The 1,6-polymerization of a triacetylene is an unknown transformation. [1, 2] Unsuccessful attempts to accomplish this polymerization were reported as early as 1972, soon after the remarkable discovery of the topochemically controlled 1,4polymerization of diacetylenes. [3] It was recognized that a successful 1,6-polymerization would require preorganization of the reactants. In 1994 Enkelmann gave a more up-to-date report of other unsuccessful attempts of the 1,6-polymerization of triacetylenes and provided a more complete analysis of the criteria necessary for a successful polymerization. [4] The structural parameters needed for a topochemical triacetylene polymerization can be derived if one assumes that the monomeric units must be preorganized with a defined simple translational distance of 7.4 , matching the geometric parameters of the expected polymer (Scheme 1). If the Scheme 1. A triacetylene should polymerize if it is preorganized with a defined geometry that matches the geometrical parameters of the expected polymer product.
monomers are spaced at this distance and tilted at an angle just under 308 then the neighboring triacetylene functionalities will be in 3.5 van der Waals contact, a condition that should maximize the chances of polymerization. These exacting structural requirements are unlikely to be met by any randomly chosen triacetylene.
The 1,6-polymerization of a triacetylene is thus a curious chemical problem. It is a long-standing synthetic problem of encapsulation complexes are rare. [16Β±18] Imprinting [19,20] in hydrogen-bonded assemblies provides another route to such systems, and recent work promises that behaviors similar to those observed here will also be found in metal Β± ligand assemblies. [8] The result augurs well for the application of these capsules in dynamic combinatorial libraries and other enantioselective processes, such as the catalysis of reactions with rates comparable to that of guest exchange. [21]