Highly Reactive Biomass Waste Humins Derived from Photocatalytic Polymerization of 5-Hydroxymethylfurfural for Self-Healing Polymers

In this article, the photocatalytic polymerization of 5-hydroxymethylfurfural (HMF) was applied in the preparation of photo-generated humins (L-H) for the first time. The molecular weight and furan ring content of L-H would be increased by introducing HMF into low-molecular-weight humins (LMW-H) extracted from spoiled HMF under visible light. Compared with humins prepared by the thermal-acid method (A-H), the molecular weight of L-H was less than 800 g/mol; therefore, L-H exhibited great solubility in various solvents and high chemical reactivity for structure modification. Also, the structure units had great differences between L-H and A-H. A-H only consisted of 2,5-dioxo-6-hydroxyhexanal (DHH) and HMF, while L-H also included the 5-hydroxy-4-keto-pentenoic acid (HKPA) generated by reacting HMF with singlet-oxygen. Then, the furan-based polymers were obtained through the classic Diels–Alder (DA) reaction to realize resource utilization of L-H. HKPA units in L-H provided more reaction sites for DA reaction. Characterizations by nuclear magnetic resonance (NMR), Fourier transform infrared (FTIR) spectrum, microscopy, differential scanning calorimetry (DSC), and tensile properties proved that the polymer DA-L-H containing D–A bonds had self-healing properties, and the recovery efficiency could reach 92.8%. Through the utilization of biomass waste humins, the recycling of resources and sustainable economic development could be realized.