Studies On Highly Efficient Polymerizable And Polymeric Benzophenone Photoinitiators

Benzophenone (BP) and its derivatives have been widely used as typeⅡphotoinitiators in UV curing technology, because of its low cost, ease of synthesis and good stability. Aimed at the property requirements for benzophenones, such as non-toxicity, low-odor, low migration in products, excellent photoefficiency and good compatibility with polymerization systems etc., novel highly efficient benzophenone photoinitiators were synthesized: polymerizable benzophenone photoinitiators through the introduction of polymerizable double bond into benzophenones, sulfur-containing benzophenone photoinitiators and polymeric benzophenone photoinitiators through homopolymerization or copolymerization method. Moreover, their photochemical and photophysical properties and photoinitiation properties were studied in detail.By introducing maleimide groups into the molecule of benzophenone, we built the new concept of chemical-bonded photoinitiators comprising the structure of maleimides (MIs) and benzophenone, and synthesized four kinds of polymerizable benzophenones containing the structure of planar N-phenylmaleimide (NPMI) and benzophenone (MPBP, BMPBP, MBP and BMBP). Compared with BP, their UV-Vis spectra have a significantly red-shifted maximum absorption and their photoefficiency was greatly enhanced when used in the photopolymerization of 1,6-hexanediol diacrylate (HDDA), and MPBP is the most efficient photoinitiator. The kinetics for photopolymerization of methyl methacrylate (MMA) using MPBP/ trimethylolpropane triacrylate (DMAEMA) was studied. The results verified that maleimide group in MPBP does not affect its photoinitiation mechanism and its kinetic equation can be expressed as Rp∝[MPBP]0.3172[MMA]0.7669[DMAEMA]0.1765. The overall apparent activation energy for the photopolymerization of MMA is 31.88 KJ/mol. The kinetics study on the polymerization of HDDA using MPBP/DMAEMA was investigated by photo-DSC. It shows that MPBP/DMAEMA concentration, temperature and light intensity have similar effect on photopolymerization. The increase in them leads to the increase in the polymerization rate (Rp) and the final conversion. At low conversion (10%), the Arrhenius plot is linear, and we can get that the apparent activation energy is 11.25 KJ/mol. By selecting 4-phenoxylbenzophenone (PBP) as the model compound, we further investigated the photochemical and photophysical properties of MPBP. The results showed MPBP and BP possess the same initiation mechanism. The large red-shifted maximalπ-π* absorption of MPBP should be addressed to phenoxyl group in MPBP instead of maleimide group. The photoefficiency of chemical-bonded MPBP is higher than the physical mixture of NPMI and BP when used in the photopolymerization of MMA and HDDA.A novel chemical-bonded polymerizable sulfur-containing photoinitiator MTPBP was synthesized. The results showed MTPBP possessed greatly red-shifted UV maximal absorption and very weak fluorescence emission in comparison with MPBP or BP. ESR spectra indicated the C-S bond in its molecule would undergo photolysis reactions to generate radicals and initiate the polymerization. Another two chemical-bonded sulfur-containing photoinitiator CMTPBP and BMTPBP were synthesized. The results showed these photoinitiators possessed greatly red-shifted UV maximal absorption due to the strong electron-donating of the thiophenyl group. Thiophenyl group may reduce the fluorescence emission intensity in MTPBP and BMTPBP in contrast to the chlorine atom in CMTPBP. Photopolymerization study of MMA, HDDA and trifunctional trimethylolpropane triacrylate (TMPTA) using these sulfur-containing chemical-bonded photoinitiators and DMAEMA as the coinitiator was studied. The results showed surprising high efficiency of these chemical-bonded photoinitiators towards different monomers in contrast to BP: MTPBP is the most efficient for MMA; BMTPBP is the most efficient for HDDA; the higher viscosity of TMPTA and the larger molecular size of BMTPBP may restrict the bimolecular H-abstraction reaction. Three novel polymerizable sulfur-containing benzophenone photoinitiators comprising the methacrylamido group (MAATPBP, CMAATPBP and BMAATPBP), and another polymerizable photoinitiator possessed the similar structure with MAATPBP (MAAPBP), were synthesized. These photoinitiators possess greatly red-shifted UV maximal absorption, and very weak fluorescence emission. The photopolymerization of HDDA and TMPTA initiated by these photoinitiators was studied. The results show that all the sulfur-containing polymerizable photoinitiators are highly efficient and different photoinitiators exhibit different behaviors towards different monomers: BMAATPBP is the most efficient for HDDA; MAATPBP is the most efficient for TMPTA. They can also efficiently initiate the photopolymerization without the coinitiator because of the photolysis at C-S bond. The photoefficiency of MAAPBP is dramatically lower than that of MAATPBP.Through the copolymerization of chemical-bonded photoinitiators containing maleimide group with DMAEMA, three polymeric photoinitiators bearing side-chain benzophenone and coinitiator amine P(MPBP-co-DMAEMA), P(MTPBP-co-DMAEMA) and P(CMTPBP-co-DMAEMA) were synthesized. These polymeric photoinitiators exhibit the usual characteristic UV-Vis absorption in comparison with their monomers, which shows that the macromolecular structure and coinitiator amine have no significant influence on the UV-Vis maximal absorption of BP moieties. The photopolymerization of HDDA and TMPTA initiated by these photoinitiators was studied. P(MPBP-co-DMAEMA) and P(MTPBP-co-DMAEMA) are more efficient than their low molecular counterpart, which further indicated that the macromolecular structure of the BP and DMAEMA moieties may favor the possibility of efficient energy migration and intramolecular reactions. P(CMTPBP-co-DMAEMA) is less efficient than CMTPBP/DMAEMA for the disappearance of double-bond in maleimide group.Novel polymeric sulfur-containing photoinitiators bearing side-chain benzophenone and coinitiator amine, copolymers of MAATPBP and of CMAATPBP with DMAEMA, as well as their hompolymers of P(MAATPBP), P(CMAATPBP) and P(DMAEMA) were prepared. Polymeric photoinitiators display similar UV-Vis maximal absorption spectra to the low molecular counterparts. With the disappearance of the carbon-carbon double bonds in these homopolymers or copolymers, their molar extinction coefficient (logε) value seems to decrease. This may be caused by the disappearance of the carbon-carbon double bonds in polymers, which lessens the conjugation of the polymers. Photopolymerization study of HDDA and TMPTA indicated that P(MAATPBP) or P(CMAATPBP) was more efficient than their monomer counterpart, which verified that polymer-bound radicals may be protected from coupling reactions by the macromolecular coiling, thus increasing their initiation efficiency as compared with their low molecular counterparts. The photolysis reaction at C-S bond greatly affects the bimolecular H-abstraction reaction in the copolymeric photoinitiators, thus decreasing the effect of interaction between BP and DMAEMA moiety in the same polymer chain and leading the higher efficiency of P(MAATPBP)/P(DMAEMA) in comparison with P(MAATPBP-co-DMAEMA). The reduced mobility of the polymeric photoinitiators in the higher viscosity reaction medium of TMPTA shortens the stage at which propagation becomes diffusion limited, leading to its shorter time to reach Rpmax and difficult to reach high final conversion.