Photopolymerization Visible Light Initiated

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 ... 

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). 

Photopolymerization of MMA was also carried out in the presence of visible light (440 nm) using /3-PCPY as the photoinitiator at 30°C [20]. The initiator and monomer exponent values were calculated as 0.5 and 1.0, respectively, showing ideal kinetics. An average value of kp /kt was 4.07 x 10 L-mol -s . Kinetic data and ESR studies indicated that the overall polymerization takes place by a radical mechanism via triplet carbene formation, which acts as the sources of the initiating radical. 

Sodium chloroaurate (NaAuCl4 2HaO) initiates polymerization of VCZ both thermally and photochemically. When the polymerization is conducted in nitrobenzene, photopolymerization under illumination of near ultraviolet to visible light proceeds with remarkable rapidity, whereas thermal polymerization starts after an induction period as shown in Fig. 8. This is a unique example of photoacceleration of cationic polymerization. Since the initiation mechanism is different from known cationic polymerization, the thermal system will be mentioned briefly before the discussion of the photochemical system. 

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. 

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. 

Gosh and Mukherjee reported that monochloroacetic, dichloroacetic and trichloroacetic acids readily initiate photopolymerization of methyl methacrylate at 40 °C in visible light when used in the presence of dimethylaniline. The inhibition period decreases with an increase in the number of chlorine substitution of acetic acid. Addition of benzophenone decreases the induction period. They believe that free-radical initiation is a result of a collapse of the exiplex (acid-dimethylaniline) and an electron transfer from the nitrogen to the carbonyl oxygen of the acid. ... 

This system is claimed to possess good thermal stability and high photochemical driving force. Upon irradiation, an electron transfer reaction between excited dye and iodonium salt leads to photo bleaching of the squarylium dye and generation of active radicals. The results obtained indicate that squarylium dye and iodonium salt systems will effectively initiate visible light photopolymerization and photocrosslinking of acrylic esters. ... 

The successful PROMP of cycloolefins, like 2-norbornene and dicyclopentadiene, with simple dialkyl-tungsten complexes by irradiation with UV- or visible light leads to the in situ formation of metal-carbenes by a Ha-abstraction reaction [20]. Metal carbenes have proven to be the active ROMP-initiators [21] and are supposed to be intermediates in the polymerization of substituted acetylenes as well [22]. Nevertheless, to the best of our knowledge, a one component catalytic system for the photopolymerization of mono- and disubstituted acetylenes has not yet been reported before our work [23]. 

A combination of benzophenone and 1,3-dioxane is a convenient hydrogen abstraction-type photoinitiator system for the free radical photopolymerization of methyl methacrylate, styrene and other monomers. As an advantage, this system does not require an additional hydrogen donor as other conventional initiators. In a similar way, mixtures of thioxanthone derivatives and fluorenes can be used as visible light absorbing oil- and water-soluble photoinitiators for free radical polymerization of methyl methacrylate, ethyl 2-(2-phosphonoethoxymethyl)acrylate and trimethylolpropane triacrylate. Photopolymerization and laser flash photolysis studies reveal that initiation occurs by intra- and intermolecular hydrogen abstraction by the thioxanthone-like triplet excited state. 

Chatterjee S, Gottschalk P, Davis PD, Schuster GB (1988) Electron-transfer reactions in cyanine borate ion pairs photopolymerization initiators sensitive to visible light. J Am Chem Soc 110 2326-2328... 

These PISs can initiate the photopolymerization of synthetic, as well as renewable, monomers/oligomers. A careful adaptation of the PISs to novel light sources avoiding the harmful mercury lamps emitting UVA and UVB or even UVC rays has been possible, and today, polychromatic visible light irradiation devices (Xe, Hg-Xe and doped lamps), quasi-monochromatic devices (LED arrangements) or monochromatic devices (laser diode arrays) can be safely used. 

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