Phloretic acid as an alternative to the phenolation of aliphatic hydroxyls for the elaboration of polybenzoxazine

Abstract

3-(4-Hydroxyphenyl)propanoic acid, so-called phloretic acid (PA), is a naturally occurring phenolic compound which can be produced by the hydrogenation of p-coumaric acid or synthesized from phloretin, a by-product of apple tree leaves. It is explored herein as a renewable building block for enhancing the reactivity of –OH bearing molecules towards benzoxazine (Bz) ring formation instead of phenol. PA is used to bring phenolic functionalities by its reaction with model molecules (ethylene glycol, as well as two polyethylene glycol molecules with molar masses corresponding to 400 g·mol⁻¹ and 2000 g·mol⁻¹) via solvent-free Fischer esterification. These phenolic groups are further reacted with a bio-based amine (furfurylamine) to form almost 100% bio-based benzoxazine end-capped molecules. Very interestingly, the whole synthesis of Bz monomers from PEG₄₀₀ and PEG₂₀₀₀ does not require a solvent or purification, and their polymerization led to a set of materials with thermal and thermo-mechanical properties suitable for a wide range of applications. These results show that renewable phloretic acid is a sustainable alternative to phenol to provide easily the specific properties of benzoxazine to aliphatic –OH bearing molecules or macromolecules. This novel approach paves the way towards a multitude of applications given the large number of –OH bearing compounds in materials science.