Chemical Synthesis of Crystalline Biodegradable Polyhydroxyalkanoates

Recently, biodegradable polymer materials have drawn much attention due to their environmental friendliness. Among them, natural polyhydroxyalkanoates (PHAs), a type of isotactic polyesters produced by microorganisms, exhibit excellent biocompatibility and biodegradability, and thus are promising alternatives to petroleum-based plastics. Chemical synthesis of PHAs possesses enormous application value due to their high synthesis efficiency, great controllability and easy control of product quality. Since the stereoregularity and crystallinity of PHAs strongly affects their thermal and mechanical properties as well as degradation rates, controlling the stereoregularity and/or crystalline PHAs is a key and also challenging issue in their synthesis process. This review provides a comprehensive overview of the advancements in the chemical synthesis of stereoregular and/or crystalline PHAs in the last 60 years from the perspective of monomer classification. It covers the stereoselective ring-opening (co-)polymerization of four-membered lactones (e.g., beta-butyrolactone (beta-BL) and its derivatives (BPL, MLA), beta-propiolactones with different alpha -, beta-, and alpha,beta-substitutions), eight-membered lactones (beta-BL's dimer, 8DL), twelve-membered lactone (beta-BL's trimer, TBL) and epoxides/CO. The review places particular emphasis on the effects of different catalyst systems on stereoselectivity, and summarizes the mechanical and thermal properties of the resulting polymers. Furthermore, it briefly touches upon the degradation and recycling strategies of PHAs, followed by an outlook on the approaches for achieving stereoselective controlled synthesis of PHAs. [Graphics] .