Polyethers containing coumarin dimer components in the main chain. I. Synthesis by photopolymerization of 7,7′-(polymethylenedioxy) dicoumarins

Eight 7,7′-(polymethylenedioxy)dicoumarins were successfully synthesized by solution condensation of 7-hydroxycoumarin (umbelliferone) or 7-hydroxy-4-methylcoumarin (4-methylumbelliferone) with various dibromoalkanes. Upon benzophenone-sensitized irradiation with 350 nm light in dichloromethane, the terminal coumarin chromophores dimerize to form polyethers containing coumarin dimer components in the main chain. The configurations of the coumarin dimer linkages were characterized by ¹H-NMR spectra. For unsubstituted dicoumarins, the resulting polyethers consist mainly of syn and anti head-to-head coumarin dimer linkages. However, the configurations are syn and anti head-to-tail for those from 4-methyl-substituted dicoumarins. The highest reduced viscosity is 0.23 dL/g obtained with polyether from 7,7′-(decamethylenedioxy)dicoumarin. The photopolymerization in dichloromethane obeys zero order and first order with respect to coumarin chromophore and benzophenone, respectively.