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 M_w = 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.