Polyethers containing coumarin dimer components in the main chain. II. Reversible photocleavage and photopolymerization

✍ Scribed by Yun Chen; Chuen-Sheng Jean

• Publisher: John Wiley and Sons
• Year: 1997
• Tongue: English
• Weight: 199 KB
• Volume: 64
• Category: Article
• ISSN: 0021-8995
• DOI: 10.1002/(sici)1097-4628(19970531)64:9<1759::aid-app12>3.0.co;2-t

✦ Synopsis

Polyethers, containing coumarin dimer components in the main chain, were irradiated successively in 1,4-dioxane with 254 and 350 nm (or 300 nm) light to investigate their reversible photoreaction properties. The coumarin dimer components cleave symmetrically under 254 nm (photocleavage) to original dioxycoumarins. Irradiating the cleaved sample with 350 nm (or 300 nm) leads to polymerization of the dioxycoumarins via dimerization of the telechelic coumarin chromophores ( l max Å 320 nm). The absorbances at 320 nm of the sample solution were used to trace the photoreactions. For both 5 and 6, the maximum absorbance (ABS) and DABS increase with the number of cycles (one photocleavage followed with one photopolymerization). However, the degenerates (percent decrease of absorbance per cycle) of 5 (ca. 50%) are much greater than those of 6 with 4-methyl substitution (ca. 29%), which has been explained by the formation of coumarin dimer components which are difficult to cleave under 254 nm. Moreover, photocleavage (254 nm) is much faster than is photopolymerization (350 nm) for both 5 and 6 due to the high-energy intensity of 254 nm light. Addition of the photosensitizer, benzophenone, enhances the photopolymerization rate with the photocleavage rate remaining almost unchanged. 4-Methyl substitution in 6 results in an increased rate for both reactions which can be attributable to its electron-releasing property. Photopolymerization under 300 nm light is more effective than under 350 nm, suggesting that the absorbing efficiency under 300 nm should be greater since it overlaps with the absorption of coumarin chromophores (320 nm).