Hydroxymethylation followed by α-bromoisobutyrylation as an effective and precise method for characterization of functional groups of hydroxymethylated lignin

Abstract

The direct use of lignin results in the synthesis and production of low-value materials. Therefore, the chemical modification of lignin increases not only its reactivity, but also improves its compatibility and better dispersion in the lignin-based polymeric materials where lignin acts as a good substitute for oil-based polyols. Systematic studies were conducted in the present work. Hydroxymethylated lignin (HML) was synthesized via its reaction with formaldehyde, and functional groups present in the sol fraction of HML were then quantitatively investigated by aqueous potentiometric/conductometry titration prior to any further modification and by ¹HNMR and FTIR techniques after reacting with α-bromoisobutyryl bromide, resulting in brominated HML (BHML). To calculate moles of hydroxyl groups per gram of HML, ¹HNMR of BHML was recorded in the presence of a given amount of N,N-dimethylformamide as an internal standard. It was found that the number of hydroxyl groups increases from 6.44 mmol/g in the lignin to 7.77 mmol/g in the HML. The moles of phenolic hydroxyl and carboxyl groups per gram of HML were calculated using aqueous titration. It was found that the moles of phenolic hydroxyl groups in the HML decrease by 19.8% compared to lignin. Due to poor solubility of the HML in tetrahydrofuran, BHML was used in the GPC analysis, from which the number average molecular weight ($\bar{M}_{\rm{n}}$) of the HML was obtained after some calculations to be 1014 g/mol. It was observed that modification of lignin with formaldehyde increases not only its functional groups but also increases its molecular weight by creating methylene bridges between the two lignin molecules.