A series of Pinus radiata kraft pulps with yields between 96% and 47%
were prepared and examined by thioacidolysis. The levels of uncondense
d beta-ether units in the residual pulp lignins dropped almost linearl
y with the decreasing lignin content of the pulp. The drop in the leve
ls of uncondensed beta-ether units in the spent liquor lignins was muc
h less rapid than in the fibre lignin. Enol ether units were formed in
the dissolved lignin during the bulk phase of cooking, and also to a
small extent in the residual fibre lignin. By examining the molecular
weight distribution of the pulp lignin thioacidolysis products using g
el permeation chromatography, it was found that the proportion of unre
active linkages increased in the residual fibre lignin as kraft pulpin
g proceeded. This was due both to breakage of reactive interunit linka
ges and to condensation.