Photoinitiators organometallic

Two types of organometallic photoinitiators for free radical vinyl polymerization are considered (1) transi-... 

It is clear from the preceding discussion that organometallic photoinitiators (metal carbonyl or chelate derivatives) can provide a convenient route for synthesizing vinyl polymers with a variety of different reactive end group or photoreactive pendant groups or side chains through the polymer chain. 

For this reaction an organometallic photoinitiator was used that absorbs at 532 nm. Photocrosslinking of the oriented polymer can be used to produce a patterned material that exhibits a stable second-order nonlinear optics signal that lasts for several weeks at 100 °C. Addition of phenol is necessary to prevent thermal crosslinking. 

Other Onium Salts and Organometallic Photoinitiators. The success of the iodonium and sulfonium salts as photoinitiators has led to the investigation of a number of analogous onium salts based on the halides and the Group VIA atoms however, these alternative initiators have not been widely used for various reasons. For example, chloronium and bromonium salts were prepared (57,58) and they were also found to function as cationic photoinitiators, but these salts are difficult to prepare and they have low thermal stability. Similarly, triarylselenon-ium salts have also been investigated and foimd to function as cationic initiators (59) however their preparation has been foimd to be expensive (60). Other onium salts such as phosphonium and arsonium salts, developed by Abu-Abdoun and co-workers for the photopolymerization ofp-methylstyrene and styrene, have also been reported (61-63) as successful cationic photoinitiators. Photopolymerization of carbazolyloxiranes with sulfonium and tropylium salts has been reported (64). Dialkylphenacyl sulfonium photoinitiator (65-67) has been reported with excellent solubility in both polar and nonpolar monomers. Pyridinium and isoquino-linium salts have also been reported and they were found useful for polymerizing both the epoxide and vinyl ether monomers (68). 

Kutal, C. (2001) Generation of bases and anions from inorganic and organometallic photoinitiators. Coord. Chem. Rev., 211, 353. 

Photoinitiation of Free Radical Polymerization by Organometallic Compounds... 

The most important practical application of the organometallic complex photoinitiators is the possibility of using these types of initiators in modifying the pre-existing polymer chain, e.g., block, graft, and crosslinked copolymers preparation. 

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. 

An atom transfer chain mechanism very similar to that in Scheme 6 is thought to occur in some photoinitiated CO substitution reactions of organometallic halide complexes. For example, [Cp2Mo2(CO)6] promotes the photosubstitution of CO by PR3 in [CpMo(CO)3X] (X = Br, I),"9 and... 

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. 

Transition metal organometallic complexes like dicarbonyl cyclopentadienyl iron [128], tricarbonyl cyclopentadienyl manganese [129] and iron-arene complexes [130,131] have also been reported as photoinitiators for photochemical crosslinking of cyanate esters. Photosubstitution of carbonyl groups by -OCN during irradiation initiates the reaction in the former case whereas photochemical dissociation of arene triggers it in the latter system. 

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). 

Organometallic molecules have been used as photoinitiators in cationic, anionic, and radical (chain) polymerization reactions. Examples of each type abound, but only selected examples will be discussed here because the fundamental principle is the same in each case irradiation is used to generate an intermediate species (an ion or a radical species) that can initiate a polymerization reaction. 

Cationic photoinitiation is generally based on a ring-opening reaction of an oxirane group (see Scheme 6). There are three main classes of cationic photoinitiator molecules, namely diazonium salts, onium salts, and organometallic complexes. When photopolymerization of epoxides is considered, the use of diazonium salts is limited due to stability issues. In addition, they release nitrogen gas that forms bubbles and pinholes in the coatings. Onium salts... 

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