---
title: "Polyphosphazines and polymeric sulfur nitride"
subject: "Engineering Chemistry"
chapter: 4
key: "semester_2/engineering_chemistry:polyphosphazine-sulfur-nitride"
source: https://www.bctengineer.com/q/semester_2/engineering_chemistry/polyphosphazine-sulfur-nitride
site: BCT Engineering
---

# Polyphosphazines and polymeric sulfur nitride

Engineering Chemistry · Chapter 4: Engineering Polymers

- Asked in: 2071 Bhadra, 2.5+2.5 marks
- Video: [Phosphazenes classification, Preparation, Properties and Structure | simplified](https://www.youtube.com/watch?v=1xdzLBv85GM) — Bholanath Academy
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_2/engineering_chemistry/polyphosphazine-sulfur-nitride
- Whole chapter: https://www.bctengineer.com/md/semester_2/engineering_chemistry/chapter-4

## Question

Explain preparation and uses of polyphosphazines and polymeric sulfur nitride.

## Short answer

Polyphosphazines are made by heating PCl5 and NH4Cl to form (NPCl2)n, then ring-opening polymerization; uses include flame-resistant coatings, elastomers, and biomedical implants. Polymeric sulfur nitride (SN)x is prepared from S4N4 via S2N2 and solid-state polymerization; it is a metallic conductor and superconductor at 0.3 K, used in microelectronics.

## Answer

#### Polyphosphazines
**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
```

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:**
- 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.

#### Polymeric sulphur nitride
**Polythiazyl (polymeric sulphur nitride), (SN)ₓ.**
Preparation:
1. S₄N₄ is made by passing NH₃ into a solution of S₂Cl₂ (6S₂Cl₂ + 16NH₃ → S₄N₄ + 8S + 12NH₄Cl).
2. S₄N₄ vapour is passed over hot silver wool at 250–300 °C to give S₂N₂ crystals: S₄N₄ → 2S₂N₂ (Ag).
3. S₂N₂ polymerises in the solid state at room temperature: n S₂N₂ → 2 (SN)ₙ (golden, lustrous fibres).

Structure: chains of alternating S and N atoms, –S=N–S=N–. (SN)ₓ conducts electricity like a metal along the chain direction and becomes superconducting at about 0.3 K.

Uses: an example of a conducting polymer without carbon, used in studies of one-dimensional conductors, microelectronic and photoconducting devices.

## Check yourself

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

1. Which reaction gives (NPCl2)n?
   A. PCl5 + NH4Cl
   B. PCl3 + O2
   C. NH3 + SO2
   D. P4 + N2
   Answer: A. n PCl5 + n NH4Cl gives (NPCl2)n + 4n HCl. (Key point: Phosphazene preparation)

2. Why are Cl atoms replaced by OR groups?
   A. P-Cl bonds are reactive; replacing gives stable polyorganophosphazenes
   B. To increase hydrolysis
   C. To make it explosive
   D. To remove nitrogen
   Answer: A. Replacing Cl with OCH3 or OC2H5 gives stable polymers. (Key point: Substitution)

3. What is the correct order for (SN)x?
   A. S2N2 then S4N4 then (SN)x
   B. S2Cl2 with NH3 gives S4N4, then S2N2 over silver, then (SN)x
   C. (SN)x then S4N4
   D. NH3 with PCl5
   Answer: B. S4N4 is converted to S2N2, which polymerises at room temperature. (Key point: Polythiazyl route)

4. Which is a use of polyphosphazenes?
   A. Solid polymer electrolytes for lithium batteries
   B. Rocket fuel
   C. Photocopier drums
   D. Fertiliser
   Answer: A. Also flame retardants, elastomers and biomedical materials. (Key point: Phosphazene use)

## Related in Engineering Chemistry

- [Polyphosphonitrilic chloride preparation and uses](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Apolyphosphonitrilic-chloride) — Engineering Chemistry, chapter 4, asked 2×
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- [Inorganic polymer characteristics; polyphosphazines](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Ainorganic-polymer-polyphosphazines) — Engineering Chemistry, chapter 4
- [Polythiazyl preparation; polyphosphazene, chalcogenide uses](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Apolythiazyl-polyphosphazene) — Engineering Chemistry, chapter 4