Poly(hydrazino-phosphine diazide)s (PHPDs): Hybrid Organic-Inorganic Polymers via Polycondensation between PN Cages and Organic Diazides

Organic polymers generally feature 1-dimensional chains or 2-dimensional rings in their backbones since the strain in small hydrocarbon cages (e.g. cubane) and synthetic challenges limit the availability of 3-dimensional units available as monomers. Inorganic cages are less strained owing to their longer and more ionic bonds, and many can be very accessible, offering an inroad into the minimally explored third dimension that is difficult to study with organic systems. However, only two families of inorganic cages – carboranes and polyhedral oligomeric silsesquioxanes (POSS) – have so far been studied and commercialized to capitalize upon the enhanced mechanical and thermal properties resulting from the use of higher dimensionality backbone components. Further exploration of this fundamentally interesting and potentially valuable parameter space requires development of new inorganic cages that are accessible on large scales, stable under ambient conditions, and polymerizable to give air- and moisture-stable materials. Here we report that a readily-assembled and bench-stable PN cage, P(NMeNMe)3P (1), undergoes clean Staudinger polycondensations with organic diazides (7R) to yield air/water-stable, solution-processable, and film-forming linear poly(trihydrazino-diphosphine diazide)s, PHPDs (8R), as a new family of hybrid organic-inorganic polymers. The solubility of PHPDs in organic or aqueous media can be controlled by choice of diazide and backbone architecture, which we rationally modify to access alternating co-polymers (12R-alt-R’) or multi-block copolymers (12R-block-R’). We also show how a tetraphosphorus cage P4(NMe)6 (2), can be used to crosslinking PHPDs to make more robust materials. Thermal analyses show Tg values for PHPDs that are comparable with rigid π-conjugated polymers (up to 152 °C) and, despite a high nitrogen content (up to 32%) and 3 N-N σ-bonds per repeat unit, decomposition temperatures exceeding 200 °C with char yields up to 60%. These data bear out theoretical predictions of high stability arising from the presence of 3-dimensional units in the backbone. As one potential application area, we show that the high char yields and PN content of a polyether based PHPD (8e) can be leveraged for halogen-free flame-retardancy. These results debut new low-carbon hybrid organic-inorganic polymers with an unusual backbone topology, reveal the design rules for controlling their microstructures and mechanical properties, and lay the foundation for future applied studies with PN cages.