Photoinitiation of ionic polymerizations

W.3 Photoinitiation of ionic polymerizations 289 Table 10.8 Chemical structures of typical cationic photoinitiators [2, 27, 52, 53]. 

W. Schnabel, Photoinitiation of Ionic Polymerizations, Chapter 7 in N. S. Allen, M. Edge, I.R. Bellobono, E. Sell (eds.), Current Trends in Polymer Photochemistry, Horwood, New York (1995). 

Kutal et reviewed the chemistry of several iron (II) metallocenes that are effective photoinitiators for ionic polymerization reactions. Photoexcitation of ferrocene and 1,1-dibenzoyl -ferrocenes in solutions of ethyl-a-cyanoacrylate produces anionic species that initiate the polymerization of electrophilic monomers. Irradiation of CsHs-Fe (t] - arene) in epoxide containing media generates several cationic species capable of initiating ringopening polymerizations. It was concluded that iron(II) metallocenes exhibit a diversity of photoinitiation mechanisms. 

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,... 

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. 

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. 

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... 

Recently, benzophenone-based initiators with hydrogen donating amine moieties covalently attached via an alkyl spacer were introduced as photoinitiators for vinyl polymerization [101,126-130] (see 1, Table 10). Although also following the general scheme of lype II initiators, the initiation is a monomolecular reaction, as both reactive sites are at the same molecule. Hydrogen transfer is suspected to be an intramolecular reaction. The ionic derivatives (2 and 3) shown in Table 10 are used for polymerization in the aqueous phase [131-133]. With 4,4 -diphenoxybenzophenone (4 in Table 10) in conjunction with tertiary amines, polymerization rates that are by factor of 8 higher than for benzophenone were obtained [134]. 

Free-radical polymerization of vinyl monomers takes place through intermediates having an unpaired electron known as free radicals. Many vinyl monomers are readily polymerized by free-radical mechanisms because free-radical polymerization is relatively less sensitive to impurities compared to ionic polymerizations. Free radicals can be generated in a number of ways, including organic or inorganic initiators and even without added initiators (e.g., thermal and photoinitiation). There are over 50 different organic peroxides and azo initiators in over 100 different formulations produced commercially. Initiators are selected based on several factors polymerization rate, reaction temperature, solubility, and polymer properties. 

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. 

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. 

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. 

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. 

Photoinitiators are classified according to the type of photopolymerization system they initiate (i.e., radical or ionic). The basic photochemical routes that produce radicals are photocleavage (type I), intermolecular hydrogen abstraction (type II), and electron transfer followed by proton transfer. Schematics of these photochemical routes are given in Scheme 5. The efficiency of these routes directly determines the monomer conversion, molecular weight of the polymer, and degree of polymerization, and hence the structural, physical, and mechanical properties of the final product. The general relationships of Pp, Pi, and DP were provided in the previous section. 

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. 

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