Ionic photoinitiators

Most of the ionic photoinitiators were developed for use in cationic polymerizations due to practical considerations. These considerations are based on the fact that the anionic photoinitiators are more sensitive to oxygen inhibition than are even the free-radical ones. They are also sensitive to moisture. Some anionic photoinitiators, however, were described in the literature. None appear to be utilized commercially. 

The block copolymer produced by Bamford s metal carbonyl/halide-terminated polymers photoinitiating systems are, therefore, more versatile than those based on anionic polymerization, since a wide range of monomers may be incorporated into the block. Although the mean block length is controllable through the parameters that normally determine the mean kinetic chain length in a free radical polymerization, the molecular weight distributions are, of course, much broader than with ionic polymerization and the polymers are, therefore, less well defined,... 

The radiolysis of olefinic monomers results in the formation of cations, anions, and free radicals as described above. It is then possible for these species to initiate chain polymerizations. Whether a polymerization is initiated by the radicals, cations, or anions depends on the monomer and reaction conditions. Most radiation polymerizations are radical polymerizations, especially at higher temperatures where ionic species are not stable and dissociate to yield radicals. Radiolytic initiation can also be achieved using initiators, like those used in thermally initiated and photoinitiated polymerizations, which undergo decomposition on irradiation. 

This reaction is based on a stoichiometric reaction of multifunctional olefins (enes) with thiols. The addition reaction can be initiated thermally, pho-tochemically, and by electron beam and radical or ionic mechanism. Thiyl radicals can be generated by the reaction of an excited carbonyl compound (usually in its triplet state) with a thiol or via radicals, such as benzoyl radicals from a type I photoinitiator, reacting with the thiol. The thiyl radicals add to olefins, and this is the basis of the polymerization process. The addition of a dithiol to a diolefin yields linear polymer, higher-functionality thiols and alkenes form cross-linked systems. 

As pointed out in Section 4.2.2, cationic polymerization processes are initiated by photoinitiators, which are essentially precursors generating Lewis and Bronsted acids. The mechanism of the process is ionic, and this chemistry does not function with the type of double bonds and unsaturation found in fhe monomers and oligomers reacting via free radical mechanism. 

In the simultaneous method, which is the one most commonly used, the substrate is irradiated while in direct contact with the monomer. The monomer can be present as a vapor, liquid, or solution. This grafting process can occur via free radial or ionic mechanism. With the simultaneous method, the formation of homopol5mier is unavoidable, but there are several systems to minimize it. The advantage of this method is that both monomer and substrate are exposed to the radiation source and both form reactive sites. The other two techniques rely upon rupture of the bond to form reactive sites, and therefore require higher radiation doses. Thus, the simultaneous method is more suitable for substrates sensitive to radiation. The simultaneous method can utilize UV radiation besides EB source. Logically, the UV irradiation requires a photoinitiator or sensitizer to achieve an acceptable level of grafting. 

Photopolymerization induced by donor-acceptor interaction has several characteristic differences from conventional photopolymerization. Firstly, the initiation is very selective. Appropriate strength of donor and acceptor is essential since the CT interaction might bring about spontaneous thermal polymerization if it is too strong. Although most charge transfer processes must be photosensitive, practically important systems are limited to those which conduct thermal reactions with negligible rates. The photopolymerization of MMA by triphenyl-phosphine should be called photoacceleration rather than photoinitiation since the rate of spontaneous photopolymerization of MMA is about half of that of polymerization photosensitized by 4 x 10 4 M of triphenyl-phosphine. Secondly, an ionic mechanism is expected. Thirdly, when both donor and acceptor are polymerizable monomers, the polymerization mixture is entirely solid and clean after polymerization. There is no initiator and no solvent. 

Commercially, the most elegant use of ionic liquids, perhaps discovered serendip-itously,51 is the BASF application for acid scavenging in their manufacture of the photoinitiator intermediate diethoxyphenylphosphine.52... 

BASF s BASIL process [15] and the Dimersol process [16] have both been commercialized. The former uses the ionic liquid as a phase transfer catalyst to produce alkoxyphenylphosphines which are precursors for the synthesis of photoinitiators used in printing inks and wood coatings. The imidazole acts as a proton scavenger in the reaction of phenyl-chlorophosphines with alcohols to produce phosphines. The Dimersol process uses a Lewis acid catalyst for the dimerization of butenes to produce Cs olefins which are usually further hydroformylated giving C9 alcohols used in the manufacture of plasticizers. Several other processes are also at the pilot plant scale and some ionic liquids are used commercially as additive e.g. binders in paints. 

The main requirements of a solvent to be utilized in SRN1 reactions are that (1) it should dissolve both the organic substrate and the ionic alkali metal salt of the nucleophile (2) it should not have hydrogen atoms that can be readily abstracted by aryl radicals (3) there must not be protons that can be ionized by the bases or the basic nucleophiles and radical anions involved in the reaction and (4) it should not undergo ET reactions with the various intermediates of the reaction. In addition to these requirements, the solvent should not absorb significantly in the wavelength range normally used in photoinitiated reactions (300-400 nm). 

Cationic cure mechanisms are an alternative approach to uv curing. This involves the photogeneration of ions, which initiate ionic polymerization. This process is not subject to oxygen inhibition, as are some of the free radical mechanisms. Cationic cure mechanisms generally also provide less shrinkage and improved adhesion. The disadvantages are that the photoinitiators are sensitive to moisture and other basic materials. The acidic species can also promote corrosion. As a result, the vast majority of uv formulations are acrylate-based and cure by a free radical mechanism. 

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. 

Hydroxytelechelic polymers can be synthesized via a photoinitiated radical process 49,50 76 77). This reaction resembles that of the redox system because an electron transfer mechanism is operative and the synthesis is carried out in aqueous solution. The reactive species is a complex ion such as Fe3+, X (OH-, Cl-, N". ..). The light absorption (hv) by the ionic species results in an electron transfer reducing the cation oxidation of the anion leads to a free radical X which initiates the polymerization. 

Metal salts and complexes continue to attract interest as radical/ionic initiators. Trisoxalatoferrate/amine anion salts have been studied as initiators of the polymerization of acrylamide. Here the anion salts react with photolytically formed COa " radicals by an electron transfer mechanism to give photoactive initiating phenyl radicals by the set of reactions shown in Scheme 9. Ferric o-phenanthroline has been shown to be a good photoinitiator for... 

Schuster and co-workers discovered that 1,4-dicyanonaphthalene solutions containing an alkyltriphenylborate salt, when irradiated, yield one-eleetron oxidation of the alkyltriphenylborate leading to carbon-boron bond cleavage and formation of free alkyl radicals [23]. In Gottschalk s hands [24, 25], it was shown that ionic salt pairs formed from cyanine dyes and alkyltriphenylborates (Figure 1) could be used as photoinitiators [26] that were active in the visible region of the spectrum. 

Figure 1. Ionic salt pairs active as photoinitiators in the visible region.

Recently a series of publications by Tazuke and co-workers 28.29,30) have disclosed photopolymerization systems in which ionic or charge transfer species act as the photoinitiators. It is interesting to note that the presence of oxygen in such systems causes less inhibition or retardation than in radical-type photopolymerizations. Donor-acceptor pairs such as vinylcarbazole and sodium aurochloride dihydrate typify the system ... 

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