Photo-crosslinking reaction

Shindo Y, Hasegawa M, Sonobe Y, Sugimura T (1994) Photo-crosslinking reaction of polydimethylsiloxane/benzophenone-containing polyimides in solution. J Photopol-ym Sci Technol 7(2) 285... 

The mechanism of pattern formation in photo-crosslinked polymer blends is not completely understood. Besides the competition between phase separation and photo-crosslinking reactions, elastic stress in the sample due to network formation, as well as an autocatalytic feedback arising from concentration fluctuations amplified by the crosslinking reaction, may play an important role in these ordering processes. 

Instead of using thermal energy to trigger the hydrogen abstraction mechanism, photo-induced reactions can be also be used to successfully crosslink acrylic PSAs [74-76], In this case, photoactive compounds, such as for example those containing benzophenone, anthraquinone or triazine nuclei are compounded with the polymer or copolymerized as one of the monomers. After drying, the adhesive... 

Immobilization of (3-D-glucosidase from almonds on photo-crosslinkable resin prepolymer (ENTP-4000) was carried out by the following procedure. One gram of ENTP-4000 was mixed with 10 mg of a photosensitizer, benzoin ethyl ether, and 110 mg of (3-D-glucosidase from almonds (3.4 units mg ). The mixture was layered on a sheet of transparent polyester film (thickness, ca. 0.5 mm). The layer was covered with transparent thin film and then illuminated with chemical lamps (wavelength range 300 00 nm) for 3 min. The gel film thus obtained was cut into small pieces (0.5 x 5 x 5 mm) and used for bioconversion reaction. 

Photo-crosslinking and the reverse process of photodissociation of pre-existing crosslinks relies on a cycloaddition reaction (and on the reverse dissociation of the cyclic adduct). For example, derivatives of vinyl cinnamic acid can form crosslinks which are dissociated by irradiation with short wavelength light (e.g. 254 nm produced by low-pressure mercury arcs). In this process the polymer chains become separated, and the polymer itself is then soluble in organic solvents. 

An example of photo-crosslinking is the reaction which follows the photodissociation of diazofluorenes, with the loss of molecular nitrogen and formation of a carbene. Two carbenes will then react to form an adduct between two polymer chains (the crosslink). In this case irradiation can be made in the near UV, for instance at 365 nm with a medium-pressure mercury arc lamp (Figure 6.13). 

With regard to thermal conversion to polyimide, we evaluated the suitable reaction conditions for thermal conversion of the photo-crosslinked patterns into completely polyimide patterns and found, that the crosslinked bridges are split off and depolymerization takes place. More than 95 % of the volatile product is the monomer hydroxyethylmethacrylate. 

Photopolymerization and photo-crosslinking are versatile and industrially useful reactions. Applications include drying or curing of coatings—paints and printing inks. In dentistry, u.v. hardening of dental filling is an example. Industrial applications of photochemical excitations are well known. 

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