This review summarizes the different categories of topochemical polymerizations for the synthesis of fully-organic polymers and their design strategies.

Conspectus Topochemical reactions are solid-state reactions that transpire under the strict control of molecular packing in the crystal lattice. Due to this lattice control, these reactions generate products in a regio-/stereospecific manner and in very high yields. In a broader sense, topochemical reactions mimic nature’s way of carrying out reactions in a confined environment of enzymes giving specific products. Apart from their remarkable specificity, topochemical reactions have many other interesting features that make these reactions more attractive than solution-phase reactions. Solution-phase reactions necessitate the use of reactants, reagents, catalysts, and solvents and often give products along with varying amounts of byproducts, necessitating complex workup and chromatographic purification using various chemicals. These inevitable chemical wastes from solution-state reactions could be avoided by topochemical reactions, as they are solvent-free and catalyst-free and often do not require any chromatographic purification in view of their specificity and high yielding nature. Also the confinement offered by the crystal lattice gives products that are not possible by solution-phase reactions. Another interesting feature of topochemical reactions is the possibility of formation of products in an ordered (crystalline) form, which imparts interesting properties. Thus, topochemical reactions have control not only at the molecular level (regio-/stereospecificity) but also at the supramolecular level (packing). Many topochemical reactions happen in single-crystal-to-single-crystal (SCSC) fashion, and crystal structure analysis of such reactions often gives mechanistic insights and knowledge about the geometrical criteria required for the reaction. Despite all these attractive features, reactions that can be done topochemically are limited. There is tremendous interest in the development of new categories of topochemical reactions and strategies to achieve reactivity in crystals. In this Account, we will summarize our attempts to develop topochemical azide–alkyne cycloaddition (TAAC) reactions. We have used hydrogen-bonding as the main noncovalent interaction for aligning azide-and-alkyne-substituted derivatives of various biomolecules in orientations suitable for their proximity-driven cycloaddition reaction in crystals. Overall, three major classes of biomolecules; carbohydrates, nucleosides, and peptides were successfully exploited for their TAAC reactions using conventional O–H···O, N–H···O, and N–H···N hydrogen bonds as supramolecular glues for controlling their assembly in crystals. The crystals of these monomers underwent TAAC reaction either spontaneously at room temperature or under heating yielding triazole-linked biopolymer mimics. The ordered packing of product molecules imparted special properties to the products formed. The legendary “cream of the crop” azide–alkyne click reaction has diverse applications in the areas of bioconjugation, material science, polymer synthesis, and so forth. Belonging to...

Different packing is a mechanism through which nature can produce materials of different properties from the same basic units. There is great interest in constructing different forms of the same polymer by utilising different packing. Common solution-synthesized polymers are amorphous and their post-synthesis crystallization into different topologies is almost impossible. Here we show solid-state polymerization of different reactive polymorphs of a monomer pre-organized in different topologies. Trimorphs of a dipeptide monomer pack in a head-to-tail fashion, placing the azide and alkyne of adjacent monomers in proximity. On heating, these crystals undergo a topochemical azide-alkyne cycloaddition reaction yielding triazole-linked polymer in three different crystalline states; one with antiparallel arrangement of polymer chains, another with parallelly oriented chains, and a third form containing a 1:1 blend of two different conformers aligned in parallel. This approach of exploiting different polymorphs of a monomer for topochemical polymerization to yield polymorphs of polymers is promising for future research.

The topochemical synthesis of a miscible polymer blend is described. The azide‐ and alkyne‐decorated tetrol 1 crystallizes as two different conformers. Both conformers exhibit self‐sorted head‐to‐tail alignment with proximally placed reacting groups such that topochemical polymerization yields two types of polymer chains, each containing only one type of conformer. The orientation of complementary reactive groups in one of the head‐to‐tail‐arranged conformers favors the formation of cis‐triazole linkages, and the other favors the trans‐triazole linkages. Crystals of 1 on heating gave a perfect polymer blend containing equal amounts of cis‐triazole‐linked and trans‐triazole‐linked polymers. As each conformer is H‐bonded to four conformers of the other kind, the polymerization yields a perfect polymer blend wherein each polymer chain is surrounded by chains of the other type. Thus, the molecular ordering in the prepolymerized state in a crystal is utilized to create a polymer blend.

Polymers are an integral part of our daily life. Hence, there are constant efforts towards synthesizing novel polymers with unique properties. As the composition and packing of polymer chains influence...