As advantageous as these reactions are, they also feature a significant disadvantage inherently, photo-induced reactions are difficult to scale up since increasing reactor volumes lead to intensity gradients in the reactors and hence loss of reaction efficiency. 13–18 Photopolymerizations feature less side product formation and give access to fast, yet simple reaction protocols.
Especially the ability to choose between different activator/sensitizer systems gives room for broad and efficient protocol optimizations. Research activities in the field are on a steep rise and constantly new variations of photoactivated RDRP methods are published. Within only a few years, photo-induced reaction routes for all above mentioned synthesis techniques have been developed and optimized. 4 To date, practically all relevant polymerization techniques have been translated to micro- or mesoflow processing, including anionic and cationic polymerization, 5,6 reversible addition fragmentation radical chain transfer polymerization (RAFT), 7 atom transfer radical polymerization, 8,9 single-electron transfer living radical polymerization (SET-LRP), 10 nitroxide-mediated polymerization (NMP) 11 or classical azide–alkyne cycloadditions 12 to name the most relevant synthesis techniques.Ī development that occurred at the same time in the field of polymer design is a renaissance of photo-induced reactions. 3 As could be shown, small diameter flow reactors allow not only for simple scale up of such polymerizations, but also allow to synthesize materials with increased precision and hence advanced properties due to the more stable reaction conditions and improved isothermicity of the reactions. In research, highly precise polymer materials are often only obtained on the milligram scale, a hurdle that must be overcome in order to give access to material testing and ultimately to application.įlow techniques take hereby a prominent role and in recent years much focus was spend not only on bulk polymer and polymer particle synthesis, 2 but also to precision polymerization techniques. While such tailor-made materials open the window to a realm of materials with unprecedented biological, physical, thermal and mechanical properties, this raises as well the need for efficient pathways to synthesize these compounds in significant amounts. The possibilities in macromolecular design are virtually endless – especially in combination with modular click chemistry 1 approaches – and almost any macromolecular architecture can nowadays be targeted in one way or another. Photoflow chemistry Controlled polymerization techniques, starting from anionic polymerization to the plethora of controlled radical polymerization techniques (referred to as reversible deactivation radical polymerization, RDRP) are without doubt the gold standard of contemporary polymer synthesis towards advanced materials. Further, the yet unexplored potential of these techniques is identified and discussed towards future development. The different photoRDRP methods are herein compared and the underlying principles of the advantage of carrying polymerization out under photoflow conditions are elucidated. Specifically the reversible deactivation radical polymerization (RDRP) techniques have gained significant interest in this respect within the past one to two years. By switching from batch to flow processing, polymerizations can be carried out with unmatched efficiency under mild reaction conditions, while concommitantly providing conditions for simple scale up of reactions.
Precision polymer design in continuous photoflow reactors is a young, yet rapidly growing research field.
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