The main chain of polyphosphazene is alternately arranged by single and double bonds of phosphorus and nitrogen, which results excellent natural flame retardancy, synergy and thermal stability. Alfa Chemistry provides you with innovative polyphosphazene-based flame retardant design services, such as linear and cyclic cross-linked structures, to help you create environmentally friendly halogen-free flame retardant solutions.
Relying on our professional and experienced chemists, Alfa Chemistry has the excellent ability to design the molecular structure of polyphosphazene-based flame retardants. According to the type of polymer molecular structure, it can be divided into linear polyphosphazene flame retardant polymer design and cyclic cross-linked polyphosphazene flame retardant polymer design.
Linear Polyphosphazene Flame Retardant Polymers Design
Linear polyphosphazene is an organic-inorganic hybrid linear polymer with alternately arranged phosphorus-nitrogen single and double bonds as the flexible main chain and with modified different functional substituents as the side chains.
Linear polyphosphazene is obtained by nucleophilic substitution between polydichlorophosphazene (PDCP) and nucleophile, which mainly includes (a) post-polymerization substitution and (b) post-substitution polymerization, as shown in the figure.
Preparation and substitution reaction of linear polyphosphazenes. 
Cyclic Cross-Linked Polyphosphazene Flame Retardant Polymer Design
The six N and P bonds in the phosphazene ring of the cyclic cross-linked polyphosphazene have the same length, and the P atoms are symmetrically distributed on the upper and lower sides of the ring. Cyclic cross-linked polyphosphazene has a six-atom ring structure and a high degree of cross-linking, showing a certain rigidity. In addition, the morphology of cyclic cross-linked polyphosphazene can be designed into various micro-nano structures such as nanotubes, microspheres, and nanosheets.
Cyclic cross-linked polyphosphazene is synthesized with hexachlorocyclotriphosphazene (HCCP) as the core and nucleophilic substitution of active chlorine with organic compounds. Cyclic cross-linked polyphosphazene is mainly prepared by precipitation polymerization, and the synthetic route is shown in the figure.
Cyclic cross-linked polyphosphazenes prepared by precipitation polymerization. 
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Danqi Yang et al. synthesized a highly cross-linked poly(cyclotriphosphazene-resveratrol) microsphere (PRV) and used it to improve the flame retardancy and mechanical properties of epoxy resin (EP). In this work, the addition of PRV significantly increased the char residue of EP composites and greatly promoted the flame retardancy. With the introduction of 3 wt% PRV microspheres, the THR and PHRR of EP decreased by 29.6% and 58.3%, respectively. At the same time, the LOI of EP with 3 wt% PRV was also improved from 25.3% to 29.7%.
The synthetic route of PRV microspheres 
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