Solvent‐Dependent Sequence‐Controlled Copolymerization of Lactones: Tailoring Material Properties from Robust Plastics to Tough Elastomers

Abstract Copolymerization stands as a versatile and potent method for tailoring polymer properties by adjusting structural unit composition and sequence distribution. However, achieving sequence‐controlled copolymerization in a one‐step and one‐pot process remains challenging. This study introduces a solvent‐dependent sequence‐controlled copolymerization strategy to produce block and statistical copolyesters from 4‐phenyl‐2‐oxabicyclo2.1.1hexan‐3‐one (4 Ph ‐BL) and ϵ ‐caprolactone ( ϵ ‐CL). The distinct kinetics of the two monomers enable the facile synthesis of diblock and triblock copolyesters, PCL‐ b ‐P(4 Ph ‐BL) and P(4 Ph ‐BL)‐ b ‐PCL‐ b ‐P(4 Ph ‐BL), in non‐coordinating solvents, such as dichloromethane and toluene. Conversely, coordinating solvents like tetrahydrofuran, 2‐methyltetrahydrofuran, 2,5‐dimethyltetrahydrofuran, 1,4‐dioxane, and 1,2‐dimethoxyethane facilitate frequent transesterifications, yielding statistical copolyesters PCL‐ stat ‐(4 Ph ‐BL) with varying ratios of heterosequences. Density functional theory (DFT) calculations confirmed that coordinating solvents stabilize the transition state for transesterification, thereby validating their role in triggering this process. By varying the microstructures and compositions, the resultant copolyesters display tunable thermal and mechanical properties, evolving from robust plastics with an ultimate tensile strength of up to 46.3±3.1 MPa to tough elastomers with >99.3 % elastic recovery. All the copolyesters exhibit remarkable thermal stability ( T d,5% =376 °C) and maintain desirable chemical circularity (>92 %), supporting a closed‐loop life cycle for sustainable material economy.