Poly(oxazoline)s with tapered minidendritic side groups as models for the design of synthetic macromolecules with tertiary structure. A demonstration of the limitations of living polymerization in the design of 3-D structures based on single polymer chains.

Abstract

The synthesis and living cationic ring-opening polymerization of 2-[3,4-bis(n-alkan-1-yloxy)phenyl]-2-oxazolines with alkan being tetradecan and pentadecan, i.e., (3,4)nG1-Oxz with n = 14 and 15, is described. The structural analysis of the resulting polymers with well-defined molecular weights and narrow molecular weight distribution was carried out by a combination of techniques, including differential scanning calorimetry (DSC), thermal optical polarized microscopy (TOPM), and X-ray diffraction (XRD). At low molecular weights both polymers self-assemble into spherical supramolecules that self-organize into a Pm3n 3-D lattice while at high molecular weights they form cylindrical macromolecules that self-organize into a p6mm 2-D hexagonal columnar lattice. Both polymers exhibit a 3-D shape change as a function of their degree of polymerization as was reported for the first time in a previous publication from our laboratory (Percec, V.; Ahn, C.-H; Ungar, G.; Yeardley, D. J. P.; Möller, M.; Sheiko, S. S. Nature (London) 1998, 391, 161). Since these polymers can be obtained via a living polymerization, a detailed mechanistic investigation of the influence of the degree of polymerization and molecular weight distribution on the formation of a 3-D spherical macromolecule from a single polymer chain, i.e., a tertiary structure, was possible. The experimental results have demonstrated that the synthesis of nonbiological macromolecules exhibiting tertiary structure is possible in at most a few percent of all macromolecules via living polymerization. This is the case even when macromolecules with very narrow molecular weight distributions and well-defined molecular weights are used. Therefore, the design of synthetic macromolecules with tertiary structure requires not only chains with well-defined molecular weight but also, in particular, macromolecules with no distribution of their chain length.