Chain-End\nControlled Depolymerization Selectivity\nin α,α-Disubstituted Propionate PHAs with Dual Closed-Loop\nRecycling and Record-High Melting Temperature

Within\nthe large poly(3-hydroxyalkanoate) (PHA) family, C3 propionates\nare much less studied than C4 butyrates, with the exception of α,α-disubstituted\npropionate PHAs, particularly poly(3-hydroxy-2,2-dimethylpropionate),\nP3H(Me)<sub>2</sub>P, due to its high melting temperature (<i>T</i><sub>m</sub> ∼ 230 °C) and crystallinity (∼76%).\nHowever, inefficient synthetic routes to its monomer 2,2-dimethylpropiolactone\n[(Me)<sub>2</sub>PL] and extreme brittleness of P3H(Me)<sub>2</sub>P largely hinder its broad applications. Here, we introduce simple,\nefficient step-growth polycondensation (SGP) of a hydroxyacid or methyl\nester to afford P3H(Me)<sub>2</sub>P with low to medium molar mass,\nwhich is then utilized to produce lactones through base-catalyzed\ndepolymerization. The ring-opening polymerization (ROP) of the 4-membered\nlactone leads to high-molar-mass P3H(Me)<sub>2</sub>P, which can be\ndepolymerized by hydrolysis to the hydroxyacid in 99% yield or methanolysis\nto the hydroxyester in 91% yield, achieving closed-loop recycling\nvia both SGP and ROP routes. Intriguingly, the chain end of the SGP-P3H(Me)<sub>2</sub>P determines the depolymerization selectivity toward 4- or\n12-membered lactone formation, while both can be repolymerized back\nto P3H(Me)<sub>2</sub>P. Through the formation of copolymers P3H(Me/R)<sub>2</sub>P (<i>R</i> = Et, <sup><i>n</i></sup>Pr),\nPHAs with high tensile strength and ductility, coupled with high barriers\nto water vapor and oxygen, have been created. Notably, the PHA structure–property\nstudy led to P3H(<sup><i>n</i></sup>Pr)<sub>2</sub>P with\na record-high <i>T</i><sub>m</sub> of 266 °C within\nthe PHA family.