Photoinitiators bimolecular

Rakitzis T P, Kandel S A and Zare R N 1997 Determination of differential-cross-section moments from polarization-dependent product velocity distributions of photoinitiated bimolecular reactions J. Chem. Phys. 107 9382-91... 

The polymerization reaction was found to develop both faster and more extensively as IQ was increased, up to a certain value above which identical RTIR curves were recorded. Consequently, the (Rp)max value reaches an upper limit, as shown in Figure 5 where (Rp)max was plotted versus Iq on a logarithmic scale. The slope of the straight line obtained at low light intensities, 0.55, is close to the 0.5 value expected for a photoinitiated radical polymerization involving bimolecular termination reactions. 

Photopolymerization. In many cases polymerization is initiated by irradiation of a sensitizer with ultraviolet or visible light. The excited state of the sensitizer may dissociate directly to form active free radicals, or it may first undergo a bimolecular electron-transfer reaction, the products of which initiate polymerization (14). Triphenylalkylborate salts of polymethines such as (23) are 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). 

Perhydroxyl radical, 75 thermal generation from PNA of, 75 Peroxy radical generation, 75 Peroxide crystal photoinitiated reactions, 310 acetyl benzoyl peroxide (ABP), 311 radical pairs in, 311, 313 stress generated in, 313 diundecanyl peroxide (UP), 313 derivatives of, 317 EPR reaction scheme for, 313 IR reaction scheme for, 316 zero field splitting of, 313 Peorxyacetyl nitrate (PAN), 71, 96 CH3C(0)00 radical from, 96 ethane oxidation formation of, 96 IR spectroscopy detection of, 71, 96 perhydroxyl radical formation of, 96 synthesis of, 97 Peroxyalkyl nitrates, 83 IR absorption spectra of, 83 preparation of, 85 Peroxymethyl reactions, 82 Photochemical mechanisms in crystals, 283 atomic trajectories in, 283 Beer s law and, 294 bimolecular processes in, 291 concepts of, 283... 

In the case of photoinitiation of radical formation the generic oxidant, [Ox], is the excited state of the dye D+, which is reduced during the process to the relatively stable radical for D- which is not an initiator, but which probably disproportionates in a bimolecular process. 

Type II photoinitiators undergo a bimolecular reaction where the excited state of the photoinitiator (acting as a photosensitizer)... 

The photoinitiation reaction (phi) is defined by the reaction Eq. (9). The dimer generation rates G2 of the diradical and asymmetric carbene dimer molecules are therefore given by the bimolecular rate equations... 

Mixtures of photoinitiators have been actively studied. Michler s Ketone and benzoyl peroxide have been shown to effectively induce the photopolymerisation of methyl methacrylate through the formation of an initial complex shown in scheme 3 7, Although the exact initiating radical does not appear to be ascertained it is almost certainly the arylalkylamino radical from the Michler s Ketone. In the interaction of benzil and thioxanthone with triethylamine in the photoinduced polymerisation of acrylic monomers their is a competition between reverse electron transfer and ketyl radical formation . As the carbonyl concentration increases the bimolecular termination rates due to radical recombination increases. The same workers also studied the same system but replaced the ketone initiators with pyrene . Their inability to identify pyrene end groups indicated that the active initiating species arise from a complex between the pyrene and the triethylamine. 

Photoinitiated SET from an electron rich aromatic such as 1,5-dimethoxy-naphthalene (DMN) to /-butyldiphenyl(phenylseleno)silane yields the corresponding radical anion (435), mesolysis of which generates the phenylselenyl anion (436) and the /-butyldiphenylsilyl radical (437). This Se-Si system has been utilised synthetically in bimolecular group transfer radical processes. For example irradiation of a solution containing /-butyldiphenyl(phenylseleno)silane, the bromo ester (438), DMN and ascorbic acid (added as sacrificial electron donor to regenerate DMN from its radical cation) results in formation of the cyclisation product (441) in 75% yield via the intermediacy of radicals (439) and (442). ... 

FIGURE 23 Experimentally determined bimolecular reaction rate constants for H + CO2 reaction photoinitiated in HI CO2 van der Waals complexes. Solid line denotes calculated RRKM decomposition rate for HOCO reaction intermediates. (From Ionov, S. I., Brucker, G. A., Jaques, C., Valachovic, L., and Wittig, C. (1992). J. Chem. Phys. 97, 9486-9489.)... 

Equation (3.9) has been derived on the assumption of bimolecular chain termination. Furthermore, this equation does not take into account the presence of gradients of photoinitiator and monomer concentration, and conversion depth on the layer of a photocomposition. These gradients are caused by the illumination gradient. That is why the presented equation will be true for the description of the process under ideal conditions. 

Bimolecular Photoinitiator Systems. Bimoiecular photoinitiators are so-called because two molecular species are needed to form the propagating radical a photoinitiator that absorbs the light and a co-initiator that serves as a hydrogen or electron donor. Photoinitiator families include benzophenone derivatives, thioxanthones, camphorquinones, benzyls, and ketocoumarins (5-9) (3). 

Water-Soluble Photolnitlators. Water-soluble photoinitiators are needed for systems snch as printing inks and emulsion processes, where the reaction system is an aqneons solution rather than simply a monomer. Since free radical photoinitiators are based on the aromatic benzoyl chromophore, the molecules are generally nonpolar and therefore incompatible with water. Research has fo-cnsed on developing photoinitiators with hydrophilic substituents that increase water solnbility (3,25). Both unimolecular and bimolecular systems have been suc-cessfnlly demonstrated. An example of a imimolecular photoinitiator is sodium 4-benzoylbenzenemethane sulfonate (13). 

Autoacceleration, where the rate of polymerization increases with conversion in isothermal conditions, is observed in both thermal- and photoinitiated free-radical polymerizations because the termination mechanisms are the same for both. As the chains grow longer, it becomes more difficult for the active centers to diffuse and imdergo bimolecular termination thus, termination frequency decreases and active centers at the chain ends can become trapped. In cases where termination is controlled by diffusion, the pseudo-steady-state assumption is no longer valid and chain length dependent termination (CLDT) may occur (67). As is discussed for chain cross-linking photopolymerizations below, more complicated kinetic treatments must then be considered, including unsteady-state kinetics. 

That is, the photooxidation rate increases as the square root of the photoinitiation rate, because the chain length of the oxidation is controlled by the bimolecular termination of peroxy radicals. As the exposure proceeds, the hydroperoxide concentration increases, eventually approaching a relatively constant level. For a stationary hydroperoxide concentration, the photooxidation rate is equal to ... 

An example of a bimolecular photoinitiator that generates free radicals by a hydrogen abstraction process is the combination of camphorquinone with a benzyl alcohol (equation [2.20]) [CRI 08], The carbon-centered free radical, 57, is capable of reducing a diaryliodonium salt in a similar manner to that shown previously in Diagram 2.5. 

Norrish type II photoinitiators are bimolecular initiators. Generally an aromatic ketone is used in combination with a tertiary amine. Both aliphatic and aromatic tertiary amines can be used. A well-known example of such an initiating system is benzophenone with dimethylaminoethanol. 

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