Acetylated lignins, molecular weight

Acetylated lignins, molecular weight distribution, 87,89-94/951,96 Acetylation, effect on lignin solubility, 284... 

This study describes the application of differential vis-cometry as a GPC detector to the problem of determining molecular weight distributions of acetylated hardwood lignins in tetrahydrofuran. Molecular weight distributions of ball-milled, organosolv, alkali-extracted/mild acid hydrolyzed, and alkali-extracted/steam exploded aspen lignins were estimated using universal calibration. 

Table I illustrates the molecular weight averages found from universal calibration (narrow standards) for four aspen lignins and two quinonemethide-derived polymers using the Unical software. The poly-dispersities were the same within the experimental errors for all lignin samples. In contrast, the polydispersities found for the quinonemethide-derived polymers by universal calibration were near 1.1. The molecular weight averages found for the four acetylated lignins studied by universal calibration were substantially larger than those determined from previous work using conventional GPC (e.g., AESE and BM lignins from refs. 7 and 12 had approximately one-third those values found by HPSEC-DV in the present study).

Recently, efforts have been made to produce calibration standards of higher molecular weight that are chemically similar to lignins, by step-wise syntheses (12), anion-initiated polymerization of quinonemethides (13), and preparative HPSEC of acetylated lignins (14). Knowledge of the molecular weights of these materials is either built into the method of preparation or determined by absolute methods such as sedimentation equilibrium measurements. 

Quantitative Acetylation of Lignins. To avoid fractionation of the lignins and loss of low molecular weight components, which occur in the usual acetylation procedures, the quantitative acetylation described by H. Chum et al. (20,21) was employed. 

Chemical methods for analysis of phenolic hydroxyl groups include determination of the increase in methoxyl content resulting from diazomethane methylation (Bjorkman and Person 1957), the increase in phenolic acetyl group content after acetylation (Lenz 1968, Mansson 1983), and low-molecular weight compounds derived from the degradation of phenolic structures (Chap. 5.2). The phenolic acetyl group of acetylated lignin may also be determined by an NMR spectroscopic technique (Lenz 1968, Robert et al. 1986) or by a selective deacetylation in pyrrolidine (aminolysis) (Mansson 1983). 

Molecular Weight Distributions of Acetylated Lignins in THF Lignin Samples... 

Molecular Weisht Distributions of Acetylated Lignin Derivatives in THF Molecular Weight Distributions from SEC... 

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