Synthesis of DOPO-Based Phosphonamidates and their Thermal Properties

Synthetic strategies for the preparation of 9,10-dihydro-9,10-oxa-10-phosphaphenanthrene-10-oxide (DOPO) derivatives are of special interest for polymer industry as well as academia involved in development of halogen-free flame retardant additives. In this work, various amino-DOPO derivatives (phosphonamidates) were synthesized by employing three different synthetic methods involving various chlorinating agents. The synthetic methods were compared with regards to purity and yield of the resulting amino-DOPO derivatives. The chlorinating agents that were used in this work are namely CCl4, trichloroisocyanuric acid and sulfuryl chloride. Some compounds, e.g., amine substituted DOPO derivative, could be synthesized using sulfuryl chloride with a yield >94% and a purity >99%. As proof of possible future industrial scale-up, up-scaling laboratory trials at the 500 g scale were also performed for two methods. The synthesized compounds were characterized for their melting point and thermal stability by using thermogravimetric analysis and direct probe insertion mass spectrometry. In general, these amino-DOPO derivatives exhibited high thermal stability with some having stability >320 °C.