Anatomy of a 2D Polymer Formation in the Single Crystal

This work addresses the anatomy of a 2D polymer formation; that is, we identify, localize, and describe the various processes required to obtain such a macromolecule in a single-crystal-to-single-crystal reaction. It contains aspects of lattice, local and integral strain, and the distribution of strain in the crystal and assembles the various bits and pieces to a full-scale mechanistic picture providing experimental evidence for why the polymerization avoids phase segregation and why the crystals do not shatter as a result. While lattice strain is investigated by Bragg scattering, most of the paper revolves around diffuse scattering and its analysis with the three-dimensional difference pair distribution function (3D-Delta PDF). Local and integral strain and the spatial distribution of all strains, which are fundamental issues in an endeavor to unravel the polymerization mechanism, are deciphered. These investigations use monomer crystals at three different polymerization conversions (0%, 22%, and 44%) to account for eventual mechanistic changes when oligomers start to prevail. The paper provides answers to how strain management is achieved by the crystal leading to an unprecedented level of insight into chemical reactions in crystals. First, it provides a comprehensive molecular-scale picture on how local strain is buffered during polymerization, and second, it gives an understanding of how the reactivity of the growth pairs is preserved despite the strong displacements observed in the average structure. The paper reaches out to chemists and therefore restricts crystallographic nomenclature to a minimum.