CONTROLLING POLYMERIC TOPOLOGY BY POLYMERIZATION CONDITIONS - MECHANICALLY LINKED NETWORK AND BRANCHED POLY(URETHANE ROTAXANE)S WITH CONTROLLABLE POLYDISPERSITY

Compared to that of model polyurethane 8 from the reaction of tetra(et hylene glycol) (6) and 4,4'-methylenebis(p-phenyl isocyanate) (7) unde r the same polymerization conditions, polydispersities (PDI) of copoly urethanes 9-11 incorporating bis(5-(hydroxymethyl)-1,3-phenylene)-32-c rown-10 (5) as comonomer were significantly higher; for these branched polymers, the PDI increased with feed ratio of 5 vs 6 up to M-w/M-n = 24 for 75% of 5. This constitutes an original method to control branc hing. The branching units in 9-11 are main chain rotaxanes formed with H-bonding between the ether moieties of macrocycle 5 and -OH groups a s a driving force and thus are mechanically linked, as directly proven by H-1 NMR spectra, NOESY, and complexation studies with a bipyridini um salt. The cavity of 5 acts as a ''topological functionality''. Sinc e solvent can either allow or disfavor such H-bonding, polymeric topol ogy, branched or linear, can be controlled by the proper choice of sol vent. Indeed, although homopolyurethanes were prepared from the reacti on of 5 and 7 under the same conditions otherwise, 12a made in diglyme had very high PDI and was highly branched, while the PDI of 12b made in DMSO was low, close to that of model polyurethane 8, and thus it wa s linear. In addition, 12c from melt polymerization of 5 and 7 is beli eved to be physically cross-linked since it is not soluble in common s olvents for 12a and 12b. Therefore, a novel strategy for controlling p olymeric topology simply by reaction conditions to afford mechanically linked network and branched polymeric materials with controllable PDI , which are essentially three-dimensional main chain polyrotaxanes, is demonstrated.