Lignin-based Non-Isocyanate Polyurethanes by transurethanisation: catalyst selection towards Covalent Adaptable Networks

Abstract

This work reports the synthesis of lignin-based non-isocyanate polyurethanes (NIPU) via transurethanisation (TU), aiming to develop biobased and recyclable covalent adaptable networks (CANs). A kinetic study of a model TU reaction enabled the selection of four catalysts for the preparation of crosslinked NIPU from a Kraft lignin-derived polyol and hexamethylene dicarbamate. All these aromatic materials exhibited dynamic covalent behavior, allowing stress relaxation and efficient recycling through multiple thermomechanical cycles. The catalyst choice significantly influenced network architecture, mechanical performance, and recyclability. Iron and bismuth/zinc catalysts emerged as promising non-toxic alternatives to conventional tin-based catalysts, promoting efficient TU while limiting side reactions such as urea formation, which otherwise can compromise the reprocessability. Chemical recycling was also demonstrated as a potential option for the end-of-life valorization. Indeed, TU emerges as a robust and versatile framework for the synthesis of biobased and circular NIPUs. This approach clearly emphasizes how strategic catalyst selection is fundamental to tailoring material properties and ensuring recyclability.