Photopolymerization sensitization

IV. Photopolymerization Sensitized by Metal Complexes Conducting Charge... 

III. Photopolymerization Sensitized by Interaction between Monomer and Organic Acceptor or Donor... 

Benzylthiosuccinic anhydride, 49 Benzyltriphenylphosphonium bromide, 231 Benzyl o-vinyl formal, MA copolymerization, 328 Betaines, MA in formation, 216, 230 Betains, MA-containing, 48, 256 Biacetyl, photopolymerization sensitizer, 243 Bibenzyl, acylation with MA, 92... 

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,... 

Photopolymerization reactions are widely used for printing and photoresist appHcations (55). Spectral sensitization of cationic polymerization has utilized electron transfer from heteroaromatics, ketones, or dyes to initiators like iodonium or sulfonium salts (60). However, sensitized free-radical polymerization has been the main technology of choice (55). Spectral sensitizers over the wavelength region 300—700 nm are effective. AcryUc monomer polymerization, for example, is sensitized by xanthene, thiazine, acridine, cyanine, and merocyanine dyes. The required free-radical formation via these dyes may be achieved by hydrogen atom-transfer, electron-transfer, or exciplex formation with other initiator components of the photopolymer system. 

Polymers in Schemes 12 and 13 were the first examples of the preparation of pyridinium and iminopyridinium ylide polymers. One of the more recent contributions of Kondo and his colleagues [16] deals with the sensitization effect of l-ethoxycarbonyliminopyridinium ylide (IPYY) (Scheme 14) on the photopolymerization of vinyl monomers. Only acrylic monomers such as MMA and methyl acrylate (MA) were photoinitiated by IPYY, while vinylacetate (VA), acrylonitrile (AN), and styrene were unaffected by the initiator used. A free radical mechanism was confirmed by a kinetic study. The complex of IPYY and MMA was defined as an exciplex that served as a precursor of the initiating radical. This ylide is unique in being stabilized by the participation of a... 

HPO group is sensitive to light, but stable to heat. Using this MAI, St was thermally polymerized at the first step, and then MMA was photopolymerized at the second step [12]. Block efficiency was 40-55% and the amount of PSt homopolymer decreased, while that of PMMA homopolymer increased, presumably due to chain transfer reaction. 

Fig. 5 shows the SEC/UV and SEC/IR traces of PMMA samples (78) which were photopolymerized with different concentrations of photogensitizer (0.05 x 10 M, 0.08 x 10 M, 0.25 x 10 M and 0.5 X 10 M). The photosensitizer used was 4,4 bis-(diethyl amino) benzophenone (DEABP). From the UV traces it is seen that the photosensitizers are chemically bound to the polymer chains. The results also seem to indicate that a greater number of sensitizer fragments reside in the lower molecular weight regions. A considerable amount of free sensitizer can be detected by the UV detector (retention volume 210 ml) when t e Initial concentration of the sensitizer is above 0.08 x 10 M. The other... 

Polymeric phospholipids based on dioctadecyldimethylammonium methacrylate were formed by photopolymerization to give polymer-encased vesicles which retained phase behavior. The polymerized vesicles were more stable than non-polymerized vesicles, and permeability experiments showed that vesicles polymerized above the phase transition temperature have lower permeability than the nonpolymerized ones [447-449]. Kono et al. [450,451] employed a polypeptide based on lysine, 2 aminoisobutyric acid and leucine as the sensitive polymer. In the latter reference the polypeptide adhered to the vesicular lipid bilayer membrane at high pH by assuming an amphiphilic helical conformation, while at low pH the structure was disturbed resulting in release of the encapsulated substances. 

Identification and preparation of oxime ester photopolymerization initiators that do not cause film discoloration or deteriorated by heat while remaining highly light sensitive. 

Observations Carbazole-based photopolymerization initiators containing the sensitive... 

Those photosensitive systems mentioned above consist of at least one vinyl compound which has an electron donating or accepting property. When both acceptor and donor are non-polymerizable, the system is not photosensitive. Photopolymerization of styrene is not sensitized by the ECZ-CH3CN pair. The definition of donor and acceptor is a matter of relativity. Styrene is by no means neutral, but there should be no objection to considering it as a weaker donor than VCZ or ECZ and a weaker acceptor than AN or CH8CN. Photoirradiation of AN, VCZ or styrene alone in a neutral solvent, such as benzene, or in bulk does not bring about any appreciable rate of polymerization. 

Table 4. Additive effects on photopolymerization of VCZ-NB system without sensitizer at 30 C, [VCZ] = 0.25 M. Irradiation by a 300 W high-pressure mercury lamp from...

Various metal nitrates, represented by silver nitrate, sensitize photopolymerization of AN, methaciylonitrile, a-chloroacrylonitrile, croto-nitrile and methyl methacrylate. The efficiency of photosensitization runs nearly parallel to the ease of reduction of the metal ion. Although there is little doubt that the monomer plays some role in the photochemical process, it is rather difficult to decide whether the primary act is direct oxidation of the monomer or electron transfer between metal ion and nitrate anion. 

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. 

A more conventional mechanism appears to be operative in the photopolymerization of ethyl acrylate [178] and methyl methacrylate [179] in aqueous solution, sensitized by fluorescein and Erythrosin, respectively. Ascorbic acid is the reducing agent in both cases and it is observed that the reaction does not proceed in the absence of buffer, usually phosphate buffer pH 6. Polymer formation starts after an induction period but its dependence on light intensity and ascorbic acid concentration has not been determined. The rate of photopolymerization is proportional to the monomer concentration and to the square root of the light intensity, dye, and ascorbic acid concentration. The authors report the order with respect to the monomer as 3/2. However, from our analysis of the data for fluorescein, which are more... 

It has been suggested in the literature that the a-amino radical is the species that initiates polymerization [210], This view is supported by our observation that, in spite of the relatively high quenching rate constant of Eosin triplet by triphenylamine (Table 5), the system Eosin-triphenylamine does not sensitize the photopolymerization of multifunctional acrylates. Thus, it is necessary that the amine contains a hydrogen at the a-carbon to be released as a proton after oxidation of the amine by the dye triplet. This deprotonation prevents the back electron transfer and forms a carbon radical that is sufficiently long-lived to be captured by the monomer. 

Recently Fouassier and Chesneau [219] studied the photochemistry of the system Eosin-PDO-MDEA in aqueous acetonitrile using steady-state irradiation and laser flash photolysis. The photopolymerization of methyl methacrylate (MMA) sensitized by the photoreduction of Eosin is investigated in acetronitrile to understand the mechanism of initiation and the enhancement in the rate of polymerization caused by the presence of PDO, 3. Rates, quantum yields of photopolymerization, and number average molecular weights of the polymer are determined with MMA (7 M), Eosin (3 x 10 5 M), and MDEA (0.1 M) in the presence and in the absence of 2 x 10-3 M PDO. 

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