Programmed Polymer Folding with Periodically Positioned Tetrafunctional Telechelic Precursors by Cyclic Ammonium Salt Units as Nodal Points

A programmed polymer folding process
has been demonstrated by employing
a pair of periodically positioned tetrafunctional, linear telechelic
poly­(THF)­s having 5-membered cyclic ammonium salt groups, i.e., N-ethyl or N-phenylpyrrolidinium groups
at both chain ends, and N,N-dialkylpyrrolidinium
groups at the two interior positions, accompanying two units of a
dicarboxylate counteranion to balance the charges, Ia and Ib, respectively. The electrostatic self-assembly
and covalent fixation process has subsequently been applied, to cause
the ring-opening reaction of the pyrrolidinium units by carboxylate
counteranions under dilution. The obtained doubly cyclized polymer
products, IIa from Ia and IIb from Ib, were characterized by 1H NMR and
by a MALDI-TOF mass technique, to indicate the formation of polymeric
constitutional isomers of either manacle-, 8-, or θ-form. The
SEC peak deconvolution analysis of IIa showed the preferential
formation of the manacle-form isomer over the 8- and the θ-form
counterparts, to accord with the polymer folding of Ia, having the equivalent chemical reactivity of the linking groups,
directed by the spatial distance between the folding points. On the
other hand, the relevant SEC analysis of IIb showed the
predominant formation of the 8-form isomer, consistent with the polymer
folding of Ib promoted by the enhanced chemical reactivity
of the N-phenylpyrrolidinium end groups over the
interior N,N-dialkylpyrrolidinium
groups.