Supramolecular chemistry with macromolecules: Macromolecular knitting, reversible formation of branched polyrotaxanes by self-assembly

Mechanically Linked branched polymer 10 was prepared via rotaxane formation by self assembly ("knitting") of poly[bis(5-methylene-1,3-phenylene)-32-crown-10 sebacate] (4) and preformed polyurethane 9 containing N,N'-bis(beta-oxyethyl)-4,4'-bipyridinium 2PF(6)(-) ("paraquat") moieties in tetrahydrofuran (THF). The interpenetrating structure resulting from "knitting" the bipyridinium units of the polyurethane through the crown ether cavities of the polyester structure was proved by its color, NMR studies and gel permeation chromatography (GPC) measurements. In accord with its equilibrium nature, the branching process (rotaxane formation) was reversible depending on the solvent and temperature. The branched material was soluble because the two polymeric components were designed to have relatively low molecular weights and the polyurethane to contain only about two paraquat units, so as to be able to carry out solution characterization. However, extension of this concept to higher molecular weight and/or more highly functionalized systems provides interesting possibilities for reversible processing of thermoset-like materials and production of new types of polymeric supermolecules with potentially interesting and controllable properties.