Slip-link polymers via entropy-driven ring-opening olefin metathesis polymerizations of rotoxanes
Slip-link polymers are a novel class of polymers in that the monomers, that comprise the polymer strands, are held together by mechanical entanglements as opposed to standard chemical bonds. As a result, slip-link polymers are expected to be uniquely responsive to external stimuli (e.g., temperature, pH, light, redox, etc.) and thus find use in molecular sensors and more complex molecular machines. In the past decade, several synthetic approaches have been reported to produce slip-link polymers; typically however, the products have been found to contain only 2 to 3 entangled monomers. In a new approach to access slip-link polymers, we have carefully designed and synthesized a self-entangled rotaxane monomer for use in entropy-driven ring-opening metathesis polymerization (ED-ROMP). We thus subjected rotaxane monomers to ROMP initiators and, as a result, slip-link polymers were obtained wherein we specifically observed structures comprised of 2 through 15 mechanically-entangled monomers. In order to study the effect of the structure of the self-entangled rotaxane monomer on the degree of polymerization, we have synthesized a series of novel copper-based self-entangled rotaxane monomers for use in ED-ROMPs. The rationale behind the changes in the design of the original rotaxane monomer, synthesis and preliminary results of using these novel rotaxane monomers, incorporated with internal olefin functionalities, in ED-ROMP are presented.