From renewable monomers to functional elastomers: sequential organocatalytic strategies for sustainable TPEs

Thermoplastic elastomers (TPEs) are a class of block copolymers characterized by a microphase-separated morphology, in which flexible amorphous segments impart elasticity while semicrystalline or glassy domains provide physical crosslinks. This dual organization enables rubber-like mechanical properties combined with melt processability. Conventional examples such as styrene–butadiene–styrene (SBS) have stimulated the development of sustainable analogues derived from biodegradable and renewable resources. Among these, aliphatic polyesters obtained through the ring-opening polymerization (ROP) of cyclic esters, including lactones and lactide, are particularly attractive [1]. However, toxicity issues associated with metal-based catalysts such as Sn(Oct)₂ have prompted the exploration of environmentally benign catalytic alternatives [2]. Hydrogen-bonding organocatalysts - especially bisurea derivatives and their ureate anions have proven highly effective, offering controlled ROP even for sterically hindered or low reactivity monomers [3]. In this work, we employed ureate anions to implement a sequential two-step polymerization strategy under mild, room-temperature conditions and in short reaction times. In the first step, various lactones were copolymerized to produce an amorphous soft segment, where combinations such as δ-hexalactone and ε-caprolactone effectively suppressed crystallinity. In the second step, lactide was polymerized to form a semicrystalline hard segment, yielding the biphasic morphology typical of thermoplastic elastomers. Overall, this approach demonstrates the potential of organocatalytic ROP to access fully renewable, structurally tunable polyester-based TPEs, paving the way toward sustainable alternatives to conventional petrochemical elastomers.