EVALUATION OF THE CONTRIBUTION OF LIGNIN STILBENE PHENOL UNITS IN THEPHOTOYELLOWING OF PEROXIDE-BLEACHED LIGNIN-RICH PULPS

In order to assess and quantify the contribution of stilbene phenols i n photoreversion of bleached high-yield pulps, a method to reduce the stilbene double bond and quantify the formed diphenylethane was develo ped on 4-benzyloxy-3,3'-dimethoxy-4'-hydroxystilbene, as lignin monome r model, and also on a more sophisticated lignin polymer model constit uted by a polystyrene framework containing 4,4'-dihydroxy-3,3'-dimetho xystilbene elements as pendent groups. The method used RhCl( PPh3)(3) as soluble catalyst to get an efficient hydrogenation and AlCl3 in ben zene to liberate the diphenylethane from the polymer framework. For th e first time a semiquantitative evaluation of the content of the stilb ene phenols formed from P-l units was given for high-yield pulps. The value of 2 p-stilbene phenols for 1000 C-9 lignin units found indicate s that they are present in very small quantities after the peroxide bl eaching. UV irradiation of the polystyrene model adsorbed on solid cel lulose matrix had revealed an efficient stabilization after the hydrog enation treatment. This was not the case for the peroxide bleached pul p showing that the p-stilbene phenols formed from beta-1 units are not the main contributors of the rapid yellowing of bleached lignin-rich pulps. (C) 1998 John Wiley & Sons, Inc.