Photopolymerization processes, oxygen inhibition

Thiol-Ene Photopolymerization The thiol-ene polymerization of suitable systems (10.84) in film is insensitive to oxygen. The reaction refers to the addition of a thiol to a double bond (e.g., vinyl, allyl, acrylate, and methacrylate) [310] and has led in these past years to a new revival of interest [311,312]. Thiol-vinyl ether or -allyl ether polymerization shows some following interesting features very fast process, low or even no oxygen inhibition effect and formation of highly cross-linked networks with good adhesion, and physical and mechanical properties. 

Essentially electron-donor substituents retarded polymerization, whereas electron-acceptor substituents accelerated the process. In fact, these workers obtained a good correlation between the Hammett a values of the substituents and the polymerization rate. Remaining with benzophenone, the photopolymerization of 1,3,5-trithiane has been found to be inhibited by amines and also the absence of oxygen,whereas in the hydrogen peroxide-initiated photopolymerization of methyl methacrylate-benzophenone has been found to be a powerful accelerator. In the latter study different solvents were found to have different effects on initiation. In solvents giving low conversions, degradative initiator transfer was found to be a dominant process. 

Photopolymerization of the monomeric composition has been carried out under the covering medical glass (0.10-0.15 mm thick) with the aim of preventing inhibition by oxygen in the surface layer of the photocomposition and in its local (at the point of a laser beam reflection) deformation caused by tensions at the phase boundary (solid and liquid) division at the frontal character of a process. 

The cationic photoinduce process present some advantages compared to the radical one [10] in particular lack of inhibition by oxygen, low shrinkage and good adhesion and mechanical properties of the UV-cured materials. Moreover, the monomers employed are generally characterized by being less toxic and irritant with respect to acrylates and methacrylates, largely employed in radical photopolymerization. 

---