2D conjugated microporous polyacetylenes synthesized via halogen-bond-assisted radical solid-phase polymerization for high-performance metal-ion absorbents

Abstract

The paper reports the first free-radical solid-phase polymerization (SPP) of acetylenes. Acetylene monomers were co-crystalized using halogen bonding, and the obtained cocrystals were polymerized. Notably, because of the alignment of acetylene monomers in the cocrystals, the adjacent C≡C groups were close enough to undergo radical polymerization effectively, enabling the radically low-reactive acetylene monomers to generate high-molecular-weight polyacetylenes that are unattainable in solution-phase radical polymerizations. Furthermore, the SPP of a crosslinkable diacetylene monomer yielded networked two-dimensional conjugated microporous polymers (2D CMPs), where 2D porous polyacetylene nanosheets were cumulated in layer-by-layer manners. Because of the porous structures, the obtained 2D CMPs worked as highly efficient and selective adsorbents of lithium (Li⁺) and boronium (B³⁺) ions, adsorbing up to 312 mg of Li⁺ (31.2 wt%) and 196 mg of B³⁺ (19.6 wt%) per 1 g of CMP. This Li⁺ adsorption capacity is the highest ever record in the area of Li⁺ adsorption.

Fig. 1. Schematic illustration of XB-assisted SPP, compounds used in this work, monomer cocrystal and polymer structures, and PXRD patterns before and after polymerization.

a Schematic illustration of XB-assisted SPP of acetylene. b Schematic illustration of polyacetylene synthesis, 2D CMP formation after linker removal, and metal ion adsorption of 2D CMP. The inserted photos show PPDA-CMP-1, 2, and 3 generated from monomer cocrystals 1·6, 1·7, and 1·8, respectively (after linker removal). c Monomers, linkers, and photo-initiator used in this work. d (A) Monomer cocrystal structure of 1·6 determined by single-crystal X-ray diffraction and a possible polymer structure expected from the monomer cocrystal structure. The figure extracts a single x-y plane of the monomer cocrystal and its possible polymer structure, which is a ladder-shaped polymer growing on the x-axis. The ladder-shaped polymer is further connected to the neighboring ladder-shaped polymers on the y-axis, forming a nanosheet in the x-y plane. The discussion of the polymer structure is given in Supplementary Information (Supplementary section 2.2 and Supplementary Fig. 1). B A possible multilayer structure of PPDA-CMP-1, showing that the nanosheets form a layer-by-layer structure on the z-axis. e PXRD patterns of pure XB linker 6 (orange), monomer cocrystal 1·6 (blue), and the polymer obtained from 1·6 via SPP (green), and their calculated PXRD patterns (in gray color and overlapped with the experimental spectra). 60% of the PXRD pattern of the polymer matched that of the monomer cocrystal. f PXRD patterns of pure solid monomer 1 (blue) and the polymer obtained from 1 via solution-phase polymerization (green), and their calculated PXRD patterns (gray). 6% of the PXRD pattern of the polymer matched that of the solid monomer.

Fig. 2. Porous structures, 2D exfoliated structures, and BET analysis of PPDA-CMP.

a Schematic illustration of three types of pores in CMPs. b SEM images of non-exfoliated PPDA-CMP-1 showing inter-grain micropores. c TEM images of exfoliated PPDA-CMP-1 (at 2 × 10^–4 wt% of CMP in GBL) showing surface (image A) and single-chain nanopores (zoom-in image B) at the outermost layer of CMP. d AFM images and schematic illustration of exfoliated PPDA-CMP-1 (at 2 × 10^–4 wt% and 0.1 wt% of CMP in GBL) with (A) monolayer (blue), (B) bilayer (green), (C) tri-layer (red), and (D) tetra-layer (pink) nanosheets. e BET analysis with a nitrogen (N₂) adsorption-desorption isotherm of non-exfoliated PPDA-CMP-1.

Fig. 3. Metal ion adsorption-desorption in PPDA-CMP.

a Schematic illustration of adsorption-desorption of Li⁺ (left) and B³⁺ (right) in PPDA-CMP. Li⁺ is selectively adsorbed from a mixture of Li⁺, Rb⁺ and Cs⁺ in PPDA-CMP-1 (bottom). b Full and Li 1 s XPS spectra of PPDA-CMP-1 after Li⁺ adsorption (purple lines) and desorption (green lines) for the pure Li⁺ system (Table 2, entry 1). c Full, Li 1 s, Rb 3p, and Cs 3d XPS spectra of PPDA-CMP-1 after metal ion adsorption (orange lines) and desorption (purple lines) for the Li⁺ + Rb⁺ + Cs⁺ system (Table 2, entry 4). d SEM-EDS mapping images of PPDA-CMP-1 after B³⁺ adsorption (top) and desorption (bottom) for the pure B³⁺ system (Table 2, entry 7); electron images (SEM, gray), summarized EDS images (EDS, multi-colors), and individual element images (carbon (C, red), oxygen (O, blue), nitrogen (N, yellow), and boron (B, purple)).