Cis / Trans Configuration-Tuned Crystallization and Mechanical Properties in Aliphatic Polyesters Synthesized from C4 Diacids and α , ω -Diols

Main-chain C═C units provide an effective structural handle for tailoring the physical structure and macroscopic property of polymers. However, how the cis/trans configuration of main-chain C═C units impacts the crystallization and physical property of polyesters is not well understood. Herein, we synthesized a series of saturated and cis/trans-unsaturated aliphatic polyesters from the four-carbon (C4) diacids (succinic acid, fumaric acid, or maleic acid) with even-numbered α,ω-diols (nCH2 = 4–14), and investigated their crystallizability, crystal polymorphism, mechanical properties, and chemical recycling. We find that the saturated and trans-polyesters with nCH2 = 4–14 all exhibit good crystallizability, while the cis-polyesters crystallize only when the CH2 spacer is sufficiently long. Compared with the saturated analogs, trans-C═C units can enhance the crystallization and melting temperatures (Tc, Tm) of polyesters. Interestingly, the long-spaced cis-polyesters (nCH2 = 12, 14) show the Tc dependence of crystal polymorphism, forming the metastable hexagonal form II at a low Tc and the thermodynamically stable form I at a high Tc. Mechanical properties of polyesters are highly influenced by the cis/trans-C═C configuration, and the cis-polyesters have much weakened mechanical strength and modulus than the trans and saturated ones. Moreover, these polyesters can be chemically recycled by methanolysis and repolymerization while preserving the C═C configuration in the recycling process. This study has shed light on the critical role of main-chain cis/trans-C═C configuration in regulating the crystallization and mechanical properties of aliphatic polyesters.