RAFT polymers as versatile macrophotoinitiators – RAFT-POP

Controlled radical polymerizations have remained underexploited so far at the industrial scale with regard to their exceptional synthetic potential. Any proposition aiming at exploiting them for new high-added-value applications, overcoming by the way their restricting extra cost compared to free radical polymerizations, would thus constitute an outstanding breakthrough in the field, likely to renew their interest from an industrial point of view.
The RAFT-POP project comes within this framework by aiming at valorizing the reversible addition-fragmentation chain transfer (RAFT) mechanism as a versatile synthetic platform for the design of well-defined polymers as new macrophotoinitiators exhibiting an enhanced photodissociation efficiency and more generally as precursors for new light-induced macromolecular engineering reactions.
This concept relies on the sensitivity to light of RAFT end-groups, resulting directly from the structure of the chain transfer agents used: these thiocarbonylthio compounds, such as dithiocarbamates or xanthates, were already commonly used as photoinitiators or photoiniferters before the discovery of RAFT. This reactivity will be here highly boosted by the introduction of a chromophore at the vicinity of the C-S photodissociable bound.
The application of the RAFT process at the industrial scale has been hindered up to now in particular by the undesirable presence in the final products of these thiocarbonylthio end-groups, which can cause for instance their coloration and remain generally sensitive to light or heat. In the RAFT-POP project, these end-groups are no longer considered as a drawback but become, on the contrary, key-functions for new possible applications of RAFT polymers.
Concretely, the chromophore is introduced straightforwardly via the RAFT polymerization mechanism, either as a side group on the terminal monomer unit, or as the Z group of the chain transfer agent. These two strategies will result in two classes of photoactive macromolecular precursors, which will be synthesized and investigated in the project.
Both of them will be first of all investigated as precursors for new macromolecular syntheses methodologies that will combine RAFT with original photo-induced reactions, as well as with other controlled polymerization mechanisms. In particular, photo-induced radical couplings or chain-end modifications, enabling the switching to another mechanism, will be considered.
In a second main field of investigation, these RAFT polymers will be implemented as macrophotoinitiators, strictly speaking. After preliminary kinetic studies, they will be used first of all for the preparation of photopolymer coatings based on multifunctional acrylates. Interestingly, some structures will be also original macrophotoinitiators for radical promoted cationic polymerization and will thus be implemented for the preparation of epoxy coatings.
The polymer nature of these macrophotoinitiators can bring several advantages over photoinitiators classically used. First of all, a lower extractability of the unreacted fraction is expected. Then, they can play the simultaneous role of additives, enabling the tuning of the final properties of the photopolymer through the length, composition and density of the chains synthesized by RAFT. Finally, it is also a means to increase the initial miscibility of a photoinitiator in a formulation.
Some precursors will be optimized macrophotoiniferters which should allow the fabrication of multilayer coatings implying covalently bound interfaces. The development of an optimized “photoRAFT” mechanism should also derive from this reactivity.
Finally, macrophotoinitiators with graftable or polymerizable side groups will be investigated as a means to reinforce interfaces between a photopolymer coating and its substrate or as a supplementary means to reduce extractability issues, respectively.