Biodegradable Polyhydroxyalkanoates by Stereoselective Copolymerization of Racemic Diolides: Stereocontrol and Polyolefin‐Like Properties

Abstract

Bacterial polyhydroxyalkanoates (PHAs) are a unique class of biodegradable polymers because of their biodegradability in ambient environments and structural diversity enabled by side‐chain groups. However, the biosynthesis of PHAs is slow and expensive, limiting their broader applications as commodity plastics. To overcome such limitation, the catalyzed chemical synthesis of bacterial PHAs has been developed, using the metal‐catalyzed stereoselective ring‐opening (co)polymerization of racemic cyclic diolides (rac‐8DLR, R=alkyl group). In this combined experimental and computational study, polymerization kinetics, stereocontrol, copolymerization characteristics, and the properties of the resulting PHAs have been examined. Most notably, stereoselective copolymerizations of rac‐8DLMe with rac‐8DLR (R=Et, Bu) have yielded high‐molecular‐weight, crystalline isotactic PHA copolymers that are hard, ductile, and tough plastics, and exhibit polyolefin‐like thermal and mechanical properties.

Hybrid monomer design for unifying conflicting polymerizability, recyclability, and performance properties

“…42 Thus, it remains a challenging task to combine several desired, but conflicting, properties into one lactone monomer. As copolymerization of two or more different monomers is considered as an effective strategy to obtain the materials with tailored properties by adjusting the chemical nature, composition, and sequence of the comonomers, often delivering improved or unattainable properties in relation to those of the constituent homopolymers, [44][45][46][47][48][49][50][51] it thus became an attractive strategy to address the trade-off issues. In this context, the LCT g-BL was copolymerized with high ceiling temperature (HCT) and strained seven-membered ε-caprolactone (ε-CL) 52,53 to produce copolymers aimed for enhanced polymerizability and thermal stability for PGBL, and degradability and recyclability for poly(ε-caprolactone) (PCL).…”

“…To improve the stereoselectivity of the ROP of rac-BiL, we next employed discrete chiral yttrium and lanthanum complexes supported by C 2 -symmetric salen ligands ( Figure 5A) as such catalysts have been shown to mediate highly stereoselective polymerization of eight-membered rac-diolides. 44,45,70 The use of rac-La-2 supported by the sterically demanding salcy ligand with bulky CPh 3 substituents at the 3-positions of the ligand rendered much more active polymerization system but the isotacticity of PBiL remained essentially the same (runs 2-4, Table 1). However, the high activity of rac-La-2/BnOH enabled the synthesis of highmolecular-weight linear PBiL (M n = 153 kg mol À1 , Ð = 1.06, run 4, Table 1) by employing a high ratio of [BiL]/[rac-La-2]/[BnOH] = 1,500/1/1.…”

N-Oxides amplify catalyst reactivity and isoselectivity in the ring-opening polymerization of rac-β-butyrolactone

“…6 High-activity catalysts capable of accessing syndioenriched P3HB have been reported, 7 yet access to isoenriched P3HB remains limited. Chen and coworkers developed an elegant strategy using 8-membered cyclic diolides to access near perfectly isotactic P3HB and stereoblock PHA copolymers; 8 however, a multi-step synthesis is required to access these monomers. Racemic β-lactones such as rac -β-butyrolactone ( rac -BBL) can be derived from abundant feedstocks via epoxide carbonylation, 9 but isoselective catalysts remain rare and generally suffer from low activity under ambient conditions (turnover frequency, TOF ∼4–25 h −1 ).…”

Carbonylative Polymerization of Epoxides Mediated by Tri‐metallic Complexes: A Dual Catalysis Strategy for Synthesis of Biodegradable Polyhydroxyalkanoates

“…Most of the resultant PHAs exhibit >24 kg mol −1 M n values, >84 % polyester selectivity, and >97 % regioselectivity (entries 6–8). The introduction of pendent‐long alkyl substituents can decrease the glass transition temperature ( T g ) of the resultant PHAs, further, it also increases the ductility and toughness to circumvent the brittleness of P3HB [6b] . Aryl groups were also tolerated in the carbonylative polymerization, and the corresponding polyester exhibits a T g value of 12 °C and an M n value of 22.9 kg mol −1 (entry 9, Figure S89 in Supporting Information).…”