The regiochemistries of glucose-based cyclic carbonates during ring-opening polymerizations (ROPs) are of essential importance, providing molecular insight and guidance into design and synthesis of polymer materials. In this work, investigations on regioselectivity were conducted for organobase catalyzed ROPs of three 5-membered cyclic carbonates with carbonate linkages formed through the 2- and 3- positions, and bearing different pendent acetal groups through the 4- and 6- positions of methyl-α-d-glucopyranoside using naturally sourced aldehydes. Regioirregularity was observed in all 4,6-acetal-protected poly(d-glucose carbonate)s (PGCs), as indicated by ¹H and ¹³C NMR spectroscopies. Explicit ratios between isomers of unimers and dimers were obtained to study the steric hindrance and/or electronic effects provided by the various protecting groups on regioselective preference during the ring-opening process. Correspondingly, the detailed proton-carbon connectivities of the unimer isomers were determined by a combination of 2D-NMR studies, including homonuclear correlation spectroscopy (COSY), ¹H-¹³C heteronuclear single quantum correlation (HSQC), and ¹H-¹³C heteronuclear multiple-bond correlation (HMBC), and further confirmed by single crystal structure analyses, which revealed a preference of O3-to-O2 polymerization direction. Moreover, comparison between the dimers revealed a significant transcarboxylation reaction coincident with the ring-opening reaction that also resulted in a regioirregular backbone connectivity. Overall, this work provides a fundamental understanding of the regiochemistry of organocatalytic ROP of 5-membered glucose carbonates with varying acetal protecting groups.

David Tran

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