Mechanically interlocked polymers remain one of the most fascinating and demanding challenges in synthetic chemistry.  Over the past decade, a vast number of interlocked macromolecules have been synthesized and characterized.  In contrast with traditional covalent polymers, mechanical bonds provide much higher degrees of freedom to the already constrained macromolecules.  Therefore, materials based on mechanically interlocked polymers are expected to display exotic physical properties.  Although the synthesis of hydrogen-bonded oligo[2]rotaxanes has been described previously, high degrees of polymerization (DPs) have never been attained.  Olefin metathesis is a powerful carbon-carbon double bond forming reaction that has become one of the most widespread synthetic tools in organic chemistry.  In particular, polymer chemistry has benefited greatly from the versatility of the reaction.  Several novel polymeric architectures have been synthesized recently on account of the high efficiency and functional group tolerance of the ruthenium-based catalytic systems.  Given the high functionality of species that undergo molecular recognition in solution and the high conversions required in step-growth polymerizations to obtain high DPs, olefin metathesis has the right credentials to perform well in the synthesis of mechanically interlocked polymeric architectures.  In this manner, poly[2]rotaxanes can be synthesized (Box) by the stoppering of a preformed daisy-chain type polypseudorotaxane (A) or by the regioselective end-to-tail polymerization of a pseudorotaxane (B).

Box.  Two different approaches towards the synthesis of daisy-chain type polymers.