Oxygen, photopolymerization

The photopolymerization of this monomer with a mercury arc89,9°) produces small yields of low molecular-weight products. In the presence of oxygen an induction period is noted and the polymers contain an appreciable amount of peroxide units in the chains9 ). The photolysis of 2-vinylfuran was briefly described by Hiraoka92 cyclopentadiene and CO were reported as products. It is not certain if free radicals are involved in this photodecomposition, but presumably they are. 

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. 

Oster [174] proposed the second hypothesis to explain his results on the photopolymerization of acrylonitrile in aqueous solution, buffered at pH 7.0, and sensitized by xanthene dyes and riboflavin using ascorbic acid as the reducing agent. Whereas the monomer is efficiently polymerized when the solution is illuminated in the presence of oxygen, irradiation in its absence leads to photoreduction of the dye to its leuco form but no polymer is formed. Therefore, the author suggests that the leuco dye reacts with atmospheric... 

Pemberton and Johnson [175,176] have performed a thorough study of the photopolymerization of vinyl acetate in aqueous methanol sensitized by ethyl Eosin and ascorbic acid. In agreement with Oster s results, these authors find that polymer is not formed in the absence of oxygen. In addition, dye photobleaching without polymer formation is observed when water is excluded. 

Other carboxylate-dye interactions have been reported. Ethylenediamine tetracarboxylic acid (EDTA) and its salts are well known reductants for a variety of dyes (54,55). The amino-acid N-phenylglycine can be photooxidized and induce polymer formation (26,56,57). Studies of the efficiency of photopolymerization of acrylate monomers by MB/N-phenylglycine combinations as a function of the pH of the medium suggest that either the amino group or the free carboxylate can act as an electron donor for the dye excited state, but that the amine functional-lity is the more efficient coinitiator (10). Davidson and coworkers (58) have shown that ketocarboxylic acids are photode-carboxylated by electron transfer quenching of dye triplet states under anaerobic conditions. Superoxide formation can occur when oxygen is present. 

An advantage of this type of photopolymerizations is that as they are non-radical chain polymerizations, they are insensitive to oxygen. In addition, as the cation is relatively stable, the reaction is able to continue in the dark. Applications of this chemistry may be found in the fields of coatings, adhesives, printing inks, and also for photocurable composites and microelectronic photoresists. 

The accelaration of styrene photopolymerization by oxygen is also explained by excitation of the DA complex of these two substances [82], A copolymer is produced which decomposes upon illumination [83]. Polymerization of methyl methacrylate is initiated by the photoexcited complex of the monomer with triethylaluminium [84]. Methyl methacrylate, acrylonitrile and acrylates in general readily produce unstable DA complexes which decompose to products quite different from the initial components. Methyl methacrylate, for example, polymerizes in the presence of quinoline and bromine. With the monomer, these pairs yield a DA complex which is unstable upon illumination [85a]... 

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. 

The complexity of the initiator mix, film thickness variation, and oxygen presence in these photopolymerizations does not... 

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