Photopolymerization process

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,... 

Finally, silicon-based polymers, especially with hydrogen lateral groups, are very interesting, but they are not yet explored sufficiently. There are many unknown properties in these materials, including the details of the photopolymerization process and a-Si formation from polysilane. Additional academic work in this field is expected and necessary to make the solution processing of silicon devices more convenient and reliable. 

This reliable control of the curing process allows for stopping the polymerization after gelation, when the system has a fixed three-dimensional structure but is still mbbery and contourable. One potential application for employing the temporal control of the photopolymerization process would be in designing contourable fraction fixation plates. Figure 3(b) are the... 

However, mechanical measurements show that especially at low intensities, the photopolymerization process continues for a considerable time at a rate which cannot be detected by DSC. At equal doses the temperatures of maximum mechanical loss, T(tan S ax), were observed to be the same. 

Photopolymerization processes have been known since the. 1820s when a permanent photographic image was obtained using a coating of bitumen on glass (B-71MI11404). Since then photopolymerization systems have received almost as much attention as silver halide processes, but only a few have been used as substitutes for the latter in information... 

In the particular case of a sensitized photopolymerization process, the expression for Rp must be modified to reflect the photophysics of the sensitizer S and the efficiencies of the processes which intervene on the way to creation of the free radical R- which will eventually initiate chain formation. For the purposes of the present discussion, consider that R is formed by the following sequence of reactions ... 

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 ... 

An alternative to sintering frits, which deserves mention here, is to form frits via UV photopolymerization of a glycidyl methacrylate and trimethylolpropane trimethacrylate solution (UV radiation, 365 nm for 1 hour) [135]. The photopolymerization process is similar to that used in the fabrication of monolithic columns (Chapters 5 and 6). Frits fabricated with this method have shown to be reproducible since there is no sintering of packing material, weakening of the capillary column by removal of the polyimide coating and/or alteration of the stationary phase at the frit are avoided. 

Further examples for a PET involving amines are quinoline-dibenzoyl peroxide [114], rhodamine 6G-dibenzoyl peroxide [115], and auramine O-dibenzoyl peroxide [116] systems. The use of coloured amines makes it possible to initiate the polymerization with visible light, which is favorable for several practical applications. Again, the photopolymerization process proceeds by a free radical mechanism, and therefore, dye moieties are incorporated as endgroups into the polymer molecules. 

Most industrial applications involving photopolymerization processes are based on free radical polymerization [43], The relatively well advanced state of... 

The classical photochemical free-radical source is a compound that is photoexcited and then undergoes bond cleavage to yield active radicals. Benzoin and its ethers are efficient radical sources and are the most commonly employed photoinitiators in industrial photopolymerization processes (42,43). The reaction is a simple Norrish type I cleavage ... 

More direct information on the energetics of the photopolymerization process is afforded by DSC measurement with UV-initiation of the reaction. Under single pulse excitation the integral heat evolved per light pulse is a measure for the product nAH and from the ratio of the signal amplitudes at t = 0 and t -+ co, respectively,... 

In recent experiments the photopolymerization process has been initiated with visible light. Sensibilization of the photopolymerization reaction is possible in diacetylene crystals by introduction of energetically low lying absorptions of the substituents via formation of mixed crystals or by doping with dye molecules Although the detailled mechanism of the sensibilization is not clear, the experiments clearly demonstrate the importance of lowlying electronic states in the polymerization reaction. 

Although benzophenone derivatives have found application in a number of photopolymerization processes (10), these have usually involved cross-linking reactions induced by energy transfer from photo-exdted benzophenones to groups (e.g. cinnamate) attached to the polymer chains. In such cases, the benzophenones act as sensitisers and not as photoinitiators. An e>fample in which hydrogen abstraction by photoexdted benzophenone has been utilized for photoinitiation is the ultra-violet induced grafting of styrene on to polyethylene (8,45). Recently, extensive studies concerned with photoinitiation of vinyl polymerization by benzophenone derivatives in homogeneous media have been carried out, and are described in detail below. 

