Photopolymerization propagation step

Photopolymerization, in general, can be defined as the process whereby light is used to induce the conversion of monomer molecules to a polymer chain. One can distinguish between true photopolymerization and photoinitiation of polymerization processes. In the former, each chain propagation step involves a photochemical process [1,2] (i.e., photochemical chain lengthening process in which the absorption of light is indispensable for... 

A polymerization process requiring a photon for the propagation step is called photopolymerization or photoinduced polymerization, when polymerization of a monomer by a free radical or ionic chain reaction is initiated by photoexcitation. 

Photopolymerization Polymerization processes requiring a photon for the propagation step. 

If the bichromophoric system is suitably chosen to eliminate intramolecular processes and to ensure the possibility of inter-molecular reaction, a polymer can be formed. Where previously the term photopolymerization was used to Indicate photoinltiated chain processes (161), the present discussion of nonconjugated blchromophores is from a different point of view. The term "photopolymerization" as defined here, is a polymerization process in which every chain-propagating step Involves the absorption of a photon (162,163). This definition Implies that photopolymerization is a stepwise reaction, in contrast to photoinltiated polymerization, which Is a chain process. A sensitized photopolymerization is a photopol ymerlzation in which the reacting... 

Participation of Excited Species in Propagation Step in Photopolymerization... 

Other workers have reported the homopolymerization of certain substituted bismaleimides in solution by successive 2+2 cycloaddition reactions, and they have appropriately defined this type of process as a true photopolymerization i.e. a polymerization in which every chain-propagating step involves a photochemical reaction. Another such example is the solid-phase 2+2 cyclopolymerization of divinyl monomers.— By contrast, the polymers described above result from a combination of photocycloadditions and Diels-Alder cycloadditions. 

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. 

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

Photopolymerization of bisimides by a photocycloaddition mechanism has also been extensively studied [271,272]. The reaction is a real stepwise condensation process since every chain propagation step involves the absorption of a photon [273,274]. 

A variety of processes exist in which photographic images are produced as a result of a photoinitiated chain reaction. Such reactions, which are characterized by three fundamental steps (initiation, propagation and termination), provide the basis for certain processes in which photopolymerization, photocrosslinking or colour formation or destruction represents the main image forming stage. 

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 photopolymerization steps can thus be divided into two main parts the photochemical event that leads to the first monomer radical, the classical chemical propagation and termination processes of the reaction. The rate of a radical polymerization is defined by Equation 10.5, where kp and kt are the propagation and termination rate constants and 7abs the amount of light absorbed. 

Initiation. In both thermal- and photopol5unerizations, the rate of initiation depends on two processes the dissociation of the initiator and the initiation of the propagating chain. The decomposition rate Ua) of thermal initiators strongly depends on temperature, with the half-life of many thermal initiators at the reaction temperature on the order of minutes or hours. In contrast, for photopolymerizations, the rate at which photons are absorbed at a specific wavelength will determine the decomposition rate of photoinitiators. This process is not temperature-dependent. Thus, in the classic initiation mechanism, the interaction between light of a specific wavelength and a photoinitiator molecule is considered. For a unimolecular photoinitiator, this reaction step can be written as follows ... 

In an efficient photopolymerization system, the excited photoinitiator L-L must be sufficiently energetic and long-lived to decompose spontaneously or to interact with a second component to produce the active free radicals. Once formed, the free radical R- (here the imidazolyl radical L- reacts with the chain transfer agent, e.g., 2-mercaptobenzoxazole, the monomer M as in thermal polymerization, undergoing propagation, chain transfer, and termination steps ... 

In the case of free radical polymerization with chemical initiation, tenperature exerts an influence at all stages of the process, with the activation energy associated with initiator deconposition generally greater than that of either the propagation or termination steps. In the case of photopolymerization, the initiator dissociation rate is expected to depend primarily on UV light intensity rather than temperature. This is evident in the data for the plateau value of G shown in Figure 2, where an increase with temperature is observed primarily at the lowest UV intensities. 

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