Conformational Control in the Cyclization of Hydrogen-Bonded Supramolecular Polymers

Bifunctional 2-ureido-4[1H]-pyrimidinone (UPy) derivatives can form small cyclic oligomers as
well as long supramolecular polymers in chloroform solutions using the quadruple hydrogen-bonding motif.
Ring−chain equilibria of a set of supramolecular monomers containing methyl-substituted alkyl linkers
between the hydrogen-bonding UPy moieties were investigated by 1H NMR spectroscopy and viscometry.
The data were characterized in terms of critical concentration (CC, denoting the onset of polymerization)
and equilibrium cyclic dimer concentration (EDC, representing preorganization of the monomer toward
selective formation of cyclic dimer). Methyl substituents in the monomer were found to promote conformations
favorable for cyclic dimerization, leading to an increase in both the EDC and the CC with respect to
unsubstituted monomer. Furthermore, we observed an odd−even effect in the CC and EDC with increasing
length of the linker between the hydrogen-bonding units. The combined results allow tuning of the critical
concentration over a broad range and offer detailed information on the correlation between monomer
structure, conformation, and polymerizability which may provide new insights for the study and design of
other ring−chain equilibria or helix−random coil transitions.