Studies of the Formation of Poly(ester rotaxane)s from Diacid Chlorides, Diols, and Crown Ethers and Their Properties

Abstract

The formation of poly(alkylene sebacate−crown ether pseudorotaxane)s by condensation of linear alkylene diols and sebacoyl chloride in the presence of crown ethers in the neat state has been studied. It was found that the average number of crown ether molecules per repeat unit in the polypseudorotaxane was a function of (a) the ring size and (b) the stoichiometric ratio of macrocycle to diol but independent of (i) the equilibration time of the diol and crown ether prior to addition of the diacid chloride, (ii) the length of the diol, and (iii) the temperature of equilibration and polycondensation. All of these observations are consistent with the involvement of hydrogen bonding between the diol and the crown ether as a driving force for threading, except the lack of temperature dependence. Dethreading of the isolated polypseudorotaxanes was shown to be extremely slow. Therefore, it was reasoned that the lack of temperature dependence was due to dethreading during the polymerization, inasmuch as once the ester bond has formed there is no strongly attractive force between the linear and cyclic species and the low molecular weights of the growing oligomeric esters would permit relatively facile dethreading. Based on this idea, a bulky tetraphenylmethane-based bisphenol was employed to make a copolymer (1:4) with 1,10-decanediol; indeed, the purified polyrotaxane contained more than twice as much crown ether as the polypseudorotaxane from the linear diol, confirming that dethreading does occur during the polymerization process. The polyrotaxanes all were capable of extracting metal ions from aqueous solutions. In the cases with high loadings of crown ether two distinct crystalline phases were detected by DSC: one due to the polyester backbone and one due to the crown ether; glass transitions were also observed for the crown ether component of the polyrotaxanes. Polyrotaxanes possess higher intrinsic viscosities than the backbone polymers of the same molecular weight due to increased hydrodynamic volume brought about by the macrocyclic components. However, differential solvation of the backbone and cyclic components of the polyrotaxanes was demonstrated; the intrinsic viscosity of the polyrotaxane decreased in a good solvent for the crown ether. The temperature dependence of the melt viscosity of a polyrotaxane was essentially the same as that of the polyester model backbone, but the absolute melt viscosity was much lower due to reduced chain entanglement.