The Science behind the project
The PHOTON DROP project is at the intersection of 3 chemistry fields: Photopolymerization, N-Heterocyclic Carbenes and Polymerization in dispersed media. You can learn more about these 3 topics by visiting the pages dedicated to each of them upon cliking on the buttons bellow.
Photopolymerization is a well-established technology showing a growing interest because of significant economical and environmental advantages. Nevertheless, more than 95% of films and coatings derived wherefrom are based on a radical process strongly inhibited by atmospheric oxygen. There is thus a high demand for novel non-radical photoinitiators. In this respect, N-Heterocyclic Carbenes (NHCs) have emerged as versatile ligands for organometallic catalyst (Grubbs’ 2nd generation) but also as powerful organocatalysts for polymerization reactions (ring-opening, step-growth, anionic) over the past decade. NHCs are usually synthesized by deprotonation of an azolium salt with a strong base, but their generation in situ and “on demand” from thermally labile progenitors has attracted a lot of interests in the past few years. Interestingly, the generation of NHCs by a photochemical process has never been reported and an efficient photolatent NHCs has yet to be fully developed.
PHOTON DROP project thus aims at developing a simple synthetic pathway to robust photolatent NHCs. They will be subsequently used as photoligands and photocatalysts in ring-opening metathesis and anionic polymerizations for key applications: film UV-curing and latex production.
To prepare the photolatent NHCs we propose a simple anion exchange between azolium salts and photosensitive salts. As pre-organocatalysts, they will be subsequently used for the ring-opening anionic polymerization (ROAP) of bio-based cyclic esters, like macrolactones. The photo-ROAP will be performed in bulk in the form of a film or in aqueous miniemulsion, leading to biodegradable films or latexes respectively. Polyesters films prepared wherefrom are expected to find application in single-use packaging items whereas polyesters latexes should be the first step to biodegradable paints.
In a more original manner, photolatent-NHCs will be also used as pre-ligands for the in-situ generation of Ru-based catalysts subsequently employed for the ring-opening metathesis polymerization (ROMP) of cyclic olefins (norbornene, dicyclopentadiene…), or/and resins derived wherefrom. Photo-ROMP will be also explored either in bulk, to form weathering resistant and hard coatings, or in aqueous miniemulsion to form cargo particles for “bioactive” coatings.
The synthesis of polymers by chain-growth polymerization is discussed here. By far, the most common example of chain-growth polymerization is that of unsaturated monomers such as vinyl monomers (The term "unsaturated" is used to designate a compound which contains double or triple bonds). Chain polymerization is initiated by a reactive species R* produced from a compound termed an initiator. In the case of a photoinitiator, the reactive species are created upon application of a radiation (in the UV or visible range) obviating the use of heat. The reactive species, which may be a free radical, cation, or anion, adds to a monomer molecule to form a new radical, cation, or anion center. Each monomer molecule that adds to a reactive center can thus regenerate the reactive center in a reaction called propagation. The process is repeated as many more monomer molecules are successively added to continuously propagate the reactive center. Polymer growth proceeds by the successive additions of hundreds or thousands or more monomer molecules. The use of monofunctional monomers (single C=C bond) results in the formation of linear polymers. Alternatively, monomers exhibiting at two unsaturations yield a three-dimensional polymer network in a reaction called cross-linking.