Excited photopolymerization

A reaction utilizing [2 + 2] dimerization that further illustrates the use of lattice control in polymer synthesis is that of distyrylpyrazine (DSP) 1 which (in its ot-poly-morph) is converted rapidly in sunlight from bright yellow crystals to a white insoluble high-molecular-weight product— Scheme 6.3. Sasada etal. showed by X-ray structural analysis that DSP packs such that nearly planar molecules form plane-to-plane stacks with adjacent molecules appropriately displaced with respect with one another. The reactive double bonds are thus separated by 3.94 A, and upon excitation photopolymerization efficiently proceeds. The process which is envisaged is shown schematically in the Scheme 6.2b. 

Photopolymerization and Plasma Polymerization. The use of ultraviolet light alone (14) as well as the use of electrically excited plasmas or glow discharges to generate monomers capable of undergoing VDP have been explored. The products of these two processes, called plasma polymers, continue to receive considerable scientific attention. Interest in these approaches is enhanced by the fact that the feedstock material from which the monomer capable of VDP is generated is often inexpensive and readily available. In spite of these widespread scientific efforts, however, commercial use of the technologies is quite limited. 

Photopolymerization. In many cases polymerization is initiated by ittadiation of a sensitizer with ultraviolet or visible light. The excited state of the sensitizer may dissociate directiy to form active free radicals, or it may first undergo a bimoleculat electron-transfer reaction, the products of which initiate polymerization (14). TriphenylaLkylborate salts of polymethines such as (23) ate photoinitiators of free-radical polymerization. The sensitivity of these salts throughout the entire visible spectral region is the result of an intra-ion pair electron-transfer reaction (101). 

The photopolymerization process taking place within a representative mixture of sensitizer, initiator, chain-transfer agent, and monomer, typical of positive Cromalin, has been studied in detail (41,42). The exact mechanism is still controversial, but a generalized reaction scheme can be postulated as follows, where L2 = biimidazole dimer, S = sensitizer, RH = chain-transfer agent, L2 = excited biimidazole dimer, L = biimidazole radical,... 

This result reveals that exciplex formation plays a principal role in the initiation of polymerization. Since the absorption band is broadened toward longer wavelengths as the result of formation of CTC between AN and aniline, a certain concentration of aniline can be chosen so that 365-nm light is absorbed only by the CTC but not by the aniline molecule. Therefore, in this case the photopolymerization may be ascribed to the CTC excitation selected. For example, a 5 x 10 mol/L aniline solution in AN could absorb light of 365 nm, while solutions in DMF or cyclohexane with the same concentration will show no absorption. Obviously, in this case the polymerization of AN is caused by CTC excitation. The rates of polymerization for different amines were found to be in the following order (Table 12) ... 

Li et al. [87,88] found that aniline will process the photopolymerization of AN either in N,N-dimethylformamide (DMF) solution or in bulk with a fair rate of polymerization only next to DMT. From UV spectra it is proved that aniline will form a CTC with AN. Using 313-nm radiation that CTC is excited to an exciplex and polymerization proceeds. N-methylaniline will polymerize AN similarly. The following mechanism was proposed ... 

The gas-phase photolysis of 2-furaldehyde in the it -n and ir <-it transitions76 proceeds with fragmentation to CO, furan and C3-hydrocarbons, but a certain amount of resinification is also noted (about 5% quantum yield with excitation of the it - n transition). The latter observation prompted a study of the vacuum liquid-phase photolysis by sunlight or by light from a medium-pressure mercury arc at room temperature24 7S. The resin obtained was submitted to fractionation and structural analysis. On the basis of the results obtained and other mechanistic evidence, the following sequence of events was postulated for the photopolymerization ... 

After the discovery of the photopolymerization of 2,5-DSP crystals, several types of photoproducts were found, not only the linear polymers, but some other derivatives, e.g. the V-shaped dimer or cyclophane (Hasegawa and Hashimoto, 1992). The photopolymerization occurs in a step-growth mechanism by cyclobutane formation between the excited olefin and the olefin in the ground state. 

Finally, we should indicate that we have not ruled out the possibility that there is a contribution to initiating photopolymerization in maleimide/vinyl ether systems from an exciplex type complex between an excited state maleimide and ground state vinyl ether. A biradical formed from such a complex might initiate free radical polymerization in lieu of cyclization to form a 2 + 2 adduct. However, we note that at present we have no evidence for such a reactive exciplex. 

