No abstract

The solution structure of hyperbranched macromolecules was investigated by means of smallangle neutron scattering (SANS). Hyperbranched polyglycerols of different molecular weight were investigated in D2O and CD3OD, and very similar molar masses and radii of gyration were obtained in both solvents. Kratky plots of the scattering intensity revealed a compact structure of the hyperbranched polyglycerols. A power law scaling relation of the radius of gyration with molar mass was observed, from which a dimension of three was obtained. These observations indicate that the hyperbranched structure prevents strong irregular association despite the high functionality of hydroxyl groups that could lead to aggregation in those solvents. Amphiphilic derivatives of the hyperbranched polyglycerols have been studied in the nonpolar solvent C 6D6. Again, molecularly dispersed polymers were found provided the degree of esterification was sufficiently high. A low degree of derivatization of only 22% was not sufficient to prevent aggregation in C 6D6. The macromolecules become more compact when the degree of esterification increases.

Despite the current broad interest in such materials, the synthesis of defined structures in the size range of 10 nm to ca. 1 μm (“mesoscopic”) is challenging. Few routes shape‐selectively afford geometrically regular structures, other than the typical spherical metal particles of 1 to 10 nm. Moreover, these few routes are largely restricted to aqueous systems, however, for catalysis and other applications dispersions in organic solvents are desirable. Carbon monoxide reduction of a palladium(II) compound in combination with stabilization by (readily available) amphiphilic hyperbranched polymers surprisingly affords dispersions of hexagonal platelets selectively with average sizes of thirty to several hundred nanometers in toluene. The size can be controlled by the polymer composition. Transmission electron microscopy (TEM), electron diffraction, and extended X‐ray absorption fine structure (EXAFS) spectroscopy demonstrate these palladium(0) platelets to be extremely thin (1–2 nm). Despite this high aspect ratio, the platelets prove quite shear resistant.

The synthesis of amphiphilic macromolecules by amidation of hyperbranched polyethylenimine was studied. Amidation with palmitic acid or the methyl ester proceeds up to 84% degree of amidation (140°C, vacuum). The primary amine end groups react preferentially. With carbonyldiimidazole (CDI)-activated acids nearly complete conversion of all primary amine end groups and secondary amine linear units can be achieved; with a corresponding limited amount of CDI the end groups can be amidated selectively. The products of these reactions are free of any unreacted carboxylic acid or other impurities ( 1 H and 13 C NMR) and can be optionally purified by pressure ultrafiltration washing with a toluene/ amine solution. Side-chain crystallization is observed (DSC), which can be supressed entirely employing branched alkyl moieties (2-hexyldecanoic acid as amidating agent). Solution structures were studied by SANS. In C6D6 radii of gyration of 2-5 nm (Guinier analysis) were observed for samples differing in PEI core molecular weight. These sizes and their observed independence of concentration in the range of 5-40 g L -1 indicate the presence of nonaggregated unimolecular inverted micelles.