Highly Active Chiral Zinc Catalysts for Immortal Polymerization of β‑Butyrolactone Form Melt Processable Syndio-Rich Poly(hydroxybutyrate) Tannaz Ebrahimi, †,‡ Dinesh C. Aluthge, † Savvas G. Hatzikiriakos, ‡ and Parisa Mehrkhodavandi* ,† † Department of Chemistry and ‡ Department of Chemical and Biological Engineering, University of British Columbia, Vancouver, British Columbia V6T 1Z1, Canada * S Supporting Information ABSTRACT: Highly crystalline poly(hydroxybutyrate) suffers from high melting point and entanglement molecular weight. This leads to low melt strength, limits processing through regular techniques, and precludes many applications. In this work we report a series of racemic and enantiopure zinc catalysts supported by variously substituted diaminophenolate ancillary ligands which form high melt strength PHBs with different molecular weights and microstructure. These complexes are active for the highly controlled polymerization of β-butyrolactone (BBL); some can polymerize 2000 equiv of BBL in less than 30 min. Changing the steric bulk of the ligand forms PHBs of varied syndiotacticity (P r = 0.75 to 0.55). These are highly robust systems capable of polymerizing an unprecedented 20000 equiv of BBL in the presence of 5000 equiv of benzyl alcohol. Thermorheological investigations reveal that the synthesized PHBs have surprisingly high melt strength at above the melting point. For processable PHBs, high density of entanglements and relatively low crystallinity are crucial. We show that the best PHBs should have high molecular weight and moderate syndiotacticity. ■ INTRODUCTION Poly(hydroxybutyrate) (PHB) is a biodegradable and biocom- patible 1 polyester which can be produced using bacterial fermentation techniques as a highly isotactic and crystalline polymer. 2 The high melting point and low melt strength of bacterial-based PHB cause instability during melt processing and limit its rheological studies and processing window. 3 As a result of its highly crystalline structure, PHB is very brittle; this limits its applications in consumer products. To improve the mechanical properties of bacterial PHB and to widen its processability window, blending with other polymers, 4 block copolymerization through microbial synthetic routes, 5 and incorporation of different organic and inorganic additives have been attempted. 3b,6 Despite the promise of these techniques, the high costs of microbial fermentation techniques and the lack of control over polymer molecular weight, dispersity, and microstructure are significant limitations in the commercializa- tion of PHB. The ring-opening polymerization of strained cyclic ester β- butyrolactone (BBL) using metal catalysts including Mg and Zn 7 Al, 8 In, 7c,9 Sn, 10 rare earth, 11 and transition metals 8b,12 as well as organocatalysts 13 has emerged as a pathway to access PHB with more varied microstructures and properties. 14 Catalysts for the copolymerization of BBL with other cyclic esters are also known. 9c,15 Some representatives are listed in Chart 1. Single site zinc(II) complexes bearing a β-diiminate ligand framework ([(BDI-Zn(μ-O i Pr)] 2 )(A) are active and robust catalysts for the nonselective polymerization of racemic BBL under mild conditions. 7j Highly active yttrium complexes supported by tetradentate dianionic aminoalkoxybis(phenolate) ligands (B) form PHB with variety of microstructures based on ligand substituents. 11d-f,h,i,16 Isotactically enriched PHB can be synthesized using chromium salophen 12c (C) and silica- supported neodymium bis(borohydride) species. 11f A series of tin complexes were reported to afford low molecular weight Received: August 31, 2016 Revised: October 27, 2016 Published: November 16, 2016 Article pubs.acs.org/Macromolecules © 2016 American Chemical Society 8812 DOI: 10.1021/acs.macromol.6b01908 Macromolecules 2016, 49, 8812-8824