Photoinitiators-radical analysis

Not enough is known for one to predict whether ionic or radical cleavage will occur. Many a-chloro and cc-bromo phenyl ketones are used as photoinitiators for polymerizations 52>, so they clearly produce radicals readily. Irradiation of chloroacetone in solution initiates the addition of CCI4 and thiols to olefins 197). Careful analysis of product structures suggests that only radical cleavage occurs. For example, in anisole the main product is orf o-methoxyphenylacetone. Radicals but not carbonium ions add preferentially ortho to monosubstituted benzenes. 

A number of nonsteady polymerization rate techniques can be used to measure ftp [11]. The most widely used method involves pulsed-laser-induced polymerization in the low monomer conversion regime. Briefly, a mixture of monomer and photoinitiator (Section 6.5.3) is illuminated by short laser pulses of about 10 ns (10 sec) duration. The radicals that are created by this burst of ligh propagate for about 1 sec before a second laser pulse produces another crop of radicals. Many of the initially formed radicals will be terminated by the short, mobile radicals created in the second illumination. Analysis of the number molecular weight distribution of the polymer produced permits the estimation of ftp from the relation... 

In the present ehapter we consider the inter- or intramolecular photoinduced electron transfer phenomenon. We mainly focus on photoinduced electron transfer processes that lead to the photoinitiation of polymerization, and on processes initiated by photoredueed or photooxidized excited states. We concentrate especially on a description of the kinetic schemes, a description of the reactions that follow the primary proeess of eleetron transfer, and the characteristics of intermediates formed after electron transfer. Understanding the complexity of the processes of photo-initiated polymerization requires a thorough analysis of the examples illustrating the meehanistie aspects of the formation of free radicals with the ability to start polymerization. 

For processes in which the rate of polymerization is not limited by the rate of electron transfer, the equation describing the rate of polymerization, one can obtain during the analysis of simple kinetics a scheme of photoinitiated polymerization. A mechanism describing a photoinitiated polymerization via PET (not considering the kinetics of free-radical formation, because this process does not affect the rates of polymerization), that contains all the major reaction steps, can be represented by Eqs. (34)-(39). 

Kinetic analysis gives additional information related to the reactivity of the free radicals obtained during the processes following a photoinduced electron transfer process. A variation in radical reactivity could be caused, for example, by their stability or their reactivity with monomer, as in the case of photoinitiation by 4-carboxybenzophenone-sulfur-containing carboxylic acid of free-radical polymerization [184]. 

The analysis of the diffusion-eontrolled features might be simplified by identifying the two types of free radieals the active and the trapped ones. Electron spin resonance speetroseopy shows that active (mobile) radicals give a 13-line spectrum and trapped (statie) radicals give a nine-line spectrum. Also, photopolymerization of a number of neat acrylate monomers used in polymer coatings for optical fiber was studied with photo DSC and with a cure monitor using a fluorescent probe. The acrylates had a functionality of one to six. It was found that conversion of monomers ranges from 40% to 100%. This, however, is depended upon functionality and structure of particular monomers. It can also be a function of the type and amount of the photoinitiator used. 

Up until this point, we have determined that the photoinitiated polymerization of the FIO monomer in the smectic phase is slower than in the isotropic phase when the continuoiis ouq>ut of a mercury lamp is us as the initiating light source. The consequences and origin of this rate phenomenon will be further explored with respect to dark polymerization in the smectic phase after the initiating light source is suddenly terminated. The results will provide direct evidence via exotherm and ESR analysis of the long-lived polymer radical chains in the smectic phase, as well as an estimate of the termination/propagation rate constant ratio in the smectic phase. 

Important insights regarding the interrelationship of exposure time and Iq have been reported for photoinitiated crosslinking (by radical polymerization) of highly functional acrylated resins. Based on simultaneous differential scanning calorimetry (DSC) and thermomechanical analysis (TMA), these elegant studies demonstrate that percent conversions are limited by rapid formation of highly crosslinked domains with immobilized radicals, the reactivity of which is essentially unaffected by continued irradiation. Conversions increased with temperature, as expected for thermal mobilization of the trapped radicals. 

The aqueous polymerization of methyl methacrylate initiated by the potassium trioxalate cobaltate (II) complex was studied by Guha and Palit [190]. At a relatively higher concentration (>0.001 mol L ), this compound can initiate aqueous polymerization of methyl methacrylate in the dark at room temperature. The complex is highly photosensitive, which can photoinitiate polymerization. A detailed end-group analysis of the obtained polymers indicated that carboxyl and hydroxyl radicals, which are from the decomposition of the photoexcited complex, are the initiating species. 

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