Visible-Light Photoinitiation of (Meth)acrylate Polymerization with Autonomous Post-conversion

Abstract

Conversion plateaus rapidly in radical photopolymerizations (RPPs) following discontinuation of irradiation due to rapid termination of reactive radicals, which restricts the wider use of RPPs in applications that involve nonuniform light access including those with attenuated light transmission or irregular surfaces. Based on our recent report of a radical dark-curing photoinitiator (DCPI) that continues polymerization beyond the cessation of irradiation by enabling latent redox initiation with photo-released amine in the presence of a suitable oxidant, we developed a new DCPI with an absorption spectrum that extends well into the visible range. Our design process involved a series of computational investigations of candidate molecules, including a systematic study of substituents and their position-dependent effects on absorption characteristics, electronic transitions, and the photochemical mechanism and its associated energetics. Our quantum chemical computations identified the target compound 5,7-dimethoxy-6-bromo-3-aroylcoumarin-DMPT/BPh₄ and predicted that it would facilitate the dark-curing mechanism by concurrent photo-radical generation and photo-induced release of an efficient redox reductant under visible irradiation. This reductant-tethered chromophore was then synthesized and optically characterized with UV-vis spectroscopy that revealed its strong visible-light absorption with a molar absorptivity of 5710 M^-1 cm^-1 at 405 nm and 50 M^-1 cm^-1 at 455 nm. We then demonstrated extensive dark-curing of >35% additional conversion over 25 min following brief activation of the shelf-stable one-part system by irradiation with a 455 nm LED that was ceased at 20% conversion. In contrast, shuttering irradiation of the control formulation at that same point resulted in immediate cessation of conversion, which plateaued at 20%. We determined a remarkable initiator efficiency of 2.82 that results from the additional redox-generated radicals with a 77% photo-reductant generation quantum yield. The combination of superior photo- and dark-curing efficiencies of this new visible DCPI is expected to open new application opportunities in RPP, especially those involving resins that are highly light attenuating, surfaces that possess irregular features that produce uneven irradiance, and production lines where continued dark-curing downstream of the light activation step enhances line efficiencies.

Figure 1.

Light- and dark-curing mechanism. The proposed DCPI undergoes a complex mechanism composed of three HM, three ET, and one PT reactions, whereby direct photolysis and latent redox reaction produce four initiating radicals over two time scales.

Figure 2.

ARC structural moiety and its potential synthetic route that facilitates facile functionalization to tune absorption. The design components of visible-light-absorbing DCPIs include diaryl ketone that mediates photochemistry, the coumarin moiety with functionalization opportunities that modify light absorption characteristics, and the methylene phenyl group that attaches to a tertiary amine as ammonium salt.

Figure 3.

(a) Simulated UV–vis spectra of benzophenone (BP) and ARC. Despite their λ_max,1 being similar at ~330 nm, the ε_max,1 of ARC is much greater than that of BP (733 vs 69 M⁻¹ cm⁻¹), making ARC a superior scaffold for visible-light-absorbing DCPIs. (b) Simulated UV–vis spectra of ARC and its methoxy-substituted derivatives. Methoxy substitutions at the C₅ and C₇ positions increased molar absorptivity more efficiently than those at the C₆ and C₈ positions. The substitution positions are described in Figure 2. Relevant photophysical constants were computed by TD-DFT.

Figure 4.

Molecular orbitals of ARC and 7-methoxy-3-aroylcoumarin (ARC-7-MeO). λ_max,1 remained unchanged because the participating orbitals (n and π*) are only marginally affected by the methoxy substituent, while λ_max,2 was bathochromically shifted from 295 to ~310 nm due to the shift of the π orbital to higher energies caused by the methoxy substituents.

Figure 5.

Illustration of the push–pull effect in ARC used to modulate light absorption in this study. Polarizability along the long molecular axis enhances the desired light absorption and increasing the push–pull characteristic of ARC increases the dipole moment, resultant polarizability, and thus the intensity of light absorption.

Figure 6.

Simulated UV–vis of ARC, ARC-5,7-diOMe, and ARC-5,7-diOMe-6-Br. ARC-diOMe and ARC-diOMe-Br exhibit better visible-light absorption than unsubstituted ARC. When para-tolyl bromination of ARC-diOMe was attempted, bromination at the sixth position between two methoxy substituents continuously occurred; therefore, ARC-diOMe-Br was instead targeted as a potential chromophore for visible DCPI after computationally confirming its viability as a visible-light DCPI.

Figure 7.

Changes in the electronic configurations of chromophore–ammonium/BPh₄ leading up to the generation of two radicals and an amine. The positions of the orbitals qualitatively represent energies of the orbitals for all chromophores studied herein.

Figure 8.

Experimentally measured optical spectra of BP-DMPT/BPh₄ and ARC-5,7-diOMe-6-Br-DMPT/BPh₄ in N,N-dimethylformamide. ARC-based PBG exhibits strong visible-light absorption while BP-based PBG is limited to near-UV absorption. Two y-axes were used to accommodate the large molar absorptivity difference between two DCPI structures. The concentration of BP-PBG is 7.50 mM and that of ARC-PBG is 0.16 mM.

Figure 9.

NMR evidence of a photobase generator from ARC-5,7-diOMe-6-Br-DMPT/BPh₄. The peaks of DMPT emerge after irradiation of the PBG, where the efficiency of photo-reductant generation is 77%. Ethylene carbonate was used as an internal standard for quantitative NMR. The concentration was 14.7 mM in deuterated dimethyl sulfoxide with 2.0 mW/cm² 405 nm LED irradiation.

Figure 10.

Methacrylate PP and dark-curing with DCPI and the control photoinitiator. The control formulation only contains PBG (ARC-5,7-diOMe-6-Br-DMPT/BPh₄) that generates amine and radicals during irradiation, excluding the dark-curing potential due to the lack of BPO peroxide. The DCPI formulation contains PBG and BPO that enables an extended conversion post-irradiation. The experimental conditions include the initiator of 0.6 wt % ARC-PBG and 1.8 wt % BPO, the resin of 80 wt % TEGDMA and 20 wt % MBL, and a 50 mW/cm² 455 nm LED.