Tunable molecular weight poly(3-hydroxybutyrate) via aluminium catalysis: from plasticizers to ultra-flexible matrices

Ring-opening polymerization (ROP) of β-butyrolactone (β-BL) represents a promising route toward sustainable poly(3-hydroxybutyrate) (P3HB) materials with tunable properties. In this work, we report a detailed study of the β-BL polymerization catalyzed by an aluminium complex [AlMeL2]. Our results demonstrate a precise control over molecular weight through modulation of the reaction conditions. High molecular weight (Mw > 50 kDa) and low molecular weight (Mw < 2 kDa) P3HB were selectively synthesized, enabling their use as polymer matrices and plasticizing agents, respectively. Density functional theory (DFT) calculations unveiled the polymerization mechanism. It reveals a ligand-assisted initiation pathway involving proton transfer to the imine functionality, followed by ring opening and formation of an active aluminium–alkoxide species. End-group analysis confirms the O-alkyl cleavage mechanism that leads to crotonate-terminated chains. Bulk polymerization under solvent-free conditions afforded high molecular weight, highly flexible polymers (Mw up to 200 kDa), highlighting a more sustainable synthetic approach. Thermal analysis shows that high molecular weight P3HB exhibits glass transition temperatures comparable to bacterial analogues, while remaining amorphous highly elastic and processable. Low molecular weight P3HB displays a significantly lower Tg (−28 °C), confirming its effectiveness as highly compatible plasticizer for bacterial P3HB. This dual-application strategy highlights the potential of the [AlMeL2] catalyst to produce tailored P3HB materials for diverse applications, ranging from flexible polymer matrices to plasticizers.