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 methacryl ate)-rotaxa-(30-crown-10)] (6) were synthesized by azobisisubutyronitrile-i nitiated free-radical bulk polymerizations of the respective monomers in ti le 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 incor porated versus 1.7 mass % (0.39 mol % with respect to the monomer) for 6. C ontrol 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 acrylat e but was relatively negligible and spectroscopically distinct. The threadi ng yields were much higher with these systems than with polystyrene, most l ikely because of the greater compatibility of the crown ether with these po lar monomers and polymers and the consequent ability to carry out the polym erizations homogeneously in the absence of added solvent; however, the thre ading process was still essentially statistical. Therefore, the polymerizat ion of methacrylate monomers 8a-8c based on tetraarylmethane moieties conne cted via diethyleneoxy or triethylencoxy spacers was examined in the presen ce of 1 in the belief that the supramolecular semirotaxane monomer 9 formed statistically in situ could be captured more efficiently and produce highe r 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 ma ss % and 2 mol % threaded crown ether, presumably trapped on the pendant st oppered side chains. Although primarily statistical in nature, the latter r otaxane syntheses afforded on a molar basis 3-7 times more efficient incorp oration of 1 than styrene (0.33 mol %), methyl acrylate (0.81 mol %), or me thyl methacrylate (0.39 mol %) monomers for tile preparation of main-chain polypseudorotaxanes and indeed even surpassed the 60-crown-20/polyacrylonit rile system (1.5 mol %). This was presumed to be due to the fact that the l oss 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)phenylmeth yl stoppers in 10b and 10c]. (C) 2001 John Wiley & Sons, Inc.