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 5,7dimethoxy-6-bromo-3-aroylcoumarin-DMPT/BPh 4 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.

A hybrid methacrylate‐norbornene‐thiol photopolymerization system with tunable physical and mechanical properties

“…The incorporation of IBOMA, at a loading of f IBOMA = 0.6 (IE7), did indeed enhance the performance of the adhesive in terms of G 0 when compared at an angular frequency of 5 rad s À1 as it was increased with IBOMA to G 0 = 2.7 Â 10 5 Pa (IE7) compared to G 0 = 4.3 Â 10 3 (PP(EG)T_0.5) for a thiol:ene ratio of 0.5. The corresponding elastic modulus 5 Pa, which suggests a strong adhesive with a high E compared to the values reported for 10 layers of 3 M Scotch tape G 0 = 6.6 Â 10 4 Pa and E ≈ 2:16 Â 10 5 Pa. 51…”

“…2,4 The low energy requirements, solvent-free formulations, and ambient reaction conditions make photopolymerization one of the most environmentally friendly polymer synthesis methods. 5 Moreover, photopolymerization reactions for synthesizing crosslinked polymer networks are highly efficient as they eliminate auxiliary chemicals, and the suggested synthesis does not produce by-products or waste. The free radical polymerization reaction has an atom economy (AE) factor of 99.8% and a waste factor of 0.001, making it attractive from a green chemistry perspective.…”

“…The advantages of photopolymerizations are many, including providing temporal and spatial control of the polymerization 2,4 . The low energy requirements, solvent‐free formulations, and ambient reaction conditions make photopolymerization one of the most environmentally friendly polymer synthesis methods 5 . Moreover, photopolymerization reactions for synthesizing crosslinked polymer networks are highly efficient as they eliminate auxiliary chemicals, and the suggested synthesis does not produce by‐products or waste.…”

The power of light – From dental materials processing to diagnostics and therapeutics

“…The authors further reported significant photo-bleaching which reduced the internal filter effect which not only increased the depth of cure prior to the redox component of the mechanistic cascade but also the speed of the light curing to 20% conversion. Importantly, the results showed >35% additional conversion in methacrylate polymers over 25 min (although it is likely to further increase with time) following the initial irradiation to 20% conversion and the ability to rescue under-cured regions post-irradiation [74]. However, whether this can be achieved in all methacrylate based dental polymers is yet to be demonstrated.…”

“…However, it has been shown that the depth of cure is product dependent and although adequate polymerisation can be achieved at depth, the bottom is usually less well cured in bulk fill materials compared with conventional materials which is likely due to the lower light irradiances delivered in deeper increments [71]. Although the cure is likely to improve during the post-cure phase [72], conversion plateaus rapidly after the light is switched off as radical production ceases while termination continues [73,74]. Due to this plateau, it has also been reported that the post-cure of some bulk fill materials may exceed the generally accepted 24 h to reach adequate conversion compared with conventional materials [75].…”