Synthesis of Poly[(R)3HB], Poly[(S)3HB], and Poly[(R)3HB-co-(S)3HB] by Phosphazene Base-Catalyzed Ring-Opening Polymerization of β-Butyrolactone Enantiomers Derived from (R)-Methyl 3-Hydroxybutyrate

Poly(3-hydroxybutyrate) (P3HB) has attracted considerable attention as an alternative to industrial polymers owing to its biorenewability and biodegradability, even in oceans. The chemical synthesis of P3HB offers advantages for the construction of molecular architectures in response to the increasing demand for materials designed for a new class of bio-related applications, including drug delivery systems and biodegradable plastics. The ring-opening polymerization (ROP) of beta-butyrolactone (BL) has been actively studied since the 1960s; however, the synthesis of isotactic P3HB remains challenging. Here, both enantiomers, (S)-BL and (R)-BL, were prepared from (R)-methyl 3-hydroxy butyrate and used for ROP with a phosphazene base (P1-t-Bu-tris). ROP proceeded with stereo inversion, and P[(R)3HB] was obtained from (S)-BL. Conversely, P[(S)3HB] was also synthesized from (R)-BL. Notably, P[(R)3HB] exhibited high isotacticity ([mm] = 0.97-0.99 and Pm = 0.97- > 0.99) analogous to the natural P3HB. Their molecular weights (Mn = 16.6-23.2 kDa) are compatible to those of previous reports for ROP of optically active BL. The copolymerization between (S)- and (R)-BL at an arbitrary ratio produced randomly distributed atactic P3HB in all cases. A mechanism for ROP using P1-t-Bu-tris was proposed, similar to the reported anionic ROP, in which a crotonic carboxylate acts as an initiator and carboxylate anions attacks the C3-carbon of BL. The proposed approach will facilitate the development of environmentally friendly alternatives to traditional industrial polymers.