Invoking Side-Chain Functionality for the Mediation of Regioselectivity during Ring-Opening Polymerization of Glucose Carbonates

The
extent of participation of side-chain functionalities during
the 1,5,7-triazabicyclo[5.4.0]­dec-5-ene (TBD) organobase-catalyzed
ring-opening polymerizations (ROP) of six-membered cyclic d-glucose-based carbonates was found to result in significantly different
regiochemical outcomes. High regioselectivity was observed for naturally
derived poly­(4,6-d-glucose carbonate)­s (PGCs) containing
carbonate side chain substituents in the 2- and 3-positions, whereas
regioirregularity was found for analogous PGCs with ether side-chain
substituents. The backbone connectivities and structural details of
these PGCs were examined through a combination of comprehensive 1D
and 2D NMR studies on unimers and dimers, verifying the ring-opening
preferences and indicating the contribution of side-chain functionalities
in regioselective ROP processes. A molecular understanding of the
curious role of side-chain functionalities was demonstrated via density
functional theory calculations, revealing stabilization effects of
intermolecular hydrogen bonding between the side-chain functionalities
and TBD in the transition states. Overall, this work provides fundamental
insights into the organocatalytic ROP of these specific six-membered
asymmetric cyclic glucose carbonates. More importantly, these findings
serve as a foundation for future design strategies that incorporate
adjacent functionalities within monomers to act as directing groups
and impart molecular interactions that define regiochemical ring-opening.