Fig. 2. (a) Two-step topochemical 1,4-polymerization of bis(diacetylene)-based monomers to form ladder-PDAs. (b) Chemical structures of the bis(diacetylene)-based monomers 1–8. (c) Favourable packing of monomer 9 in crystals for the formation of a ladder-PDA upon TP. In the crystal packing of 9, the DA units are represented as a ball-and-stick model and H atoms are omitted for clarity, except for N–H.