Copolymerization of glycolic, d,l-lactic and d,l-2-hydroxybutyric acid mixtures present in kraft black liquors

A series of copolymers of glycolic acid (GA), d,l-lactic acid (dl-LA) and d,l-2-hydroxybutyric acid (dl-2HBA) were prepared via melt condensation polymerization. The utilized monomers represent mixtures that are present and potentially obtainable from kraft black liquor by fractionation. Polymerizations were performed in 165 °C at nitrogen atmosphere and reduced pressure using stannous octoate as catalyst. Copolymerizations were conducted with varying dl-2HBA content in order to observe how the presence of the dl-2HBA monomer affects the properties of the polymers. Prepared copolymers were characterized with 1H NMR and 13C NMR, size exclusion chromatography, differential scanning calorimetry and thermogravimetric analysis. The obtained copolymers had molecular weights between Mw = 3500–10,000 g/mol. dl-2HBA had a lowering effect on the glass transition temperatures and molecular weights of the copolymers. However, significant lowering of molecular weight was observed only when the amount of dl-2HBA in the feed exceeded 60%. The results indicate that dl-2HBA can be copolymerized with GA and dl-LA and therefore the corresponding kraft black liquor fractions can be utilized for polymerizations.