A Recyclable Polythioester With α ‐Gem‐Dimethyl Substitution: Instantaneous Crystallization Triggered by Large and Rapid Stretching

ABSTRACT Designing and fabricating recyclable polymers combining closed‐loop depolymerizability with high mechanical performance remains a major challenge in sustainable materials. To reach both high recyclability without compromising the mechanical properties, precise controlling of the crystallization behaviors emerges as a crucial strategy. Here, we demonstrate unexpected crystallization behavior in a depolymerizable polythioester (PTE), PaGMTE, and report the first crystal structure of this kind. Derived from organocatalytic ring‐opening polymerization of α ‐gem‐dimethyl‐ β ‐thiolactone, PaGMTE exhibits exceptionally slow quiescent crystallization, which stands in sharp contrast to its polyester analog polypivalolactone that crystallizes rapidly. However, large‐amplitude rapid stretching (strain ~600%, strain rate ≥ 10 s −1 ), where the Deborah number based on the Rouse time of entanglement strand exceeds 1, triggers its crystallization, accelerating the crystallization rate by > 200,000 fold and achieving ~40% crystallinity within 1 s. This approach yields highly oriented fibers with outstanding mechanical properties (Young's modulus of 0.80 GPa, tensile strength of 120 MPa, breaking strain of 70%), high transparency, and the potential as a waveguide. We unveil PaGMTE's orthorhombic crystal structure (space group P 2 1 2 1 2 1 , a = 1.075 nm, b = 0.622 nm, c = 3.618 nm) with antiparallel‐packed homochiral 8 3 helices. The slow quiescent crystallization shall arise from entropic barriers associated with conformational adjustments toward the helix. Our findings highlight sulfur substitution's profound impact on polymer properties and prove that significantly enhancing the crystallization process holds promise for making PTE a high‐value‐added recyclable material.