Simple One-Pot Strategy for Viscosity Reduction in Itaconic Acid-Based UPRs via End-Group Modification

Improving the processability of bio-based unsaturated polyester resins without compromising their performance remains a key challenge for their broader industrial adoption. This study investigates how post-polymerization modification reactions influence their molecular structure, intermolecular interactions and rheological behavior. A solvent-free, one-pot post-polymerization modification reaction using dimethyl itaconate was developed to modify the prepolymers directly after synthesis, without any additional purification steps. Acetylation served as a reference post-polymerization modification reaction. Four prepolymers with varying contents of hydroxyl groups in excess (10–40 mol.%) were prepared and then modified. Both approaches resulted in substantial reductions in viscosity up to 71% for end-capping with dimethyl itaconate and up to 61% for acetylation (relative to the corresponding non-modified prepolymers), markedly enhancing processability. However, mechanical properties were sensitive to the post-polymerization modification reactions: while acetylation led to a general decrease across all measured mechanical properties, end-capping resulted in retained or increased tensile modulus despite the reduction in tensile strength, highlighting the trade-off between flow behavior and structural integrity. This study demonstrates that targeted end-group modification can effectively tune resin processability, providing insight for the design of sustainable resin systems.