Didier Le Nouen

Insights into photoinduced sol-gel polymerization: an in situ infrared spectroscopy study.

Photoacid-catalyzed sol-gel polymerization is now recognized as a powerful single-step synthetic approach to the synthesis of hybrid films, which can be distinguished from conventional sol-gel methods by higher reactivity and a solvent-free process. Despite its utility, the mechanism is not yet understood, in particular what chemical, physical, and photochemical parameters determine the precise sequence, kinetics, and advancement of this UV inorganic photopolymerization. Here, using mainly transmission real-time Fourier transformed infrared (RT-FTIR) spectroscopy, we characterize in situ the hydrolysis-condensation reactions of oligomeric silicon alkoxides and the formation of byproducts. Systematic review and assessment of numerous processing variables (relative humidity, film thickness, precursor structure, nature, and the concentration of photoacid generator) prove that the reaction kinetics are controlled by the two independent phenomena: the intrinsic chemical reaction rates and the water vapor permeation into the film.

Photoinduced Cross-Linking of Dynamic Poly(disulfide) Films via Thiol Oxidative Coupling.

Initially developed as an elastomer with an excellent record of barrier and chemical resistance properties, poly(disulfide) has experienced a revival linked to the dynamic nature of the S-S covalent bond. A novel photobase-catalyzed oxidative polymerization of multifunctional thiols to poly(disulfide) network is reported. Based solely on air oxidation, the single-step process is triggered by the photodecarboxylation of a xanthone acetic acid liberating a strong bicyclic guanidine base. Starting with a 1 μm thick film based on trithiol poly(ethylene oxide) oligomer, the UV-mediated oxidation of thiols to disulfides occurs in a matter of minutes both selectively, i.e., without overoxidation, and quantitatively as assessed by a range of spectroscopic techniques. Thiolate formation and film thickness determine the reaction rates and yield. Spatial control of the photopolymerization serves to generate robust micropatterns, while the reductive cleavage of S-S bridges allows the recycling of 40% of the initial thiol groups.

Novel applications of fluorescent brighteners in aqueous visible-light photopolymerization: high performance water-based coating and LED-assisted hydrogel synthesis

Commercial fluorescent brighteners (styrene-based, coumarine-based and 2,5-bis(benzoxazolyl)thiophene-based derivatives) are shown to be active photoinitiators in aqueous visible-light photopolymerization. The proposed systems are characterized by: (i) a low intensity visible-light photopolymerization reaction of acrylates in water using a unique photoinitiating system consisting of a catalytic amount of commercially available optical brighteners and diphenyliodonium salt and (ii) a category of more ecofriendly acrylate formulations enabling the LED-assisted synthesis of hydrogels that typically show high swelling capability. Herein, styrene-based brighteners show their potential as the most promising photoinitiators for the synthesis of tack free waterborne coatings and hydrogels under mild light irradiation conditions, i.e. safe and cheap irradiation devices were selected.

Synthetic strategies towards mycolactone A/B, an exotoxin secreted by Mycobacterium ulcerans

Mycolactone A/B, a complex polyketidic macrolide secreted by Mycobacterium ulcerans, has been synthesized using catalytic and/or asymmetric reactions. Several palladium-catalyzed reactions were investigated for forging key σ-bonds as well as a particularly challenging hydroboration reaction of a functionalized and sterically demanding 1,1-disubstituted exo-alkene. The full account of these investigations is discussed.