Photoinitiators-Organometallic Compounds

Photoinitiation of Free Radical Polymerization by Organometallic Compounds... 

Different techniques have been used to study the products of photoreactions of organometallic compounds for example, irradiation of the arene complexes [CpFe() -arene)]+ resulted in the substitution of the arene by solvent or other potential ligands present in solution. In solutions containing an epoxide monomer, this photochemical reaction generated a species that initiated polymerization. Ion cyclotron resonance Fourier transform mass spectrometry and electrospray ionization mass spectrometry were used to elucidate the mechanism of these photoinitiated polymerizations. 

Organometallic compounds which can act as highly efficient photoinitiators for epoxide monomers have been described in the literature (25-27,69-71). The most commonly used organometallic compounds are the photoinitiators based on iron arene complex developed by the investigators at the Ciba-Geigy Corp. (37,69-71). These salts are generally prepared from ferrocene, as reviewed by Lohse and Zweifel (37). The molecular structure of a typical iron-arene complex is shown as (3). 

In addition to the above two classes of commonly used radical photoinitiators, a wide assortment of azo, sulfur, and heterocyclic compounds, organometallic compounds, and charge transfer complexes are known to generate radicals on irradiation with light. These systems are also occasionally used to initiate radical photopolymerizations and may have advantages in specific applications due to their long-wavelength absorption. 

Along with the photochemical generation of bases, Crivello and Dietliker have reviewed the field of photoinitiated anionic polymerizations. Until this point in time, there has been no report of the photochemical generation of carbanionic species equivalent to those obtained from organometallic compounds... 

Organometallic compounds have great potential as photoinitiators—many of them have satisfactory absorption characteristics and undergo photoin-duced chemical reactions, which may be utilized for initiation of polymerizations. However, the use of many organometallic compounds is hampered... 

Cafionic photoinitiation is based on the ring opening of the oxirane group. The photoinitiators of practical importance belong to three main classes of compounds diazonium salts, onium (e.g., iodonium and sulfonium) salts, and organometallic complexes, which upon irradiation by UV light decompose and yield an acid catalyst. 

By virtue of their absorption characteristics, many of the compounds listed in Table 10.4 can be employed in conjunction with visible light sources. As the research in organometallic chemistry gained momentum, the potential advantages of organometallic complexes as photoinitiators were also explored, and two such compounds, a ferrocenium salt and a titanocene, were commercialized (see Chart 10.3). 

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