Poly(methylene)s Bearing Pendant Benzoxazines via C1 Polymerization: Maximizing Functional Group Density for Enhanced Cross-Linking

Abstract

We report the design, synthesis, and study of poly- (methylene)s with benzoxazine groups attached to every repeat unit of the main polymer chain. Diazoacetate monomers functionalized with pendant benzoxazine groups were first prepared and characterized. The length of the spacer between the diazoacetate and the pendant benzoxazine proved to be a critical design factor. When the spacer was relatively short (i.e., one methylene unit), the corresponding polymer underwent premature decomposition by emitting gaseous carbon dioxide at elevated temperatures. The use of longer (i.e., ethylene) spacers effectively circumvented premature decomposition. Each monomer was transformed into its respective poly(methylene) using a C1 polymerization methodology that employed a Pd-based catalyst. A poly(vinylene) analogue, which features benzoxazine groups attached to every other carbon unit of the main chain, and a bis(benzoxazine) that is commonly used to prepare thermosets were also prepared as controls. The structures of the monomers and polymers were elucidated using NMR and Fourier transform infrared (FT-IR) spectroscopy, and the molecular weights of the polymers were determined using size exclusion chromatography (SEC). Exposure of the benzoxazine-functionalized polymers or the bis(benzoxazine) to elevated temperatures, typically >200 °C, afforded mechanically robust, cross-linked resins. Thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and dynamic mechanical analysis (DMA) were used to characterize the thermal and physical properties of the cured materials. The thermally cured poly(methylene)s exhibited higher cross-link densities (25,937 mol m−3) than those derived from the poly(vinylene) analogue (12,904 mol m−3) or the bis(benzoxazine) (3513 mol m−3). The cured poly(methylene)s also exhibited a higher storage modulus (329.8 MPa) than the controls (145.8 and 44.6 MPa, respectively). Although the control thermosets were stiffer at room temperature (5.4−5.5 GPa vs 4.4 GPa), the higher cross-link density of the cured C1 polymers provides superior high-temperature performance and thermal stability. These findings demonstrate how incorporating benzoxazines into every repeat unit of a polymer backbone effectively increases crosslink density and enables the development of robust, high-performance thermosets with potential utility in advanced structural and electronic applications.