---
title: "Polyphosphonitrilic chloride preparation and uses"
subject: "Engineering Chemistry"
chapter: 4
key: "semester_2/engineering_chemistry:polyphosphonitrilic-chloride"
source: https://www.bctengineer.com/q/semester_2/engineering_chemistry/polyphosphonitrilic-chloride
site: BCT Engineering
---

# Polyphosphonitrilic chloride preparation and uses

Engineering Chemistry · Chapter 4: Engineering Polymers

- Asked in: 2073 Bhadra, 2.5 marks; 2067 Mangsir, 3 marks (asked 2 times)
- Video: [Phosphazenes ( P-N compounds) ; structure, preparation & properties](https://www.youtube.com/watch?v=vmgTv33kIh0) — Priyanka Jain
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_2/engineering_chemistry/polyphosphonitrilic-chloride
- Whole chapter: https://www.bctengineer.com/md/semester_2/engineering_chemistry/chapter-4

## Question

Write the preparation and uses of polyphosphonitrilic chloride.

## Short answer

Polyphosphonitrilic chloride is prepared by heating PCl5 with NH4Cl at 120–150 °C to give cyclic trimer (NPCl2)3, then heating this at 250 °C yields the linear polymer (NPCl2)n. It is used as a precursor for polyorganophosphazenes, flame retardants, and derivatives for fire-resistant elastomers and biomedical materials.

## Answer

#### Preparation
**Phosphonitrilic compounds (phosphazenes)** contain the –N=PX₂– unit (X = Cl, F, Br, OR, NR₂) and are formed as cyclic oligomers (NPX₂)₃, (NPX₂)₄ and long chain polymers (NPX₂)ₙ.

**Preparation of phosphonitrilic chloride**
1. Phosphorus pentachloride is heated with ammonium chloride at about 120–150 °C in a solvent (chlorobenzene or sym-tetrachloroethane):
   n PCl₅ + n NH₄Cl → (NPCl₂)ₙ + 4n HCl
   The main product is the cyclic trimer (NPCl₂)₃ with some tetramer.
2. On heating the trimer at about 250 °C in a vacuum (or sealed tube), ring-opening polymerisation gives the high polymer, polydichlorophosphazene (NPCl₂)ₙ, a rubber-like substance called "inorganic rubber".

```text
        N
     /     \
  Cl₂P       PCl₂      (NPCl₂)₃ trimer:
     |       |         six-membered ring of
     N       N         alternating N and P
      \     /
       PCl₂
```

#### Uses
- It is the starting material for polyorganophosphazenes: the Cl atoms are replaced by OR, NHR or OCH₂CF₃ groups, which give stable, useful polymers.
- Used directly as an inorganic rubber (hydrolytically unstable, so its use is limited) and as a flame retardant.
- Its derivatives are used as fire-resistant materials, elastomers for seals and hoses, biomedical materials, membranes and solid polymer electrolytes.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Which reactants give the cyclic trimer (NPCl2)3?
   A. PCl3 and NH3 gas
   B. PCl5 and NH4Cl heated in a solvent
   C. POCl3 and water
   D. PCl5 and NaOH
   Answer: B. PCl5 is heated with NH4Cl at about 120-150 C in chlorobenzene, releasing HCl. (Key point: Step 1 reagents)

2. How is the high polymer (NPCl2)n obtained from the trimer?
   A. Dissolving it in water
   B. Treating with sodium methoxide
   C. Heating at about 250 C in vacuum causing ring-opening polymerisation
   D. Cooling it below 0 C
   Answer: C. Ring-opening polymerisation at about 250 C gives rubber-like polydichlorophosphazene. (Key point: Polymerisation step)

3. Why is polyphosphonitrilic chloride mainly used as a starting material?
   A. Because it is totally stable to water
   B. Because its Cl atoms can be replaced by OR or NHR groups to give stable polymers
   C. Because it is a metal
   D. Because it conducts electricity
   Answer: B. The reactive P-Cl bonds are substituted to make stable polyorganophosphazenes, whereas the chloride itself hydrolyses. (Key point: Why it is a precursor)

4. What is the ring structure of the trimer (NPCl2)3?
   A. Three-membered ring of P only
   B. Six-membered ring of alternating N and P
   C. Six-membered ring of carbon
   D. Chain of Cl atoms
   Answer: B. The trimer is a six-membered ring of alternating N and P atoms, each P carrying two Cl. (Key point: Structure)

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