Different Gelation and Self‐Sorting Properties of Two Isomeric Polyamides Owing to the Parallel versus Anti‐Parallel Alignment of Backbone Dipoles

Abstract

Two isomeric bottlebrush polyamides P-1 and A-1, with the same repeating monomer dipole units aligned along the polymer backbone in pseudo-parallel and pseudo-antiparallel fashion, respectively, were synthesized and characterized. Both polymers can form thermoreversible gels with aromatic solvents but P-1 was found to show inferior gelation strength compared with that of A-1. Furthermore, despite their close structural resemblance, a 1:1 mixture of the P-1 and A-1 polymers was shown to exhibit self-sorting in the gel state. Gel formation was found to be a kinetically trapped process through hydrogen bonding, π-π stacking interactions, and side chain interdigitation. The different gelation and self-sorting properties can be explained by the local dipole-dipole interactions originating from the different modes of backbone dipole alignment. In single gel systems, the antiparallel-aligned dipoles in A-1 facilitated a more compact molecular packing owing to the enthalpically more favorable polymer chain association. On the other hand, the parallel-aligned dipoles in P-1 gave rise to a less stable head-to-head packing, which had difficulties to convert to the more stable head-to-tail packing in a kinetically trapped environment. In the mixed gel system, it is the unfavorable hetero-polymer mismatch dipole-dipole interaction that inhibited the mixing of the A-1 and P-1 polymers and led to self-sorting.

Differential Metal Ion-Sensing Properties of Isomeric Crystalline Polyacylhydrazone Organogelators

“…The second is that the presence of Hg 2+ can promote gel strength. As mentioned earlier, we reported that isomeric polymers having different molecular dipole alignment patterns could exhibit very different organogelating and self-sorting properties due to the additive dipole moment effect. However, in the present study, given that P-1 and P-2 are isomers that only differ in the relative disposition of the two alkyl side chains (i.e., the sterically bulkier R 2 and the smaller R 1 ), it is rather perplexing to find out that their different metal ion-responsive properties were due to such a small perturbation of the steric factor.…”

“…It was noted that such differential properties of isomeric polymers, although not commonly seen, had been reported before. In previous cases, the origin of the differential properties was due to the different alignment patterns of the monomer dipoles along the polymer backbone, resulting in a difference in their organogelating and self-sorting properties …”

“…In previous cases, the origin of the differential properties was due to the different alignment patterns of the monomer dipoles along the polymer backbone, resulting in a difference in their organogelating 28 and self-sorting properties. 29 To promote the crystallinity of the polymers, 2,5dihydroxyterephthalamide units were used for the rigidity of the polymer backbone via intramolecular hydrogen bonding. At the same time, isopentyl (R 1 ) and racemic 4-butyldecyl (R 2 ) side chains were attached to the polymer backbone to improve their solubility.…”

Self-sorting assembly of artificial building blocks

“…In another example, Chow et al reported the self-sorting of two polyamines with different dipole alignments. 142 These polymers aggregated via strong hydrogen bonding and π–π stacking to form gels. By mixing the two polymers upon heating and cooling to room temperature, self-sorted gels could be formed.…”

Organogelating and narcissistic self-sorting behaviour of non-preorganized oligoamides

“…Narcissistic self-sorting can also be manifested in the form of gel formation in multi-component gel system 30. In this context, we earlier showed that self-sorting occurred in a mixture of oligoamide gels 3 and 4 ,27 and also in polyamide gels 5 and 6 28. In the case of the oligamides, self-sorting was attributed to the difference in the number of amide units of the two structurally homologous hexamide 3 and dodecaamide 4 .…”