The nonlinear response of material to optical excitation is the main requirement for formation of 3D patterns by photopolymerization. The appU-cation potential of such patterns in microphotonic, photonic crystal, MEMS, and microlluidic appUcations is described in our contributions to [58]. 

Condensed monolayer films of pure 6 polymerized rapidly, as did mixed 6/DSPE films of up to 75% DSPE, provided the monolayers were in the condensed state [33], In the liquid-expanded state, polymerization did not occur. In the condensed state, lateral diffusion of individual lipids within the monolayer is severely restricted compared to the liquid-like state. This precludes initiation of polymerization by diffusive encounter between excited-state and ground-state diacetylene lipids. In order for polymerization to occur in the condensed state, the film must be separated into domains consisting of either pure 6 or pure DSPE. A demonstration that the rates of photopolymerization for pure 6 and mixed 6/DSPE monolayers are equal would be a more stringent test for separate domains of the lipids, but no kinetic data have been reported for this system. 

Photopolymerization of acrylamide by the uranyl ion is said to be induced by electron transfer or energy transfer of the excited uranyl ion with the monomer (37, 38). Uranyl nitrate can photosensitize the polymerization of /S-propiolactone (39) which is polymerized by cationic or anionic mechanism but not by radical. The initiation mechanism is probably electron transfer from /S-propiolactone to the uranyl ion, producing a cation radical which propagates as a cation. Complex formation of uranyl nitrate with the monomer was confirmed by electronic spectroscopy. Polymerization of /J-propiolactone is also photosensitized by sodium chloroaurate (30). Similar to photosensitization by uranyl nitrate, an election transfer process leading to cationic propagation has been suggested. 

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. 

Just as the oxidizing power of a dye increases on excitation, so does the reducing power. However, examples of the use of dye excited states to initiate photopolymerization via dye photooxidation are less common than photoreducible dye sensitization. 

Macrae and Wright (96) demonstrated that visible light irradiation of xanthene dyes (eosin, erythrosin, rhodamine B, or RB) in ethanolic solutions of 4-(N,N-diethylamino)benzene-diazonium chloride (as the zinc chloride double salt) resulted in decomposition of the diazonium salt. Electron transfer from the dye excited state(s) to the diazonium salt was postulated and dye-diazonium salt ion pair formation in the ground state was shown to be important. Similar dyes and diazonium salts were claimed by Cerwonka (97) in a photopolymerization process in which vinyl monomers (vinylpyrrolidone, bis(acrylamide)) were crosslinked by visible light. Initiation occurs by the sequence of reactions in eqs. 40-42 ... 

The photopolymerization of furfural by UV radiation has not received much attention. Although the products of heat polymerization of furfural are branched polycondensates with highly conjugated structures, the photopolymer of furfural is a linear polyaddition product (8,9). The gas-phase photolysis of furfural in the n — 7r and ir — 7r transitions (10) proceeds with fragmentation to carbon monoxide, furan, and C3 hydrocarbons, but a certain amount of resinification has also been noted (about 5% quantum yield with excitation of the n — 7r transition). Vacuum liquid-phase photolysis by UV radiation at room temperature has produced linear polymers (4) with a degree of polymerization of about... 

The first reaction describes the excitation of uranyl ions. The excited sensitizer can lose the energy A by a non-radiative process (12b), by emission (12c) or by energy transfer in monomer excitation to the triplet state (12d). Radicals are formed by reaction (12e). The detailed mechanism of step (12e) is so far unknown. Electron transfer probably occurs, with radical cation and radical anion formation these can recombine by their oppositely charged ends. The products retain their radical character. Step (12g) corresponds to propagation and step (12f) to inactivation of the excited monomer by collision with another molecule. The photosensitized initiation and polymerization of methacrylamide [69] probably proceeds according to scheme (12). Ascorbic acid and /7-carotene act as sensitizers of isoprene photoinitiation in aqueous media [70], and diacetyl (2, 3-butenedione) as sensitizer of viny-lidene chloride photopolymerization in a homogeneous medium (N--methylpyrrolidone was used as solvent) [71]. 

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]... 

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