SOME MECHANISTIC ASPECTS OF THE LIGHT-INDUCED YELLOWING OF LIGNIN - AFURTHER STUDY OF THE PHOTOCHEMICAL-REACTIONS OF ALPHA-GUAIACOXY-BETA-HYDROXY-PROPRIOVERATRONE, AND SOME SUBSTITUTED METHOXYBENZENES

The primary photoreduction of a model lignin system alpha-guaiacoxy-be ta-hydroxypropioveratrone occurs via a triplet mechanism leading to po larized ketyl radicals which undergo beta-ether cleavage similarly to the photochemical reactions of alpha-guaiacoxyacetoveratrone. Comparis on of the photoyellowing rates of solutions of the two lignin models s howed a slightly greater rate for the derivative with the beta-hydroxy methyl function, with some subtle differences in behavior between the two compounds probably due to the intramolecular hydrogen bonding in t he former. In addition it was found that photooxidation of several met hoxybenzenes was considerably slower than that of corresponding methox yphenols. This is reassuring that the ubiquitous presence of methoxybe nzene groups in lignin matrices is unlikely to contribute significantl y on the timescale relevant to the photoyellowing of CTMP pulps.