Photopolymerization processes, oxygen

An usual way to generate a strong acid as an initiator of cationic polymerizations is by the UV decomposition of a complex aromatic salt of a Lewis acid. Cycloaliphatic epoxy monomers are used in this reaction because they exhibit higher reactivities than those of glycidylether epoxies such as DGEBA. These formulations are used in photopolymerization processes whose main advantage apart from the fast reaction rate is the insensitivity to oxygen (contrary to free-radical polymerizations). 

Light-induced cationic polymerizations are often retarded by impurities such as water or basic substances but will continue in the dark and in the presence of oxygen for a considerable time. Light-induced radical polymerizations, on the other hand, will start rapidly but suffer from the presence of molecular oxygen. Onium salts initiate both photopolymerization processes since they yield radicals as well as cationic species upon photolysis. 

Photocrosslinking. The second class of photopolymer chemistry that is used in some commercial products is based on the reaction of unsaturated moieties attached to an organic polymer. These photopolymer materials include the [2+2] cycloaddition of the ethylenic groups in poly(vinyl cinnamate) polymers and in the newer styryl pyridinium (10) and thiazolium (77) derivatives of poly(vinyl alcohol). The main advantage of this chemistry is that, unlike free-radical photopolymerization, they are insensitive to the presence of oxygen. This photopolymer mechanism is principally used in applications employing a washout development process (e.g. resists). 

The photopolymerization of mixtures of maleimides and vinyl ethers is shown to be an efficient, rapid process in the absence of external photo initiators. Polymerization proceeds both in the presence and absence of oxygen. Films produced by the photopolymerization of maleimide/vinyl ether systems exhibit little absorbance at wavelengths greater than 300 nm. The thermal stability of these films are also excellent. 

Photodimerization of acetylene to give vinylacetylene (butenyne) and formation of polymers in the photolysis of alkynes generally are examples of photoaddition to alkynes. Photopolymerization of di- and poly-ynes has been studied, and for both conjugated diynes or triynes the polymerization process is a 1,4-addition reaction (equation 34). The products are highly unsaturated, and they tend to contain a high proportion of oxygen after exposure to the atmosphere. 

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. 

In fact, the importance of the cationic photopolymerization lies beyond epoxides (39). Many oxygen-containing compounds, e.g., vinyl ethers, tetra-hydrofuran, oxetane, lactones, trioxane, and some unsaturated compounds (Fig. 3) can be polymerized by the same mechanism to form adhesives or coating materials. Crivello (43) reviews the scope of cationic photopolymerization, providing us with a perspective on this promising 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. 

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