Five organosoluble visible light benzophenone derivatives (BPs), incorporated different arylamine as electron donating groups have been synthesized and investigated for their roles as photoinitiating systems for free radical photopolymerization of acrylate monomer upon the UV and LED exposure. All the target compounds (BP-1-5) have confirmed through 1 H NMR, HR-MS/EI-MS spectra and elemental analysis. BPs displayed red-shifted absorption, higher molar extinction coefficient and better thermal properties as compared to reference benzophenone (BP) compound. BP and BPs in combination with hydrogen donor, triethylamine (TEA), are prepared and investigated their electron spin resonance (ESR) spectroscopy and photo-DSC (photo-differential scanning calorimetry). ESR spectra of BP-1/TEA package showed the highest radical intensity among the test photoinitiator packages. In addition, BP-1-based formulation exhibited the best double bond conversion efficiency than other BPs and comparable to the BP for the free radical polymerization (FRP) of TMPTA under similar UV light source. We then selected BP-1/TEA and BP/TEA package for FRP under LED light irradiation. Interesting, the BP-1/ TEA system exhibited better efficiency and shorter time at maximum heat flow than BP/TEA. This result indicates BP-1 photoinitiator not only displays good light harvesting, thermal property, but exhibits conversion efficiency under the irradiation of UV and LED.
In this work, free radical photopolymerization (FRP) kinetics for series of different phenylamine oxime ester structures (DMA–P, DEA–P, DMA–M, TP–2P, TP–2M and TP–3M) was investigated. Steric hindrance and branched substituents were prepared to realize the corresponding electronic and photopolymerization effects. The photophysical, electrochemical, thermal properties and radical concentration were investigated by UV‐visible spectroscopy, cyclic voltammetry (CV), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC) and electron paramagnetic resonance (EPR). Furthermore, the structure–reactivity relationships were also studied in detail through photo‐DSC experiment. We demonstrate that the introduction of alkyl chains and/or numbers of oxime esters affects significantly the photoreactivity. Under the same weight ratio of formulation and irradiated condition, TP–3M containing three oxime esters in its structure and methyl group in the periphery exhibits the highest double‐bond conversion efficiency. TP–3M‐based formulation also shows a wide operation window under different contents and light intensities. Importantly, the photoreactivity of the TP–3M‐based system was found to be better than the commercial photoinitiator (OXE–01) under LED@405 nm at a low concentration. This work could provide some significance to the design of oxime esters with enhanced photoreactivity.
Three novel visible-light absorbing benzophenone-based hydrogen acceptors (BPD-D, BPDM-D and BPDP-D) were designed on the basis of a donor–benzophenone–donor structural backbone. Mono or diketone units and double diphenylamine electron-donating groups in para-or meta-positions were introduced to comprehend the electronic and structural effects on free radical photopolymerization (FRPP). Such a structural change leads not only to a red-shift of the absorption maxima but strongly enhances their molar extinction coefficients compared to the commercial phototinitiators such as benzophenone (BP) and 4,4′-bis(diethylamino) benzophenone (EMK). In addition, excellent melting points and thermal decomposition temperatures were achieved for those novel compounds. Further, the photochemical reaction behavior was studied by cyclic voltammograms (CV), photolysis and electron spin resonance (ESR) spectroscopy. Finally, benzophenone derivatives in combination with an amine (TEA, triethylamine) as a co-initiator were prepared and initiated the FRPP of trimethylolpropane trimethacrylate (TMPTMA) using a UV lamp as a light source. When used in stoichiometric amounts, the BPDP-D/TEA had the best double bond conversion efficiency among all the compounds studied, and were even superior to the reference compounds of BP/TEA and EMK/TEA. The results and conclusions could provide the fundamental rules applicable for the structural design of benzophenone derivative-based photoinitiators.
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