Supramolecular organogel formation behaviors of beads-on-string shaped poly(azomethine)s dependent on POSS structures in the main chains

Abstract

Beads-on-string-shaped polymers incorporating cage silsesquioxanes in the main chain, especially double-decker-shaped phenyl-substituted silsesquioxanes (DDSQ), have received significant attention in the past few decades. Difunctionalized isobutyl-substituted cage octasilsesquioxanes (T₈) have also been generating this type of polymer architecture. However, direct comparisons between main-chain-type polymers comprising phenyl-substituted DDSQ and isobutyl-substituted T₈ units have seldom been reported. In this study, we prepared bis(3-aminopropyl)-DDSQ (6) via the hydrosilylation of 3,13-dihydrooctaphenylhexacyclodecasiloxane (dihydro-DDSQ) with t-butyl N-allylcarbamate and deprotection with trifluoroacetic acid. Both para-bis(3-aminopropyl)hexaisobutyl-substituted T₈ cage (1), which was developed by us, and 6 were polymerized with terephthalaldehyde (2a), isophthalaldehyde (2b), and 4,4′-oxydibenzaldehyde (2c) to produce the corresponding T₈-poly(azomethine)s (3a–c) and DDSQ-poly(azomethine)s (7a–c), respectively. Although precipitates were observed when the polymerization solution of 7 was concentrated under reduced pressure, 3 underwent organogel formation. Solubility test results for heptaisobutyl–hydride–POSS (8) and dihydro-DDSQ (9) as model compounds for the POSS units in 3 and 7, respectively, suggested that the wider solubility spectrum of the isobutyl-substituted T₈ moieties in 3 may provide a suitable balance for organogel formation, and the narrower spectrum of solubility for the phenyl-substituted DDSQ moieties in 7 may prevent the existence of a specific solvent region for suitable balanced gel formation. The concentration (c) dependences of zero shear relative viscosity (η_0,r) for the polymerization solutions of 3a–c, containing various linker structures were well described by an equation with a single value of the characteristic exponent using different values of gelation threshold concentration.