Triplet sensitized crosslinking

This paper describes the successful synthesis and examination of polyfr-(amino /9-thiosulfate) ether] (PATE), a water soluble photolabile polymer. Evidence has been presented that the PATE polymer is zwitterionic and forms weak associations in aqueous solutions. Heat treatment of PATE films result in extensive crosslinking, presumably through a disulfide bond. This work presents strong evidence that PATE is activated by deep UV radiation, and that a disulfide crosslink is formed. Sensitization experiments demonstrate that the crosslinking reaction can be induced by a triplet sensitizer. Finally, preliminary results point out the potential for application of PATE films as active photoimaging systems. 

Polyvinyl cinnamate gives crosslinking of the cyclobutane ring by irradiation, as shown in Equation (1.109). Polyvinyl cinnamate absorbs UV light near 280 nm. Therefore, polyvinyl cinnamate shows sensitivity to visible light. Triplet sensitizers such as Micheler s ketone and 5-nitroacenaphthene are effective to extend sensitivity to visible light. 

The photosensitized crosslinking of amorphous polymeric films containing dimerizable cinnamate moieties show quantum yields of dimerization of about 0.04 (4). Sensitization of such films by ketocoumarins proceeds via triplet-triplet energy transfer (2) (see Scheme V). The irradiations were performed at 405 nm at a wavelength... 

Lastly, the polyimide has some inherent photosensitivity. The use of a photosensitive polymer requires fewer processing steps, which also implies an increased yield for the circuit. Ciba-Geigy announced the formulation of inherently photosensitive polyimides in 1985.(5) The use of an inherently photosensitive polyimide is attractive because no sensitizers are added to contaminate the final resin, no functionality is added which degrades the thermal and chemical resistance, and fewer volatiles are present to contaminate the resin or equipment than in some other approaches to photosensitive polyimides. Recent work has shown that these polyimides crosslink on irradiation through hydrogen abstraction by triplet benzophenone and subsequent coupling of the resultant radicals. ... 

As a result of luminescence-quenching experiments, Curme, Natale and Kelley 46> concluded that cinnamate did not accept singlet energy from Michler s ketone or picric acid at 77° K. Again it was suggested that triplet energy was donated by the sensitizer to cinnamate ester, which then underwent dimerization-type crosslinking. 

The rates of the crosslinking reactions can be greatly accelerated by addition of photosensitizers. They are molecules that, after absorption of light and attaining the triplet state, transfer the energy to other molecules. The effectiveness of sensitizers in accelerating the reactions of poly(vinyl cinnamate) is illustrated in Table 8.1. 

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