Isodimorphic‐Like Crystallization and Mechanical Properties of Unsaturated <i>Poly(butene succinate)</i> : Crucial Role of <i>Cis</i> / <i>Trans</i> Isomeric Contents

Geometric configuration of C═C bonds along the polymer backbone plays a crucial role in determining the physical properties of the resulting materials. However, the key role of cis/trans-C═C units in the crystallization and physical property of stereoisomeric polymers has not been well understood. Herein, we report a unique series of linear unsaturated polyesters, poly(butene succinate) (PBuS), with the precisely controlled cis/trans-C═C unit contents synthesized via melt polycondensation. High-molecular-weight cis/trans-polyesters without detectable isomerization or saturation of C═C bonds were prepared in the used polycondensation method. We find that the synthesized PBuS was crystallizable across the entire range of cis/trans unit ratios and showed the unique isodimorphic-like crystallization behavior, with an asymmetric pseudo-eutectic point at ∼31% cis-unit content. Notably, cis-PBuS crystallized much faster than its trans-counterpart, highlighting the profound effect of C═C isomerism on crystallization behavior. Mechanical properties of PBuS correlated strongly with the crystallinity and crystalline structure, showing low strength and modulus but high flexibility around the pseudo-eutectic composition. This work elucidates the crucial role of cis/trans isomerism in polymer crystallization and provides a rational way for designing the aliphatic polyesters with tailored physical properties.