An investigation of the photoinitiated cationic ring-opening polymerizations of various epoxy
monomers was conducted using real-time infrared spectroscopy and optical pyrometry. Several types of
behavior were observed and interpreted on the basis of the relative stabilities of the secondary and tertiary
oxonium ion intermediates that are generated during the ring-opening polymerization of these monomers.
Based on the mechanistic interpretations proposed, attempts were made to accelerate the photopolymerizations of epoxy monomers that undergo overall slow reaction as a result of the presence of an
induction period. Three general methods were effective for the acceleration of the photopolymerizations
of these and other epoxy monomers: (1) carrying out the photopolymerizations at higher temperatures,
(2) copolymerization with more reactive monomers, and (3) the use of free radical photoinitiators.
Curcumin, a naturally occurring, intensely yellow dye extracted from the spice turmeric, is an efficient photosensitizer for diaryliodonium salt photoinitiators at wavelengths ranging from 340 to 535 nm. With curcumin as a photosensitizer, it is possible to carry out the cationic photopolymerization of a wide variety of epoxide, oxetane, and vinyl monomers with long-wavelength UV and visible light. An example of the photopolymerization of an epoxide monomer with ambient solar irradiation is provided. Several other curcumin analogues were synthesized, and their use as photosensitizers is examined. With such photosensitizers, the range of spectral sensitivity can be extended well into the visible region of the electromagnetic spectrum. V V C 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 5217-5231, 2005
Summary: Curcumin, an intensely yellow dye isolated from Indian turmeric is an excellent electron-transfer photosensitizer for cationic vinyl and ring-opening polymerizations. Especially attractive features of curcumin are its strong absorption, broad spectral sensitivity, efficient electron-transfer, low toxicity and low cost. Using curcumin as a photosensitizer, the cationic photopolymerizations of a variety of monomers were carried out including such naturally derived monomers as epoxidized terpenes and epoxidized vegetable oils. The impact of this technology on applications such as housing, boat-building and water distribution is discussed.
The photoinitiated cationic ring-opening polymerizations of 3,3-disubstituted oxetanes display a characteristic induction period followed by very rapid thermally accelerated polymerization. When a thin-film sample of such a monomer is irradiated for a time within the induction period and then allowed to stand at room temperature, no appreciable further conversion of monomer to polymer takes place. However, when heat is applied to a small portion of the films rapid polymerization takes place as a front, which propagates rapidly throughout the entire reaction mass. To characterize these frontal polymerizations, a new monitoring technique, employing optical pyrometry has been instituted. This method provides a simple, rapid means of following these polymerizations and quantitatively determining their frontal velocities.
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