The single-crystal-to-single-crystal (scsc) synthesis of a 2D polymer based on photochemically triggered [2 + 2]-cyclo addition is reported. Both monomer and polymer single crystals are analyzed by X-ray diffraction, which is the first case of a scsc two-dimensional polymerization based on this cycloaddition and the third ever case for a scsc synthesis of a 2D polymer. The product crystals at quantitative conversion are wet-exfoliated under mild conditions and afford countless features that are single and double layers as judged by their AFM heights of h(AFM) approximate to 1.2 +/- 0.5 and 2.2 +/- 0.5 nm, respectively. The X-ray-structure based molecular weight of the 2D polymers and their degree of polymerization per mu m(2) are M = 360 MDa and P-n = 464 900, respectively. The sheet size is on the order of 5 X 5 mu m(2).