Abstract

A method for selective, fast, and near-quantitative methylation of carboxyl groups in the presence of aliphatic hydroxyl groups of cellulose was developed. The main advantage of the method is the complete maintenance of cellulose integrity; no degradation of cellulose whatsoever during the methylation procedure was observed, making the protocol extremely useful in advanced cellulose analytics. Trimethylsilyl diazomethane (TMS-DAM) was used as the methylation agent. The methylation conditions were pre-optimized by means of carbohydrate model compounds with low-molecular weight, such as derivatives of glucuronic acid and hexenuronic acid, and then re-optimized for methylation of cellulose. The influence of reaction parameters, such as time, temperature, solvents, and concentration, was also determined. The reaction kinetics was monitored by fluorescence labeling of the remaining carboxyl groups according to the common FDAM protocol. A general protocol for the fast methylation of carboxyl groups in pulp samples is presented. The methylation with TMS-DAM is a very fast process, both in comparison to general heterogeneous reactions on cellulosic pulps, and especially in comparison to other methylation approaches. In dissolving and paper grade pulps, methylation of xylan carboxyls proceeded faster than methylation of cellulosic carboxyls. The methylation rate and kinetics are dependent on the pulp type. As application examples, electrokinetic measurements were used to demonstrate the “blocked” reactivity of carboxyl groups in spin-coated films of methylated pulp samples, and ICP-MS was used to demonstrate the drastically decreased metal ion binding capacity of pulp after such methylation.