Polymers with Pendant Azide Groups
Both azide and sulfonyl azide groups are photo-reactive and decompose into active nitrene groups and nitrogen upon irradiation. The dissociation of the azide moiety follows almost every transition from an excited n → π* state to a high vibrational level of the ground state [170, 171]. Introductions of pendant azide and sufonyl azide groups into polymeric structures are possible in a variety of ways and many publications describe different approaches. Thus, Tsuda and Yamaoka [81] introduced azide groups into a phenolic novolac resin by the following scheme:
Tsunoda and Yamaoka [173, 174] also condensed formyl-1-naphthyl azide with poly (vinyl alcohol). All steps in the synthesis were not disclosed. The preparation was illustrated as follows:
Preparations of azide derivatives from styrene-maleic anhydride copolymers, cellulose, and gelatin by attaching aromatic azide compounds are described in the literature [175]. Most of the resultant polymers cross-link rapidly when exposed to light of 260 mm wavelength. Also, as much as 90% of the hydroxy groups of poly (vinyl alcohol) can be esterified with p-azido-benzoyl chloride. These reactions must be carried out in mixtures of chloroform and aqueous sodium hydroxide [175]. Earlier, Merrill and Unruh [175] described formation of derivatives from poly (vinyl alcohol) and attachment of aromatic azide groups. Most of the seazide polymers photocross-link at a faster rate than does poly (vinyl cinnamate), when exposed to light of 260 mm. In addition, they responded well to photosensitization. Also, it was observed [50] that the 4-isomer of azido phthalate shows greater speed increase than does the 3-isomer. In general, the poly (vinyl alcohol) derivatives were reported to exhibit higher cross-linking speeds than do other azide functionalized polymers [176]. The reaction product of p-azido benzoyl chloride with polyvinyl alcohol was investigated by Tsuda and coworkers [179]. In the polymer studied by them, over 90% of the hydroxy groups were esterified. The photocross-linking reaction was followed by observing changes in the ultraviolet and infra-red absorption spectra. It was shown that the simple photochemical reaction occurs stochiometrically upon irradiation. Also, an absorption band was observed at 1,500/cm in the infra-red region of the irradiated and cross-linked polymer. This band is due to N¼N stretching vibration of the azo group. Based on that, Tsuda concluded the cross-linking reaction takes place by dimerization, as expected [179]. There are reports in the literature that photosensitive azide polymers formed from polymers substituted with isocianate groups [176, 177]. The preparation and properties of many other polymers containing pendant aryl azide groups were described by Delzenne and Laridon [178]. The polymers were prepared by interfacial polycondensation of azido-substituted acid chlorides with diols and diamines. Also, in one experiment, a cinnamate moiety was combined with an azide group, together in one pendant functional structure [178]. It was found, however, that addition of the azide groups to the cinnamate side chain does not increase reactivity. A marked wavelength dependence was observed on π → π* and n → π* transitions that occurs in both functional groups [179].
