Polyrotaxanes by free-radical polymerization of acrylate and methacrylate monomers in the presence of a crown ether

Poly[(methyl acrylate)-rotaxa-(30-crown-10)] (5) and poly[(methyl methacrylate)-rotaxa-(30-crown-10)] (6) were synthesized by azobisisubutyronitrile-initiated free-radical bulk polymerizations of the respective monomers in the presence of 30-crown-10 (1; equimolar; 5 times the monomer mass). For 5, 3.8 mass % (0.81 mol % with respect to the monomer) of the crown was incorporated versus 1.7 mass % (0.39 mol % with respect to the monomer) for 6. Control reactions with 18-crown-6, which is to small to be threaded, showed that chain transfer to the crown ethers was detectable only for the acrylate but was relatively negligible and spectroscopically distinct. The threading yields were much higher with these systems than with polystyrene, most likely because of the greater compatibility of the crown ether with these polar monomers and polymers and the consequent ability to carry out the polymerizations homogeneously in the absence of added solvent; however, the threading process was still essentially statistical. Therefore, the polymerization of methacrylate monomers 8a-8c based on tetraarylmethane moieties connected via diethyleneoxy or triethylencoxy spacers was examined in the presence of 1 in the belief that the supramolecular semirotaxane monomer 9 formed statistically in situ could be captured more efficiently and produce higher threading yields, presumably of side-chain polyrotaxanes, than the simple (meth)acrylate monomers. Azobisisobutyronitrile-initiated polymerizations either neat or in toluene produced polyrotaxanes 10 with up to about 1.6 mass % and 2 mol % threaded crown ether, presumably trapped on the pendant stoppered side chains. Although primarily statistical in nature, the latter rotaxane syntheses afforded on a molar basis 3-7 times more efficient incorporation of 1 than styrene (0.33 mol %), methyl acrylate (0.81 mol %), or methyl methacrylate (0.39 mol %) monomers for the preparation of main-chain polypseudorotaxanes and indeed even surpassed the 60-crown-20/polyacrylonitrile system (1.5 mol %). This was presumed to be due to the fact that the loss of the crown ether, once it was threaded onto the monomer to form 9 and the latter was polymerized, was either retarded (by the tetraphenylmethyl stopper in 10a) or prevented completely [by tris(p-t-butylphenyl)phenylmethyl stoppers in 10b and 10c]. (C) 2001 John Wiley & Sons, Inc.