Divergent synthesis, convergent recycling: A single-monomer platform for tuneable polyurethanes
The transition from a linear “take-make-dispose” plastics economy to a circular one represents one of the most formidable challenges in modern materials science. Central to this transition is the development of polymers that combine performance parity with existing materials—necessary for adoption—and intrinsic chemical recyclability—necessary for sustainability. Yet these two objectives have often proven difficult to reconcile. Polymers with precisely tuneable properties typically require copolymerization of multiple monomers, generating complex microstructures that resist depolymerization back to original feedstocks. Conversely, polymers designed for facile recycling often compromise on performance or processability. Recently, in Nature Materials, Jian-Bo Zhu et al. at Sichuan University reported an elegant strategy that sidesteps this trade-off, demonstrating that a diverse family of high-performance polyurethanes (PU) can be divergently synthesized from a single monomer while maintaining the ability to converge back to that same monomer at end of life.
To our knowledge, the concept of a single-monomer platform with chemical recyclability was first demonstrated by Qu and co-workers using thioctic acid. Qu et al. reported that poly(disulfides) can be depolymerized back to the monomer in dilute NaOH solution, establishing a dual closed-loop recycling system (Figure 1A). Zhu and colleagues take this concept further by introducing time-controlled regioselective polymerization and convergent recycling of mixed waste streams (Figure 1B).
The conceptual advance lies in recognizing that polymer microstructure need not be fixed at the moment of chain growth. By exploiting the interplay between regioselective ring-opening polymerization (ROP) and post-polymerization covalent bond exchange, Zhu et al. demonstrated that polymer composition—and hence material properties—can be varied simply by modulating reaction time. This approach stands in marked contrast to conventional copolymerization strategies, where composition is dictated by monomer feed ratios and remains largely invariant once polymerization is complete, and reflects the principle that base-catalyzed covalent bond exchange can drive a net compositional shift among urea, carbonate, and urethane linkages.
Dynamic covalent chemistry meets regioselective polymerization
The monomer tetramethyleneurethane (TeU) is a seven-membered cyclic urethane. Its unsymmetrical ring allows opening at either the ester or amide bond. With potassium tert-butoxide, polymerization proceeded rapidly at 30°C, reaching 89% conversion within 4 min. Quenching early yielded PU35 (where 35 represents the percentage of urethane linkages), and the remainder are urea and carbonate units. Allowing the reaction to continue evolved the composition: PU48 after 30 min and PU81 after 96 h. This drift arose from base-catalyzed exchange between urea/carbonate and the more stable urethane linkages. By varying reaction time, the authors accessed PUx polymers with urethane contents from 35% to 81%—all from one monomer, one catalyst, and one parameter. This stands in contrast to orthogonal polymerizations requiring two catalysts and a photo-stimulus.
