Fabrication of Polyelectrolyte Sheets of Unimolecular Thickness via MOF-Templated Polymerization

Abstract

Two-dimensional (2D) electrolytes have intrigued researchers due to their anisotropic structures and unconventional properties. This report introduces the bottom-up synthesis of a polymer-based 2D electrolyte utilizing a metal–organic framework (MOF), [Ni­(Hbtc)­(bpy)] n (1), with a pillared-layer structure as a template. Crosslinking polymerization of methyl methacrylate (MMA) within MOF nanochannels comparable in size to the vinyl monomer resulted in poly­(methyl methacrylate) (PMMA) monosheets. Following hydrolysis of this precursor polymer successfully gave 0.6 nm thick poly­(methacrylic acid) (PMAA) sheets. Despite the crosslinked structure, the obtained polymer (PMAA-1) is highly soluble as monosheets dispersed in water at a pH higher than 5.7. PMAA-1 shows viscous liquid-like nature with the addition of the Al3+ ion, which inherently promotes hydrogel formation of conventional linear PMAA. The topological effects of the 2D polyanion network structure on the uptake of cations in the matrix are likely responsible for these remarkable solution and viscoelastic properties of PMAA-1.

Nanoconfinement Approaches to Two-Dimensional Polymeric Materials

“…(c) An example of observation on conformational change of a 2DPM: pH-responsive hydrodynamic radius (⟨ R H ⟩) change of flexible 2D PMAA in aqueous solution. Reprinted and edited with permission from ref . Copyright 2023 American Chemical Society.…”

“…The dimensionality of the MOF nanopores is transcribed to the polymer structure. When monomers are polymerized in the 2D nanospace of a pillared-layer MOF are separated by coordination layers with the distance of the “pillar” molecules, 2DPMs were prepared on a gram-scale. ,, A similar synthetic trial of a 2DPM was performed in an earlier study using sodium montmorillonite clay; however, the obtained polymer was neither cross-linked nor isolated by removing the host . When we performed the cross-linking polymerization of styrene and ethylene glycol dimethacrylate (cross-linker) in a pillared-layer MOF, [Ni­(Hbtc)­(bpy)] n (btc = 1,3,5-benzenetricarboxylate, bpy = 4,4′-bipyridyl), we obtained 2D polystyrene (2D PS) after MOF removal (Figure a) .…”

“…For example, a 2D poly­(acrylonitrile) (2D PAN) and a 2D poly­(methyl methacrylate) (2D PMMA) were fabricated using the same method. , Both neutral and charged polymers are available. The hydrolysis of precursor 2D PMMA yields 2D poly­(methacrylic acid) (2D PMAA) in the form of a sheet-shaped polyelectrolyte . Based on the stability of the obtained 2DPMs, further chemical modification may be performed to further diversify its applications.…”

“…In solution, the 2D network greatly influences dispersibility. PMAA linear chains in basic aqueous solutions behave as polyelectrolytes to form transient aggregates. , On the other hand, 2D PMAA exhibits high dispersity . This is possibly due to the suppressed entanglement and screening effects resulting from cation trapping in the 2D network.…”

“…One potential solution is the statistical analysis of AFM images, which delivers intuitive results for the average size and distribution. We have recently shown that mass photometry can also deliver relatively accurate number-averaged and weight-averaged molecular weights for 2DPMs . The advantage of mass photometry is its ability to acquire a large number of data points (>10,000 counts) when using just a few microliters of very dilute solution (0.1–1 μM).…”