Photosensitized polymerization

On this basis, five classes of different polyphosphazenes are considered as outstanding examples of this type of macromolecules, in which skeletal and substituent features overlap to the highest extent. The reported materials are elastomers, flame retardants and self-extinguishing macromolecules, polymeric ionic conductors, biomaterials, and photosensitive polymeric compounds all of them based on the polyphosphazene structure. 

Photoexposure through mask onto photosensitive polymeric Him... 

Fig. 32 Diagram of image formation within a photosensitive polymeric film containing PAG, and acid-sensitive fluorophore, allowing two-photon-induced, dual-channel fluorescence imaging...

Oxime photoinitiators, (II), were prepared by Tanabe et al. (2) and used in preparing photosensitive polymeric materials. 

Finally, one more interesting reaction, the mercury-photosensitized polymerization of acetylene should lie mentioned. iShenvood and Gunning (1) showed that a stable mercury compound, probably a mercury carbide, forms in this reaction and that an optimum isotopic composition of 82% 2ll2Hg was obtained in the solid reaction product. These results suggest that, in addition to the primary step leading to the formation of an excited acetylene molecule which was proposed earlier (39), the following two additional primarv routes must also be considered (40) ... 

Pyrolysis of acetylene to a mixture of aromatic hydrocarbons has been the subject of many studies, commencing with the work of Berthelot in 1866 (1866a, 1866b). The proposed mechanisms have ranged from formation of CH fragments by fission of acetylene (Bone and Coward, 1908) to free-radical chain reactions initiated by excitation of acetylene to its lowest-lying triplet state (Palmer and Dormisch, 1964 Palmer et al., 1966) and polymerization of monomeric or dimeric acetylene biradicals (Minkoff, 1959 see also Cullis et al., 1962). Photosensitized polymerization of acetylene and acetylene-d2 and isotopic analysis of the benzene produced indicated involvement of both free-radical and excited state mechanisms (Tsukuda and Shida, 1966). 

Kabanov et al. have studied the radical polymerization of acrylic acid in aqueous solutions [61] (see Chap. 4, Sect. 1.4). A crystaline, syndiotactic polymer was formed at pH 10.2-10.8. The kinetics of the photosensitized polymerization of acrylic acid was studied by Galperina et. al. [62], They observed a strong solvent dependence for the rate constant of propagation. At 293 K, kp[mol-1 dm3 s-1] = 22 500 in water, 4 200 in formamide, and 500 in dimethylsulphoxide. 

When an emitting light at a wavelength of 200 nm is used as the light source, a satisfactory pattern cannot be formed using photosensitive compositions containing aromatic substituents. To address this concern, non-aromatic photosensitive polymeric compositions containing diamantane have been prepared. 

The mercury photosensitized polymerization of acetylene has been studied by Steacie and LeRoy. They investigated the effect of nitric oxide on the reaction, and deduced that it proceeds at least partially by a free radical mecha ism. If this is so, the reaction must be... 

Kristensen S, Wang RH, Tonnesen HH, Dblon J, Roberts JE. Photoreactivity of biologicaUy active compounds. VIII. Photosensitized polymerization of lens proteins by antimalarial drugs in vitro. Photochem Photobiol 1995 61 124—130. 

It has been generally assumed that the polymer is formed by a free-radical chain which is initiated by H atoms or alkyl radicals. While this mechanism may be true for the mercury-photosensitized polymerization, recent results on the direct photopolymerization suggest that the two systems may be very similar and that a free-radical chain mechanism is not tenable for the latter case. It is probably most logical to examine this system by first studying the polymer that is formed, and then establishing the kinetics and mechanism of its formation. 

When a polymerizable monomer is exposed to UV light in the presence of a photosensitizer, polymerization may be initiated by radicals generated by dissociation of the excited sensitizer, e.g. benzoin methyl ether. Alternatively, the excited photosensitizer may transfer its excitation energy to the monomer which, in turn, undergoes excitation and radical initiated polymerization. 