There are many review articles on various aspects of photoinitiated polymerization [5-11], eovering both applications of dye-sensitized polymerization and basie researeh studies. However, there are essentially no reviews describing, in detail, the meehanistie aspeets of photoinduced electron transfer proeesses as applied in photoeuring teehnologies, the PET process applied to photopolymerization processes, or the latest photoredox pairs to be used as photoinitiators. 

Figure 1.40. The Philips photopolymerization process for replicating video disks, (a) The liquid layer L is spread over the mold Mo by deforming the substrate S to make it slightly convex, (b) Exposure to UV light polymerizes the liquid coating, (c) The substrate unth coating is separated from the mold, (d) The information layer is coated with a mirror M and protective layer P. Reproduced with permission from reference 55. Copyright 1982 Philips Re-search Laboratories.)...

There are two types of photopolymerization processes (1) photopolymerization with photocatalytic systems and (2) direct photoexcitation of the monomer. The former has been demonstrated by the formation of polythiophene and polypyrrole either by n-type... 

Next, we discuss the concept of phonon-assisted reactions. In relation to thermal reactions, they can be assisted by phonon-mode softening leading to large-amplitude overdamped oscillations. In the case of a photochemical reaction, a strong electron-phonon coupling can assist in polymerization. Then some non-linear spectroscopic studies are discussed which illuminate on the dynamics of photopolymerization process. Then follows a discussion of results on reaction in a different kind of molecular assembly, the Langmuir-Blodgett films. Finally, some gas-solid interface reactions which produce polymers in a doped state are discussed. 

A more recent method of inducing SCSC [2+2] photoreactivity involves the use of crystals of nanometer-scale dimensions [70]. The method has been successful for the construction of polymers of 2,5-distyrylpyrazine, as well as methyl p-phenylene-diacrylate. Presumably, the nanoscopic crystal size provides the reactant crystals with enough flexibility to withstand strain induced by the photopolymerization process. 

Because of the conformation change and defect formation during the photopolymerization process, long coherence length of the columns in the columnar phase was difficult to maintain. Therefore, the excellent carrier transport characteristics in the columnar phases of monomeric triphenylene derivatives could not be retained in the polymerized films. 

Few things are known in the control of photopolymerization reactions. In a light-induced reaction, a photoiniferter can be used. Both the initiation and the reversible termination are photoinduced. The mechanism of a classical living radical photopolymerization process is recalled in e21. 

The topological control of such polymerization reactions has been the subject of recent extensive reviews,52 and the same phenomenon in 2,5-distyrylpyrazine has been further discussed.53 The true photopolymerization process discussed here is typified by the u.v.-induced polymerization of AW -polymethylenebis-(maleimides) discussed above,51 and a complete study of the kinetics of the process is included in this series of papers. The conclusion is reached that the triplet state of the chromophore is the reactive species this can be quenched by the addition of ferrocene or 3,3,4,4-tetramethyl-l,2-diazetine 1,2-dioxide. The kinetics of a further example of true photopolymerization, the reductive photocopolymerization of diaryl ketones, have been compared with those of the photoreduction of the model compound benzophenone and found to be similar.53 The photoreductive poly-recombination of the diaryl ketones shown in Schemes 2 and 3 has been studied extensively.54... 

The photopolymerization systems, which are used for Image recording, are photolnltlated processes. They are erroneously classified as photopolymerization processes this term should be reserved for the true chain-lengthening processes where light Is Indispensable for each propagation step and from which many examples are known (15). This true photopolymerization did not find applications In photography due to the low sensitivity of the system, which comes from the necessity to use a photon for each addition step. 

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). 

The final two entries in Table III are representative of the lamp sources currently available from the laser industry. Although both are extremely important as tools for investigation of basic photopolymerization processes, their widespread use as sources for UV curing is limited by the small cross section of the beam to specialty applications in the imaging and electronics industries. 

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