Stereoregular cyclic poly(3-hydroxybutyrate) enabled by catalyst-controlled tacticity and topology

Abstract

Chemocatalytic routes to poly(3-hydroxybutyrate) (P3HB), a natural biodegradable polyester that has long been considered as a more sustainable alternative to non-(bio)degradable polyolefins, can deliver P3HB materials with catalyst-controlled diverse stereomicrostructures for tunable properties. P3HB topology brings in another way of modulating its properties, but selective synthesis of cyclic P3HB is challenging; yet it is harder to simultaneously control both tacticity and topology. Here we introduce an effective strategy to selectively synthesize cyclic isotactic or syndio-rich P3HB by using bulky C₂-symmetric salen-ligated metal catalysts bearing an initiating ligand that is also a good leaving group, and a relatively long and ionic metal–polymer bond, both facilitating chain end-to-end cyclization. The resulting cyclic P3HB topology has been extensively characterized and differentiated from its linear counterpart, including direct visualization. When compared to its linear counterpart, cyclic P3HB of both stereomicrostructures shows noticeably superior properties, including higher melting and decomposition temperatures, better processability, and higher overall mechanical toughness.