Titanocenes photoinitiators

Thickness of the laminar layer is deterrnined both by the need to reproduce fine detail in the object and by the penetration depth of the actinic laser light into the monomer bath (21,76). There is thus a trade-off between precision of detail in the model and time required for stereohthography, ie, the number of layers that have to be written, and an optimum Light-absorbing initiator concentration in the monomer bath corresponding to the chosen layer thickness. Titanocene-based initiators, eg, bis-perfluorophenyltitanocene has been recommended for this apphcation (77). Mechanistic aspects of the photochemistry of titanocenes and mechanisms of photoinitiation have been reviewed (76). 

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). 

Scheme 10.6 Photoinitiation of the free radical polymerization of an alkyl acrylate with the aid of a fluorinated titanocene [47].

The fluorinated titanocene presented in Chart 10.3 is a very effective photoinitiator that functions without a co-initiator when irradiated with visible light. It is thermally stable (decomposition at 230 °C) and absorbs light up to 560 nm with maxima at 405 and 480 nm [2]. According to mechanistic studies, the com-... 

Superieure de Chimie, Mulhouse, Fr. J. Photopolym. Sci. Technol. (2000), 13 (2), 237-242 (Eng.). The interactions of excited states of a coumarin or a ketocoumarin photosensitizer with a bisimi-dazole deriv., mercaptobenzoxazole photoinitiators, and titanocene were studied by laser absorption spectroscopy and by photocond. The redox potential of the compds. was measured and used in calcn. of the free enthalpy change for a possible electron transfer reaction. The coumarin forms radicals through an electron transfer reaction, while the ketocoumarin undergoes an energy transfer reaction with bisimidazole and a hydrogen abstraction reaction with a benzoxazole deriv. Thus, coumarin or ketocoumarin/free radical initiator combinations are suitable as initiators of radical polymn. reactions, esp. those applicable to laser imaging. [1707-68-2]. 

Two and Three Component Photoinitiating Systems Based on Coumarin Derivatives. AUonas, X. Fouassier, J.P. Kaji, M. Miyasaka, M. Hidaka, T. Departement de Photochimie Genetale, UMR No. 7525, Ecole Nationale Superieure de Chimie, Mulhouse, Fr. Polymer (2001), 42 (18), 7627-7634 (Eng.). Several coumarin or ketocoumarin/additives combinations (bisimidazole deriv., mercapto-benzoxazole, titanocene, oxime ester) can initiate quite efficiently radical polymn. The interactions between the excited states of coumarins or aketocoumarin and various additives have also been studied by laser absorption spectroscopy, time resolved fluorescence and photocond. the redox potentials of these different compds. have been measured and allowed the calcn. of free enthalpy variations for a possible electron transfer reaction. The whole results explain the interaction mechanism and show that the coumarins are able to form radicals through an electron transfer reaction with the different additives whereas the ketocoumarin leads to an energy transfer with bisimidazole and to an hydrogen abstraction with the benzoxazole deriv. 

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