Controlling Polymeric Topology by Polymerization Conditions:  Mechanically Linked Network and Branched Poly(urethane rotaxane)s with Controllable Polydispersity

Abstract

Compared to that of model polyurethane 8 from the reaction of tetra(ethylene glycol) ( 6 ) and 4,4‘-methylenebis( p -phenyl isocyanate) ( 7 ) under the same polymerization conditions, polydispersities (PDI) of copolyurethanes 9 − 11 incorporating bis(5-(hydroxymethyl)-1,3-phenylene)-32-crown-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 branching. The branching units in 9 − 11 are main chain rotaxanes formed with H-bonding between the ether moieties of macrocycle 5 and −OH groups as a driving force and thus are mechanically linked, as directly proven by 1 H NMR spectra, NOESY, and complexation studies with a bipyridinium salt. The cavity of 5 acts as a “topological functionality”. Since solvent can either allow or disfavor such H-bonding, polymeric topology, branched or linear, can be controlled by the proper choice of solvent. Indeed, although homopolyurethanes were prepared from the reaction 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 was linear. In addition, 12c from melt polymerization of 5 and 7 is believed to be physically cross-linked since it is not soluble in common solvents for 12a and 12b . Therefore, a novel strategy for controlling polymeric 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.