Abstract
Covalent long-range ordered (crystalline) sheets called 2D polymers have recently been synthesized by irradiating single crystals of suitably packed monomers. To have such an action proceed successfully, billions of bond formation processes have to be mastered exclusively in two dimensions within 3D crystals. This raises questions as to how to elucidate the mechanism of these unusual polymerizations as well as their entire strain management. The article will show that single crystal X-ray diffraction based on both Bragg and diffuse scattering are powerful techniques to achieve such goal. The very heart of both techniques will be explained and it will be shown what can be safely concluded with their help and what not. Consequently, the reader will understand why some crystals break during polymerization, while others stay intact. This understanding will then be molded into a few guidelines that should help pave the way for future developments of 2D polymers by those interested in joining the effort with this fascinating and emerging class of 2D materials.
Highlights
Covalent long-range ordered sheets called 2D polymers have recently been synthesized by irradiating single crystals of suitably packed monomers
Local structural information is important when monitoring the course of a chemical reaction within single crystals with X-ray diffraction (XRD) (3) To learn that powder XRD, transmission electron microscopy (TEM) and electron diffraction are alternative methods to single crystal XRD for structural elucidation but that each of these methods has specific strengths and weaknesses which renders them comprehensive to one another rather than exchangeable
(4) To understand that the mechanism of a 2D polymerization in a single crystal is a complex 3D process that involves all components the crystal is composed of: monomers, templates, and solvents
Summary
How to use X-ray diffraction to elucidate 2D polymerization propagation in single crystals†. Covalent long-range ordered (crystalline) sheets called 2D polymers have recently been synthesized by irradiating single crystals of suitably packed monomers To have such an action proceed successfully, billions of bond formation processes have to be mastered exclusively in two dimensions within 3D crystals. When synthesizing 2D polymers in single crystals, an external stimulus (most often light) converts a layered monomer crystal into a crystal composed of stacks of regularly covalently connected layers.[2,3] Fig. 1 shows a prototype example all the way from the macroscopic single crystal to the molecular structure of the product obtained.[4] Subsequent exfoliation of these layers provides access to thin sheet stacks and individual sheetlike macromolecules The latter are called 2D polymers because of their molecular structure being tile-covered by topologically planar repeat units (RU),[5,6,7] very much in analogy to linear polymers the molecular structure of which is composed of a sequence of topologically linear RUs.8 2D polymers with sizes of say a few tens of mm[2] require formation of millions of covalent
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