Organocatalytic Synthesis of Bio-Based Thermoplastic Polyester Elastomers

Thermoplastic elastomers (TPEs) are versatile materials characterized by a phase-separated architecture of soft and hard domains, combining rubber-like elasticity with melt processability. Traditional systems like styrene–butadiene–styrene (SBS) have inspired the design of sustainable analogues using biodegradable and renewable building blocks. In this context, aliphatic polyesters obtained via ring-opening polymerization (ROP) of monomers such as lactones and lactide represent a promising alternative [1–3]. However, the toxicity of conventional metal-based catalysts like Sn(Oct)2 has driven increasing interest in greener catalytic solutions [4]. Hydrogen-bonding organocatalysts, especially bisurea derivatives and their ureate anions, have proven to be highly efficient for ROP, enabling controlled polymerizations even with sterically hindered or low-reactivity monomers [5-6]. By combining lactones of varying ring sizes and functionalities through a sequential copolymerization strategy, thermoplastic elastomers with tunable mechanical and thermal properties can be produced [7]. The opportune selection of the initiator (e.g., monofunctional alcohols for linear chains or polyols for branched architectures) allowed a further control over polymer topology. In our work, we employed ureate anions as catalyst to promote a two-step sequential polymerization strategy (typically challenging to control) under mild conditions at room temperature and in short reaction times. Initially, different lactones were copolymerized to form an amorphous soft block; the combination of monomers (e.g., δ-hexalactone and ε-caprolactone) effectively disrupted crystallinity. Subsequently, lactide was added to grow a semicrystalline hard block, generating a biphasic morphology typical of thermoplastic elastomers. We also explored the use of a functional cyclic carbonate monomer bearing an allyl pendant group (AOMEC), copolymerized with ε-caprolactone to yield a central block with post-polymerization modification potential. This modular approach broadens the design space for biodegradable, functional polyester materials derived entirely from renewable resources.