Homolytic scission of interunitary bonds in lignin induced by ultrasonic irradiation of MWL dissolved in dimethylsulfoxide

An electron spin resonance (ESR) method combined with a spin trapping technique was applied to trap and characterize unstable radicals which were generated by ultrasonic irradiation of the dimethylsulfoxide (DMSO) solution of a softwood, Yezo Spruce (Picea jezoensis carr.) lignin. It was found that an unstable secondary carbon radical, similar to CH . in the solution was trapped as the stable nitroxide spin adduct when the DMSO solution was subjected to ultrasonic irradiation in the presence of a spin trapping reagent: 2,4,6-tri-tert-butylnitrosobenzene (BNB) at 50 degrees C for 30 min. This means that the alkyl phenyl ether bonds, similar to CH-O-phenyl, known as interunitary bonds in lignins were homolytically cleaved by the ultrasonic irradiation, although the phenoxy radical Ph-O ., called guaiacoxy radical, i.e. the counter radical of the secondary carbon radical, was not trapped by the BNB spin trap. This suggests that the trapping of the guaiacoxy radical, having a methoxy group in an ortho-position, by the BNB molecule, carrying two bulky butyl groups in the ortho-positions, is sterically hindered.