Abstract
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.
Highlights
Polyamides (PAs) are a commercially important class of polymers with applications in many industries
Using nucleophiles in the initiation of the aza-Michael polymerization of acrylamide allows for the preparation of polyamide 3 (PA 3) featuring a branching point at approximately every second repeat unit
The reaction is carried out in the melt of acrylamide, and no solvents are involved in the preparation
Summary
Polyamides (PAs) are a commercially important class of polymers with applications in many industries Their utility primarily arises from the amide groups and hydrogen bonding among them, creating strong interchain interactions. The preparation uses melted acrylamide and is initiated with low amounts of sodium tert-butoxide (NaOtBu). The reaction conditions are much more sustainable when compared to usually employed protocols for the synthesis of hyperbranched polymers.[14−16] In this case, the key for obtaining a high degree of branching is to use nucleophiles for initiating the polymerization. Acrylamide (1, 1.00 g, 14.07 mmol) was placed in a Schlenk tube, a magnetic stirrer bar was added, and the reaction vessel was heated to 83 °C using a temperature-controlled oil bath. NaOtBu (Table 1, entry 6). (c) 1H NMR spectrum in DMSO-d6 of PA 3 prepared with 1 mol % DBU (Table 1, entry 8). (d) 15N{1H} NMR spectrum of 15N-PA 3 prepared with 2 mol % DBU
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