How to determine the molecular weight of cellulose by terminal group method

The basic principle of determining the molecular weight of cellulose by terminal group analysis is to quantify specific functional groups at the end of the polymer chain through chemical or physical methods, and then calculate the average molecular weight. The terminals of cellulose often contain groups such as hydroxyl groups (-OH), so molecular weight can be estimated by detecting the concentration of these terminal groups.

Detailed steps:

1. Sample preparation

Dissolve the cellulose sample in the appropriate solvent to ensure complete dissolution. Cellulose molecules are not easy to dissolve, and a suitable solvent system is required to choose, such as dimethylacetamide-lithium chloride system (DMAc/LiCl), or alkaline aqueous solution.

2. Chemical reaction calibration terminal groups

Commonly used methods include esterification or acylation reaction, which modify the hydroxyl group (-OH) at the end of the cellulose molecule. For example, acetylation reaction is used to react with the terminal hydroxyl group to form an acetate. The product generated after the reaction is then determined by titration or other quantitative analysis techniques, thereby indirectly calculating the concentration of terminal hydroxyl groups.

3. Spectrophotometry or titration method determination

The specific product generated after the reaction can be measured by titration methods such as acid-base titration, or the terminal groups can be quantified by measuring the absorption peak at a specific wavelength by UV-visible spectrophotometer.

4. Calculate molecular weight

Based on the measured concentration of terminal groups and combined with the mass of the sample, the number average molecular weight (Mn) is calculated using the following formula: Mn=m /( n×f ), where:

• m is the mass of the cellulose sample,

• n is the number of terminal groups (according to the concentration measured experimentally),

• f is the number of specific terminal groups on each polymer chain (cellulose is usually 2 terminal hydroxyl groups).

Notes:

• Solution selection : The solubility of cellulose is low, and the appropriate solvent needs to be selected.

• Error control : Since cellulose is a natural polymer, uneven chain lengths can lead to errors, it is usually used in combination with other methods such as gel permeation chromatography to verify the results.