A Novel Bio-Based Unsaturated Copolyester With Tunable Structure and Properties: Synthesis, Characterization, and Performance of PPeSAB and Its Nano-Silica Composites

Unsaturated polyesters (UPs) are widely used thermosetting polymers due to their excellent mechanical strength, chemical resistance, and processability. In this study, a novel bio-based unsaturated copolyester, poly(1,5-pentanediol-co-3,4-butanediol succinate-co-adipate) (PPeSAB), was synthesized from bio-derived 3,4-butanediol, 1,5-pentanediol, succinic acid, and adipic acid via a two-step melt polycondensation. The synthesis was optimized to minimize side reactions, yielding high-molecular-weight copolyesters (M_n = 7.7 × 10⁴ g/mol) with well-controlled composition. By adjusting the monomer feed ratios, the material properties could be tuned from rigid plastic to soft elastomer, and the copolyesters exhibited excellent low-temperature performance (T_g < −61°C). Incorporation of nano-SiO₂ further improved mechanical strength, achieving tensile values comparable to linear low-density polyethylene. The fully aliphatic ester backbone also suggests potential environmental advantages, associated with its highlighting the material's promise for sustainable polymer applications. These findings provide a rational strategy for designing bio-based unsaturated polyesters with tunable thermal and mechanical properties.