Photoinitiation processes

Note that the initiator decomposition makes the largest contribution to E therefore photoinitiated processes display a considerably lower temperature dependence for the rate of polymerization. 

Not all initiating radicals (/ ) succeed in initiating polymerization, recombination of these radicals in the solvent can decrease the efficiency (/) to a value lower than 1. Detailed kinetic treatment of photoinitiation processes are discussed by Oster and Yang [3]. 

Both R and MMA radicals are found to be responsible for the photoinitiation process. Chaturvedi and coworkers [54,55] introduced phenyl dimethyl sulfonium-ylide cupric chloride and chromium thiophene carboxylate as the photoinitiator of styrene and MMA. No reaction mechanism was given for these systems. 

Since most of the dental applications employ a photoinitiated process (visible or ultraviolet light), the rate of initiation may be expressed as [92]... 

These photoinitiation processes which depend on the formation of free radicals in some photochemical reaction lead to chain reactions, since each molecule of initiator can promote the addition of many monomer units to a polymer chain. The quantum yield of monomer addition can therefore be much larger than unity, but it cannot be controlled since the growth of a polymer chain is then limited by termination reactions in which two free radicals react to produce closed-shell molecules. 

Sulfinates. Arylsulfinate anions were shown to be effective activators for photoreducible dye polymerization systems by Hiller, Margerum, and Rust (41). The work is described with applications in a number of patent publications (20,41a,42). These workers also studied the mechanism of the photoinitiation process (43). 

Zn(mnt)2]2 as a photocatalyst for hydrogen production from water was described briefly in Section HA. The actual photocatalyst is believed to be zinc sulfide formed from the metal dithiolene in a photoinitiation process (75, 76). 

Parallel to the photoinitiation processes (with hv) photoaddition processes are observed, as shown for example in the Figs. 4, 5, 11 to 13. After dimer initiation, trimer formation from the dimer is possible etc. The chain propagation within the DR or AC series is performed by photoaddition of monomer molecules M adjacent to the reaction centres, given by the dimer (DR2 or AC2), trimer (DR3 or AC3),... molecules. The molecules M are lowered in energy by the perturbation introduced by the reaction centres. They form a trap for the optical excitation energy. They may be excited directly (M + hv -> M ) or indirectly via nonperturbed monomer molecules (M -f- hv -> M ) and subsequent energy transfer (M + M M + M ). The chain propagation reaction therefore is in competition with the chain initiation reaction. 

Photoinitiated and optical electron-transfer processes and their relationship to corresponding ground-state thermal processes provide important new tests of theory, especially when comparisons are made for a given DBA system, of charge separation (CS) and charge recombination (CR), or of thermal and optical electron transfer (e.g., [27]). Photoinitiated processes have also been valuable in providing access to the dynamics of electron transfer in the activationless and inverted kinetic regimes (e.g., [43, 44]). 

Section 4.4) on the photoinitiation process, one can anticipate that under certain conditions (identical free radicals formed), the rules regulating the primary processes can also be applied for the secondary processes. The results presented in Figure 8 confirm this expectation. It is clear from the data (Figure 8) that the rate of polymerization as initiated by the series of cyanine borates in Table 2 increases as the driving force of the electron transfer increases. This behavior is predicted by the classical theory of photoinduced electron transfer. 

All of these chromophores absorb beyond 290 nm, but are present in PP at low concentrations (<5xl0 M for >C=0 and -OOH, <1x10 for Ti). The key photoinitiation process in a given PP sample will depend on production and processing conditions as well as the subsequent history of the article ( ). 

Not only radical ions, generated via electron transfer from a photoinitiator (Scheme 6.275), but also photoproduced ions or radicals can be the key intermediates in the photoinitiation processes. Today, the term photoinitiator is mostly connected with photoinitiated (chain) polymerization reactions,1134,1470,1471 in which the reactive intermediates are generated from relatively small amounts of an excited initiator (initiation step) to start a chain reaction (propagation steps). Initiation of polymerization by light, rather than by heat, has several advantages, such as high reaction rates at ambient temperature and spatial control of the process.1492... 

Radiative and nonradiative relaxation processes (characterized by a first-order rate constant k) arise from the S i and T states according to (10.68) and compete with the photoinitiation process as already shown in Scheme 10.1 ... 

Mechanism (27) distinguishes fast photoinitiated processes, viz. (a) and (b) leading to vibrationally or electronically excited O from the slower thermally assisted desorption in step (c) and from the recapture of electrons by 02, which together would account for t 100 ms and low 0. 

The acetylacetonate complexes of cobalt(II) and manganese(111) are efficient catalysts for the thermally intiated oxidation of tetralin, but do not influence the photoinitiated process. The reverse situation is observed for the iron(III) and cobalt(III) complexes [70a]. The thermal oxidation can be influenced by the addition of free-radical initiators like t-butyl hydroperoxide or 2,2 -azobisisobutyronitrile [70b]. 

The process of free radical polymerization can be divided into three steps initiation, propagation, and termination. Fig. 1 shows a simplified scheme of the photoinitiation process. The photoinitiator (PI) absorbs radiation over the range of 250-450 nm, resulting in the formation of the electronically excited state photoinitiator (PI ). As most photoinitiators are derivatives of aromatic ketones, the active excited state is generally the triplet state. The details of this photochemistry will not be reviewed in this paper as Pappas and McGinniss have described the nature of the excited states involved during photoinitiation (2). 

Many other examples can be sited. Following are some basic kinetic considerations of the reactions of photoinitiators and photoinitiating processes. Based on Beer s Law, the fi action of absorbed light by a solution (or a light curable composition) can be expressed as follows ... 

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