Structurally Diverse and Recyclable Isocyanate-Free Polyurethane Networks from CO2-Derived Cyclic Carbonates

Abstract

Isocyanate-free chemistry and the introduction of dynamic bonds are a promising combination toward the development of more sustainable polyurethane (PU) networks. Here, we present the synthesis of reprocessable, isocyanate-free PU networks through atom-economical reactions for both monomers and polymers, valorizing CO2 as a building block. The monomers employed were five-membered cyclic carbonates prepared through an efficient (>99% conversion) reaction between CO2 and epoxides with high yield (up to 97% at a 100 gram scale), catalyzed by the moisture-tolerant and easily prepared DBU·I2 complex. The structural diversity of the monomers and the utilization of cystamine bearing the dynamic S-S motif realized PU networks with a finely tuned profile of thermal (Tg from −9 to 44 °C), mechanical (E from 0.2 to 1700 MPa), and viscoelastic properties (E′ from 0.03 to 5.5 MPa at 1 Hz). Facile recycling (100 °C, 20 min) of the networks was enabled thanks to the rapidly exchanging disulfide bonds and the modulated cross-link density. Moisture-induced plasticization of the networks was identified, and its effect on the properties of the networks was elucidated. The atom economy and energy-efficiency methodology, avoiding toxic reagents and preventing waste generation, make this approach an attractive and greener pathway to PU networks taking a step toward a circular plastic economy.