An interesting application is photosensitized polymerization of pyrrole (431) using [Ru(bpy)3]3 + to give a conducting polymer, polypyrrole (432), in aqueous solution or in a polymer matrix (Scheme 6.205).1236 The acting ground-state electron acceptor, [Ru(bpy)3]3 +, is obtained in the initial electron transfer step between an excited [Ru(bpy)3]2+ and [Co(NH3)5C1]2+ ion. 

The possible site of action for the antimalarials as potential photosensitizers will be various parts of the eye and the skin. Photohemolytical properties photosensitized polymerization of lens proteins and interaction with melanin were therefore selected as in vitro test parameters in the further screening of these compounds. According to their photohemolytical capability, quinacrine, quinine, and primaquine need to be considered as potential in vivo photosensitizers (Kristensen et al., 1994a). 

Photosensitized polymerization of lens proteins can be selected as a measure of eye phototoxicity (Chapter 11). Proteins isolated from calf lens are suitable to simulate the conditions in the human eye. The reaction mechanisms can be evaluated by adding various quenchers to the reaction medium during irradiation. Chloroquine, hydroxychloroquine, mefloquine, and quinacrine induced polymerization under the given experimental conditions (Kristensen et al., 1995). These compounds have a large apparent distribution volume and a long elimination half-life and must therefore be considered as potential photosensitizers in the eye. Primaquine and quinine were also shown to induce polymerization of lens proteins in vitro but are less likely to reach the eye in vivo due to fast elimination from the body. 

G. Manivannan and J.P. Fouassier, Primary processes in the photosensitized polymerization of cationic monomers. J. Polym. Sci. A Polym. Chem. 1991, 29(8), 1113-1124. 

Table 2.7 Typical Reactions Used in Photosensitive Polymerization and Photoresists...

In the cyclic acetal-photosensitized polymerization of styrene and methyl methacrylate, the conversion was found to increase with an increase in the number of cyclic acetal groups in the initiating molecule. Asakura et have described in detail the homo- and co-polymerization of )S-allyloxypropionaldehyde by direct photoexcitation. Although no mechanism was postulated it was certainly free radical in nature. Triallylidene sorbital (7) has also been found to polymerize by direct photoexcitation by the radical mechanism shown in Scheme 2>P ... 

Photopolymerization of acrylonitrile in the presence of naphthalene and its derivatives,19 of 9-vinylanthracenes,20 and of sulphonyl activators for dye-photosensitized polymerization,21 and the polymerization of MMA photo-initiated by anthraquinone (AQ) and 2-t-butylanthraquinone22 have been reported. Some of the important steps in this latter process are shown in reactions... 

Since the late 1980s several innovative syntheses of polypyrroles have been discovered. The photosensitized polymerization of pyrrole in aqueous solution and in polymer matrices using tris(2,2 -bipyridine)ruthenium(II) as a photosensitizer has been reported <89CC132>, and PPy can be photochemically deposited on to any type of surface under visible light irradiation conditions <89CC657, 90CC387). The preparation and potential applications of surface-functionalized polypyrrole-silica nanocomposite particles have been discussed <94PP217>. 

Figure 2. Mechanism of photosensitized polymerization of pyrrole and optical microscope photographs of (A) ptdypyrrole synthesized on Nafion film by photosensitized polymerization (B) photo mask used for photoinadiation (width of line = 10 im).

It has been shown that for NVK and PVK as photosensitizers, both the monomolecular and the polymeric photosensitizer behave similarly. Copolymers of NVK with diethylfumarate show excellent solubility in these monomers. A marked improvement of the photo-response of the photosensitized polymerization using broadband UV light was observed in comparison to experiments in the absence of a photosensitizer. The copolymers can be prepared with AIBN. 

Figure JO Photosensitized polymerization of an acrylate/epoxide monomer blend. [Darocur 1173] = 2 wt% [DAI] = 2wt% [ITX] = 0.5 wt%. Light intensity 60 mW cm. Film thickness 30 pm. Atmosphere air...

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