A Clear Path for Polymer Crystallization

Visible light polymerizes aggregates of colored organic molecules to create transparent polymer microcrystals. [Also see Report by Dou et al. ] The interior of a molecular crystal may not seem like the ideal environment for setting up a chemical reaction. The molecules in the crystal are held rigid, and there is no way to introduce new reagents or catalysts. However, for some reactions, a crystalline environment brings molecules together in a preferred orientation that leads to products that could not be formed in solution ( 1 ). The periodicity and high degree of order that characterize crystalline materials are particularly well suited to polymerization reactions, which can stitch monomers together into a highly ordered, nearly defect-free chain. The most commonly studied class of polymers made in this way are polydiacetylenes, prepared from crystals of molecular diyne monomers ( 2 , 3 ). On page 272 of this issue, Dou et al. ( 4 ) report that a new class of polymers can be prepared by solid-state polymerization of organic dye molecules. Both the crystallization and the polymerization in this system are so favorable that the reaction can be carried out in highly concentrated solutions, where micro- or nanocrystalline aggregates control the chemistry.