Acetylation, lignins

Scientific name/Common name Glucan Xylan Galactan Arabinan Man nan Uronic anhydride Acetyl Lignin Ash... 

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).

Although evidence exists that concentration effects may be important with even acetylated lignins in THF, the effect of increasing column loadings from 1 to 2 mg seems unlikely as the cause of the variance in MW shown in Table I. This observation illustrates the more general problem in current SEC-based absolute MW measurement that of a limited concentration window for analysis. The limiting value for sample concentration appears to be near 1 mg per injection for HPSEC-DV, which is comparable to the 0.2-1 mg per injection range usable in HPSEC-LALLS (33). For studies... 

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. 

From the spectra of acetylated lignins (15) the relative amounts of primary, secondary, and phenolic OH groups are determined for the average benzene ring ... 

Figure 3. Relationship between % of crystallinity and lignin content for blends prepared with the acetylated lignin series. The degree of substitution of hydroxy functionality is given with each curve.

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

Fig. 4.1.15. Acetyl bands of acetylated lignins before and after deconvolution (Faix 1987). Acetylation in Ac20/pyridine. (Conditions described in legend to Fig. 4.1.11 caption)...

It is a good solvent for acetylated lignins and exhibits only a small band around 1210cm-1 and a broad band between 780-720cm-1. 

Unknown, the peak is absent in the spectrum of NaBH4-reduced and acetylated lignin... 

Lundquist K, Stomberg R, von Unge S (1987) Stereochemical assignment of the threo and erythro forms of 2-(2,6-dimethoxyphenoxy)-l-(3,4-dimethoxyphenyl) 1,3 propanediol from X ray analyses of the synthetic intermediates (Z)-2-(2,6-dimethoxyphenoxy) 3 (3 4 dimethoxypheny )-2-propenoic acid and threo-2-(2,6 dimethoxyphenoxy) 3 (3 4-dimethoxyphenyl)-3-hydroxypropanoic acid Acta Chem Scand B41 499-510 Lundquist K, von Unge S (1986) NMR studies of lignins 8 Examination of pyridine-d, solutions of acetylated lignins from birch and spruce by H NMR spectroscopy Acta Chem Scand B40 791-797... 

Estimation of the different types of hydroxyl groups can be performed readily on acetylated lignin samples (Fig. 5.4.9) (Robert and Brunow 1984,... 

Alternatively, 100 ml to toluene is added to the reaction mixture, which is transferred to a 250-ml round-bottom flask. The acetic anhydride, acetic acid, and pyridine are removed as an azeotropic mixture at 40 °C by rotary evaporation (Gierer and Lindeberg 1980, Chum et al. 1985). A small amount of acetone is added to the solution to stabilize the acetylated lignin during removal of the toluene. If necessary, this operation may be repeated to insure complete removal of the acetylation reagents. The residual toluene is in turn similarly removed azeotropically from the mixture by adding 70 ml of ethanol-acetone (1 1, v/v). The product is dried in a drying pistol at 50°C under vacuum. Yield, —130 mg (Note 3). 

Since the hydroxyl groups are converted to O-acetyl groups by acetylation on a one-to-one molar basis, the total hydroxyl content of the lignin is equal to the total O-acetyl content of the acetylated lignin. Provided the methoxyl content of the acetylated lignin is known (see Chap. 7.6.3), the total hydroxyl content of the lignin expressed in terms of methoxyl content can be calculated ... 

Total —CCH3/OCH3 (Moles/mole, based on acetylated lignin)... 

The total hydroxyl content of the lignin/C9 unit may also be calculated directly from the elemental composition of the acetylated lignin, i.e., % C, % H and the total O-acetyl and methoxyl contents (Freudenberg 1962). 

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). 

The acetylated lignin, dissolved in 1.0 ml of dioxane containing 5 mg of an internal standard, is treated with 1.0ml of a dioxane-pyrrolidine (1 1, v/v) solution which initiates the aminolysis reaction. The rate of the 1-acetylpyrrolidine formation is followed by periodically injecting 1-2/d of the reaction mixture into the gas chromatograph. The initial rapid phase is generally complete within 20 min, but a total of 60min is required to complete the deacetylation process. 

The rate of 1-acetylpyrrolidine formation is followed as in the case of acetylated lignin, but the overall process is much slower and a 6-7-h reaction period may be required as illustrated in Fig. 7.2.1A for Norway spruce wood meal. 

---