A Tetrafunctional Epoxy Monomer Based on Thymol from Facile Synthesis to High-Performing Thermosets Empowered with Excellent Thermal Performances and Low Moisture Absorption

Developing heat-resistant biobased epoxy resins with low water uptake and high biobased contents plays a crucial role in advancing sustainable epoxy chemistry and technology. Herein, thymol is readily co-condensated with less hazardous terephthalaldehyde to form C–C bond-linked tetraphenol (TE) of 83% biomass-derived carbon content, followed by efficient O-glycidylation transformation to obtain a crystalline tetrafunctional epoxy monomer (TEEP). TEEP is cured into thermosets by using high-temperature hardeners 3,3′-/4,4′-diaminodiphenyl sulfone (33/44DDS) and compared with a standard bisphenol A epoxy (DGEBA). TEEP-33/44DDS shows lower reactivity but much higher Tg (241–255 °C), an ∼60 °C increment compared to DGEBA-33/44DDS. The storage modulus at the rubbery state increases by orders of magnitude, while the thermal expansion coefficient also decreases. TEEP-33/44DDS also demonstrates reduced dielectric constant, loss factor, density, and water absorption (∼2%), with gel content >99% and adhesion strength over 8 MPa. In addition, 33DDS and 44DDS considerably influence the curing reactivity toward TEEP, heat resistance, and swelling. In summary, TEEP could be readily synthesized in good quality from bulk biobased stocks with more thymol blocks incorporated and exhibits its advantage in performances, especially thermal and water resistance.