Investigations of low molecular weight and high molecular weight lignin fractions

Milled wood lignin (MWL) samples isolated from spruce wood and birch wood w ere divided into a low molecular weight (MW) fraction and a high molecular weight (MW) fraction by gel permeation chromatography. The chemical composi tions of the fractions were similar in many respects. Degradation by thioac idolysis showed that the number of end groups of the benzaldehyde and benzo ic acid types was comparatively large in the low MW fractions. Thioacidolys is of spruce lignin samples (low and high MW fractions, MWL and borohydride -reduced MWL) gave the same yield of dihydroconiferyl alcohol and 3,4-divan illyltetrahydrofuran. This reinforces the opinion that these compounds corr espond to true lignin constituents. The xylan content was higher in the hig h MW birch lignin (4.1%) than in the low MW birch lignin (1.3%). H-1 NMR sp ectroscopy showed that the number of phenol groups was larger in the low MW spruce lignin than in the high MW spruce lignin. Mass spectrometric examin ation of degradation products formed on reduction of MWL with NaBD4 and sub sequent degradation by thioacidolysis/Raney nickel treatment provided proof of the presence of end groups of the glyceraldehyde 2-aryl ether type.