---
title: "Phosphonitrilic compounds and polyphosphazenes"
subject: "Engineering Chemistry"
chapter: 4
key: "semester_2/engineering_chemistry:phosphonitrilic-phosphazenes"
source: https://www.bctengineer.com/q/semester_2/engineering_chemistry/phosphonitrilic-phosphazenes
site: BCT Engineering
---

# Phosphonitrilic compounds and polyphosphazenes

Engineering Chemistry · Chapter 4: Engineering Polymers

- Asked in: 2071 Magh, 1+2+1+1 marks
- Video: [Basic character, structure and Uses of Phosphazenes](https://www.youtube.com/watch?v=ucrt-iekUbE) — Gain Chemistry
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_2/engineering_chemistry/phosphonitrilic-phosphazenes
- Whole chapter: https://www.bctengineer.com/md/semester_2/engineering_chemistry/chapter-4

## Question

What is phosphonitrilic compounds? How phosphonitrilic chloride can be prepared? Write the monomers of polydimethoxy and polydiethoxy phosphazines. Mention the uses of phosphazines.

## Short answer

Phosphonitrilic compounds (phosphazenes) contain –N=PX2– units. Phosphonitrilic chloride is prepared by heating PCl5 and NH4Cl to give (NPCl2)n, then ring-opening polymerization. Monomers: N=P(OCH3)2 and N=P(OC2H5)2. Uses: flame-resistant materials, elastomers, biomedical implants, battery electrolytes, and water-repellent coatings.

## Answer

#### Phosphonitrilic compounds and preparation of the chloride
**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
```

#### Monomers of polydimethoxy and polydiethoxy phosphazines
The P–Cl bonds are very reactive. Replacing Cl by organic groups gives stable polyorganophosphazenes:

(NPCl₂)ₙ + 2n NaOCH₃ → [NP(OCH₃)₂]ₙ + 2n NaCl   (polydimethoxyphosphazene)
(NPCl₂)ₙ + 2n NaOC₂H₅ → [NP(OC₂H₅)₂]ₙ + 2n NaCl  (polydiethoxyphosphazene)

The repeating units (monomers) are therefore N=P(OCH₃)₂ and N=P(OC₂H₅)₂.

#### Uses of phosphazines
- Flame-resistant and non-burning materials and coatings.
- Elastomers (fluoroalkoxy phosphazene rubber) that stay flexible from –60 °C to 180 °C: seals, O-rings, fuel hoses and gaskets in aircraft and the oil industry.
- Biomedical uses: biodegradable implants, drug delivery, artificial skin, dental materials.
- Solid polymer electrolytes for lithium batteries; membranes for gas separation.
- Water-repellent coatings, lubricants, hydraulic fluids and low-temperature insulation.

## Check yourself

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

1. Which repeating unit characterises phosphonitrilic compounds?
   A. –N=PX2–
   B. –P=S–X2
   C. –N=SiX2–
   D. –P–O–PX2–
   Answer: A. Phosphonitrilic compounds (phosphazenes) contain the –N=PX2– unit, with X = Cl, F, OR, NR2 etc. (Key point: Phosphazene unit)

2. How is phosphonitrilic chloride (NPCl2)3 prepared?
   A. Heating PCl3 with NaOH in water at 25 °C
   B. Heating PCl5 with NH4Cl at about 120–150 °C in chlorobenzene or sym-tetrachloroethane
   C. Burning phosphorus in nitrogen gas
   D. Reacting PCl5 with NaOCH3 at 250 °C
   Answer: B. PCl5 and NH4Cl are heated in an inert solvent, giving mainly the cyclic trimer plus HCl. (Key point: Preparation of chloride)

3. What happens when the trimer (NPCl2)3 is heated at about 250 °C in vacuum?
   A. It decomposes to P4 and N2
   B. It forms the stable polydimethoxyphosphazene directly
   C. Ring-opening polymerisation gives polydichlorophosphazene, 'inorganic rubber'
   D. It hydrolyses to phosphoric acid
   Answer: C. Heating opens the ring and gives long-chain (NPCl2)n, a rubber-like polymer. (Key point: Ring-opening polymerisation)

4. What is the repeating unit (monomer) of polydimethoxyphosphazene?
   A. N=PCl2
   B. N=P(OC2H5)2
   C. P(OCH3)3
   D. N=P(OCH3)2
   Answer: D. (NPCl2)n reacts with NaOCH3 replacing Cl by OCH3, so the unit is N=P(OCH3)2. (Key point: Monomer of polydimethoxyphosphazene)

## Related in Engineering Chemistry

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