Poly(hydrazinophosphine diazide)s (PHPDs): Hybrid Organic–Inorganic Polymers via Polycondensation between PN Cages and Organic Diazides

Abstract

Organic polymers generally feature 1-dimensional chains or 2-dimensional rings in their backbones since synthetic challenges limit the availability of 3-dimensional monomers. Inorganic cages are less strained and more accessible, offering an alternative route to explore this parameter space. However, only two families─carboranes and polyhedral oligomeric silsesquioxanes (POSS)─have been well-studied, revealing materials with valuable mechanical and thermal properties. Further exploration of this frontier requires the development of new inorganic cages that are accessible, stable, and polymerizable. Here we report that an easily assembled, bench-stable PN cage, P(NMeNMe)₃P, undergoes Staudinger polycondensation with organic diazides to yield robust, solution-processable, and film-forming linear poly(trihydrazino-diphosphine diazide)s─PHPDs─as a new family of hybrid organic–inorganic polymers. Their solubility can be controlled by diazide choice and backbone architecture, which we rationally modify to access alternating or multiblock copolymers. We also show how a tetraphosphorus cage, P₄(NMe)₆, can be used to cross-link PHPDs. The T_g values for PHPDs are comparable to those of rigid π-conjugated polymers (>150 °C), and, despite a high nitrogen content (up to 32%) and three N–N σ-bonds per repeat unit, they show decomposition temperatures >200 °C with char yields up to 60%. These data support hypotheses of high stability arising from the presence of 3-dimensional backbone units. We further show that PHPDs may be leveraged for halogen-free flame retardancy. Collectively, the results debut new low-carbon polymers with an unusual backbone topology, reveal the design rules for controlling their microstructures and properties, and lay the foundation for future applied studies.