Stereoregular functionalized polysaccharides via cationic ring-opening polymerization of biomass-derived levoglucosan

We report the facile synthesis and characterization of 1,6-alpha linked functional stereoregular polysaccharides from biomass-derived levoglucosan via cationic ring-opening polymerization (cROP). Levoglucosan is a bicyclic acetal with rich hydroxyl functionality, which can be synthetically modified to install a variety of pendant groups for tailored properties. We have employed biocompatible and recyclable metal triflate catalysts - scandium and bismuth triflate - for green cROP of levoglucosan derivatives, even at very low catalyst loadings of 0.5 mol%. Combined experimental and computational studies provided key kinetic, thermodynamic, and mechanistic insights into the cROP of these derivatives with metal triflates. Computational studies reveal that ring-opening of levoglucosan derivatives is preferred at the 1,6 anhydro linkage and cROP proceeds in a regio- and stereo-specific manner to form 1,6-alpha glycosidic linkages. DFT calculations also show that biocompatible metal triflates efficiently coordinate with levoglucosan derivatives as compared to the highly toxic PF5 used previously. Post-polymerization modification of levoglucosan-based polysaccharides is readily performed via UV-initiated thiol-ene click reactions. The reported levoglucosan based polymers exhibit good thermal stability (T-d > 250 degrees C) and a wide glass transition temperature (T-g) window (<-150 degrees C to 32 degrees C) that is accessible with thioglycerol and lauryl mercaptan pendant groups. This work demonstrates the utility of levoglucosan as a renewably-derived scaffold, enabling facile access to tailored polysaccharides that could be important in many applications ranging from sustainable materials to biologically active polymers.

Automated summary

This study demonstrates the facile synthesis of functional stereoregular polysaccharides with 1,6-alpha linkages from biomass-derived levoglucosan through cationic ring-opening polymerization. Computational studies show that metal triflate catalysts exhibit high coordination efficiency with levoglucosan derivatives and low toxicity compared to previous catalysts. The reported polymers have good thermal stability and a wide glass transition temperature window.