Photopolymerization singlet

With ketones in the long-lived singlet state (e.g. TX, Ac, MAc), the singlet charge-transfer complex generated by amine quenching is expressed mainly as a back electron transfer and deactivation rather than proton transfer and initiation. Therefore, such a reaction course is a loss in efficiency of the photopolymerization. 

The photopolymerization of the polymers studied is influenced by the sensitizer as well as the intensity of light used. Aromatic hydrocarbons, like anthracene and tetracene, are used as singlet sensitizers whereas 1-chlorothioxanthone (CTX) is a triplet sensitizer. From the comparison of the polymerization rates of the systems studied as a function of sensitizer it follows that there is a dependence on the type of sensitizer. As seen from Fig. 2, CTX as triplet sensitizer has been found to be the best sensitizer for vinyl ether systems. 

Photochemical generation of the radical cations derived from A -vinylcar-bazole/acceptor charge-transfer complexes and subsequent polymerization is well known. Perhaps somewhat more interesting are the cationic photopolymerizations of styrene and a-methylstyrene. With these monomers of relatively weak electron donor character photolysis of the charge-transfer complexes formed with tetracyanobenzene and pyromellitic dianhydride produces monomer radical cation species from both singlet and triplet states, and the photophysics of the primary processes have been elucidated in some detail. ... 

In actual photopolymerization, each individual propagation step is photochemically activated. Here, a reactive ground state or an excited singlet or triplet state may react. 

Singlet states are involved in the photopolymerization of anthracene derivatives, which represents a (47t + 47t) cycloaddition ... 

Valderas et al., also studied the photopolymerization of methyl methacrylate initiated by 2-chlorothioxanthone in the presence of various amines of different structures. Here too, the photoinitiation efficiency of these systems was found to be highly dependent on the structure of the amine. The polymerization rate increases with the amine concentration and reaches a constant value at an amine concentration range of 10—30 mm. At these amine concentrations, aliphatic hydroxyaUcyl amines are more efficient photoinitiators than the corresponding trialkyl-substituted compounds. Dimethylanilines with electron acceptor substituents in the 4-position give higher polymerization rates than electron donor substituted anilines. Their data also show that the singlet and triplet excited states of thioxanthones are efficiently deactivated by the amines. Rate constants correlate well with the oxidation potentials of the amines. The effects of the chemical structure of the amine on the polymerization rates of 2-... 

Oxygen inhibits free radical polymerization through two pathways. First, an oxygen molecule can quench the active triplet state of the photoinitiator and be excited to singlet state itself The seeond path is scavenging of the photoinitiator and polymer radicals through oxidation with the yield of peroxy radicals. The overall chain reactions of photopolymerization with the presence of O2 is schematically shown as follows... 

In order to produce free radicals that initiate polymerization, photoinitiators absorb light of a certain frequency. Upon absorption, the photoinitiator molecule is promoted from the ground electronic state to either a singlet or triplet excited electronic state. This excited molecule then undergoes either cleavage or reaction with another molecule to produce initiating free radicals. Numerous photoinitiators have been developed to meet the needs of a variety of photopolymerization systems, as described in a number of recent papers and reviews (3-6,8-12). 

The mechanism of photopolymerization in many application, generally involves the light-induced activation of a sensitizer material, such as an aromatic ketone, for example, Michler s ketone or a polyaromatic, to its singlet state, followed by crossover to form its triplet-excited state, followed by energy transfer to an initiator. The activated initiator usually splits into two radicals, one or both of which react witir tire monomer causing polymerization to occur. Norrish type I and type II reaction mechanisms are examples of this science [3]. Acrylates and methacrylates are commonly the monomers of choice because of their high polymerization rate, low cost, and acceptable toxicity (Figure 7.5). Additionally, acrylates and methacrylates... 

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