Esterification of kraft lignin inherently addresses its potential for thermoplastic applications either on its own or as a component of polymer blends. In this effort, we have investigated the selectivity of softwood kraft lignin toward esterification via acylation. LignoBoost kraft lignin was esterified with acetyl (C 2 ), octanoyl (C 8 ), lauroyl (C 12 ), and palmitoyl (C 16 ) chlorides at various molar ratios with respect to the total hydroxyls present. Quantitative 31 P NMR spectroscopy, Fourier transform infrared spectroscopy (FTIR), and gel permeation chromatography (GPC) were used to evaluate the selectivity and efficiency of these reactions on the various hydroxyl groups present. The C 8 −C 16 acyl chlorides showed distinct enhanced reactivity toward the aliphatic hydroxyl groups, whereas C 2 acyl chloride was found to react uniformly with any available OH irrespective of their chemical nature. The effects of long chain acylation on the polymer and material properties were also examined using solution viscosity, thermal, and rheological measurements. Polymer blends were also produced and studied by melt extrusion. The long aliphatic chains when installed on the lignin displayed peculiar association effects in solution and enhanced the melt flow characteristics of the lignin−polymer blends.
Stereochemical studies on the formation of the diastereomers of arylglycerol-fl-aryl ether structures during lignin biosynthesis have been carried out with model compounds. The addition of water to quinone methides of the fl-syringyl ether type gives arylglycerol fl-syringyl ethers with a predominance of the erythro isomer when the pH of the medium is low. Since erythro forms of arylglycerol fl-syringyl ethers are prevalent in hardwood lignins, this indicates that the pH of the medium in which lignin biosynthesis occurs is lower than has been assumed until now. Equilibration studies with non-phenolic model compounds of the arylglycerol-fl-guaiacyl ether and fl-syringyl ether types under acidolysis conditions indicate that the erythro predominance observed in the syringyl ethers in lignins does not correspond to equilibrium conditions. A remarkable resistance to acidolysis is observed in the model compounds of etherified syringylglycerol fl-syringyl ether type.
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