Hydroxypropylation of lignin and lignin-like model compounds

Copolymerization reaction of propylene oxide and Kraft lignin with various catalysts, catalyst concentrations, at several temperature levels were studied. The reaction rate of propylene oxide was estimated from the pressure vs time diagram of the reaction and the copolymerization extent was evaluated on the basis of H-NMR spectroscopy in terms of molar substitution of purified lignin/propylene oxide copolymers.

The investigation included studies with lignin-like model compounds under conditions of varying KOR concentrations at 190°C. Reaction rate and molar substitution were determined.

Homogeneous ( completely liquefied ) Kraft lignin products were . 0 only obtained at reaction temperatures above 190 C or temperatures above the glass transition temperature of the particular lignin preparation. Various lignins including Kraft lignin, methylated Kraft lignin, and demethylated Kraft lignin were employed to reacted with propylene oxide at 190°c. The effect of lignin structure variations on both reaction rate and molar substitution were determined.

Results revealed that homopolymerization was the major reaction although copolymerization also took place during the hydroxypropylation. Reactions with model compounds indicated that the reaction rate depended on the ratio of total alkalinity to total acidity in the system which was found to be related with KOH concentration, lignin content, and lignin structure. Particularly, carboxyl groups of lignin resulted in high acidity and low homopolymerization rate.

It was also found that molar substitution varied between zero and three and results from model compound study indicated that conjugated aliphatic hydroxyl and certain phenolic hydroxyl groups reacted with particular ease with propylene oxide. Therefore, reaction products from lignins rich in these types of functional groups can be expected to produce derivatives with low glass transition temperature and improved solubility.