Hafnium-Catalyzed Controlled Polymerization of 1,5-Hexadiene toward Polyolefin-Based Graft Copolymers for Poly(lactic Acid) Toughening with High Transparency

This study developed an efficient strategy based on hafnium catalysts for the synthesis of poly(lactic acid) (PLLA) toughening agents. The homopolymerization of 1,5-hexadiene (HD) catalyzed by selected hafnium complexes successfully prepared poly(1,5-hexadiene) (PHD) with reactive C═C double bonds in the side chains (up to 11 mol %), and the catalytic activity was as high as 10⁶ g(polymer)·mol^–1(Hf)·h^–1. Hydroxylated PHDs (PHD_OH) were obtained by functionalizing the PHD with β-mercaptoethanol using thiol–ene click chemistry. Subsequently, a series of graft copolymers (PHD-g-PLLAs) with different PLLA side chain lengths was synthesized from ring-opening polymerization of l-lactide (l-LA) with PHD_OH as the macromolecule initiator and cyclic trimeric phosphazene base as the organocatalyst. The graft copolymer, as an efficient toughening agent, can significantly enhance the toughness of commercial PLLA, showing a maximum elongation at break of up to 183%, which is approximately 30-fold that of neat PLLA. Meanwhile, the resultant toughened commercial PLLA maintained high transparency and rigidity. This method provides a promising approach for the development of PLLA materials with both excellent toughness and optical transparency.