The synthesis of polycatenanes is considered a very challenging task. The formation of poly-[2]-catenanes can be achieved by polymerization of functionalized [2]-catenanes. Also the synthesis of [3]-catenanes, [5]-catenanes, [6]-catenanes and [7]-catenanes is reported in many articles. The synthesis of poly-[n]-catenanes has instead some practical issues. To this purpose, molecular dynamic simulation is very used as a tool for the design of the optimal synthetic path toward the desired product by predicting the final topology.
There are two main synthetic routes: the Statistical approach and the Template-Directed approach.
The statistical approach is based on a stochastic methodology. When the reactants are together, there is a probability that they will fit together first and then close on top of each other in a process of cyclization. The catenation of two rings into a catenane is already complex, thus, as expected, the interlocking of multiple cycles into a polycatenane is statistically improbable. Being an unfavored entropically process the product is obtained in very small amount. Also, the cyclization process requires high dilutions, but the elongation of the chain is favored at high concentrations, making the synthesis even more difficult.
The Template directed approach is based on the host-guest interactions that can direct the cyclization of pre-organized linear unit upon the existing macrocycle. These interactions can be hydrogen bonds, π-π interactions, hydrophobic interactions or metal ions coordinations. In this way the synthesis can be enthalpy-driven, obtaining quantitative results.
The yield and selectivity are restrained by the kinetic or thermodynamic control of the reaction.
Generally the kinetic control induces the formation of a product after short reaction times because it is favoued by irreversible reactions (or equilibrium reaction moved very much toward the formation of the products). The thermodynamic product is obtained for longer reaction times for reversible processes. In this case the units have the time to rearrange themselves toward the most stable state, in a sort of error-checking process. This is obtained by breaking covalent and coordination bonds and forming the most stable ones.