance,the possible mechanism for generating active radical upon the appropriate wavelength LED exposure is proposed.The results of curing depth experiment imply that CA-IDs have great potential applications in deep curing materials.Based on the structural characteristics of carbazole and the structural design principle of photoinitiators in the third chapter,two visible photoinitiators CA-CZs with photobleaching properties are designed and synthesized in the fourth chapter.Their structures are confirmed by nuclear magnetic resonance(NMR)and high resolution mass spectrometry(HR-MS).The results of electron paramagnetic resonance and visible photolysis show that CA-CZ-2 could yield active radical and occur visible photolysis,and the rate of photolysis could be increased by adding the additional hydrogen donor or cationic initiator.The study of photopolymerization kinetics experiments illustrate that CA-CZ-2 has higher photoinitiating performance than CA-CZ-1.In addition,CA-CZ-2/Iod(Diphenyliodonium hexafluorophosphate)system could efficiently initiate the polymerization of TMPTA(Trimethylolpropane triacrylate)/EPOX(3,4-Epoxycyclohexylmethyl 3,4-epoxycycohexanecarboxylate)and lead to higher initiation activity.The results of structural optimization experiments show that it is also an effective way to design cinnamyl-carbazole photoinitiator with electron-donating ortho-substitution of double bond in the carbazole ring to improve the efficiency of free radical polymerization.Combined with the analysis of photophysical and chemical behaviors,electron paramagnetic resonance experiments,the possible mechanism for generating active radicals upon LED@455 nm exposure is proposed.The curing depth experiments show that CA-CZ-2 and CA-CZ-2/Iod have photobleaching properties in photocuring components,and the introduction of cationic initiator(Iod)could improve the photobleaching performance in acrylic/epoxy resin,extending its application prospect in photocuring materials.In the fifth chapter,polysiloxane-carbazole macromolecular photoinitiator(SCZ)is prepared by the nucleophilic substitution reaction of CZ-OH andγ-chloropropylmethylpolysiloxane-co-dimethyl-polysiloxane(PSO-Cl).Its structure is fully characterized by nuclear magnetic resonance(NMR),infrared spectroscopy(FT-IR),Gel permeation chromatography(GPC)and ultraviolet spectroscopy(UV-vis).The results of photopolymerization show that SCZ has a higher photoinitiating efficiency compared with low molecular photoinitiator(CZ).The polyurethane diacrylate films are prepared by using SCZ and CZ as photoinitiators to initiate the photolinking of polyurethane diacrylate(PUA)prepolymer,and their properties are extensively analyzed.The results show that SCZ can improve the surface properties,thermal stability and resistance to water and ethanol of the cured PUA films.The high-resolution surface patterns are obtained by employing macromolecular SCZ as a photoinitiator.By comparing analysis of the ~1H NMR,FT-IR and UV-vis spectra of the original and recovered photocurable components,they are basically consistent.Therefore,carbazole-polysiloxane macromolecular photoinitiator can be used in photoresist materials with ethanol as the developer and the components without crosslinking can be recycled.