Derivatisation of Polyphenols

Abstract

Polyphenols, such as tannins, offer potential as a bio-derived chemical feedstock. Their present utilisation is limited mainly to leather tanning and wood panel adhesives. However, appropriate derivatisation may alter both the chemical and physical properties and thereby allow further utilisation of polyphenols.

Derivatisation of polyphenols was achieved by esterification and etherification of the phenol groups. Esterification was achieved by alcoholysis of acid chlorides and transesterification with vinyl esters, while etherification was achieved by the ring opening of propylene oxide. The polyphenols used were resorcinol, catechin, Pinus radiata bark tannin, and Schinopsis lorentzii tannin. The products were characterised using a range of techniques including NMR (1H, 13C and 2D NMR in both the solution and solid state), ESI-MS, GPC, DSC, TGA, and rheology.

The preparation of polyphenolic esters by alcoholysis provided model compounds to establish the key chemical, spectroscopic, and physical features. A range of simple polyphenol esters such as resorcinol dilaurate and catechin pentalaurate were prepared using lauroyl chloride. Furthermore, tannin lauroyl esters were prepared with varying degrees of substitution. A transesterification method was developed for the preparation of polyphenol esters. Ester interchange occurred effectively in the presence of base catalyst in aqueous solution or dimethyl sulfoxide with short or long chain vinyl esters. This included the first report of the base-catalysed transesterification of flavonoids by vinyl esters to give products such as catechin mono- and di-laurate. Transesterification occurred preferentially at the B-ring as shown by NMR spectroscopy. Subsequently, this transesterification procedure was used to prepare tannin esters. The chemical and physical properties of polyphenol esters were assessed using thermal, antioxidant, and UV/VIS light absorption analysis. Thermal analysis indicated melt/flow properties for some of the polyphenol esters. In some cases, the thermal stability was also shown to increase upon esterification. The antioxidant activity was shown to decrease upon transesterification of pine bark tannin with vinyl laurate, while the UV/VIS absorption was shown to increase. These properties may lend the products towards applications as polymer additives or pharmaceuticals.

Polyphenol ethers were prepared by the Williamson ether synthesis and ring opening of propylene oxide. However, the Williamson ether synthesis, a common route to prepare ethers, proved unsuitable for flavonoids. Catechin and tannin hydroxypropyl ether derivatives of varying substitution were prepared by the ring-opening of propylene oxide in the presence of triethylamine. Upon hydroxypropylation the thermal properties of the polyphenol were altered. For example, catechin hydroxypropyl ethers showed a glass transition, which was dependent upon the molar substitution, while rheology showed melt behaviour for several of the tannin hydroxypropyl ethers.