Melt Polymerization of Acrylamide Initiated by Nucleophiles: A Route toward Highly Branched and Amorphous Polyamide 3

The melt polymerization of acrylamide initiated by nucleophiles allows for the preparation of polyamide 3 (PA 3) with a branching factor of about 1.5. The high share of branching units imparts a fully amorphous morphology featuring a low glass-transition temperature of 67 °C and renders the polymer water-soluble (430 ± 20 g/L at 25 °C). The disclosed method provides an easy, resource-efficient, and green access to a polymer interesting for applications in biological and biomedical systems. The obtained PA 3 was characterized by several NMR techniques, matrix-assisted laser desorption/ionization time-of-flight mass spectrometry, size exclusion chromatography, thermal analyses, and powder-X-ray diffractometry. Preparation and characterization of a 15N-marked polymer complemented the elucidation of the polymer structure. Mechanistically, the polymerization can be considered as an aza-Michael polymerization of acrylamide involving zwitterionic species as the key intermediates being responsible for the high degree of branching that can be controlled by the temperature at which the polymerization is carried out. The highest branching is obtained at 83 °C and decreases upon increasing the temperature.