Turning down the heat: catalyst-free, low-temperature chemical degradation of thermoplastic polyurethanes

Abstract

Thermoplastic polyurethanes (TPUs) constitute one of the most industrially relevant polymers because of their versatile and tunable properties, which makes them very interesting for a wide range of applications. However, their chemical degradability and recyclability is only achievable through hydrolysis or glycolysis of the urethane moieties, which typically occurs when using catalysts and elevated temperatures, thus resulting in a poorly cost- and energy-efficient process. Herein, we report a novel strategy where variable amounts of chemically degradable β-amino ester (BAE) moieties are incorporated into the main backbone of the thermoplastic urethanes. The amino group in the β-position of the ester was thought to act as an internal, covalently bonded catalyst that not only enables the preparation of materials without the use of external and potentially leachable toxic catalysts, but also the chemical degradation of the resulting TPUs under relatively mild conditions and shorter times via a straightforward transesterification process. The addition of different amounts of BAE groups has been investigated for both aliphatic and aromatic polyurethane thermoplastic backbones. Their chemical, structural and thermal properties have been investigated in-depth, and kinetic studies have been additionally performed with low molar mass model compounds to find the optimal conditions for their chemical degradation. The final aim of this bottom-up molecular design was not only to enable a catalyst-free polyurethane synthesis but also to open new avenues for developing TPUs with an enhanced, cost-effective and energy-efficient degradation process.