Inorganic polymer characteristics; polyphosphazines

General characteristics of inorganic polymers

An inorganic polymer is a polymer whose backbone is made of atoms other than carbon (e.g. Si–O, P–N, S–N, B–N). Examples: silicones, polyphosphazenes, polythiazyl, plastic sulphur.

General characteristics:

• High thermal stability (strong backbone bonds such as Si–O, 452 kJ/mol) and non-flammability.
• High chemical and oxidation resistance, and weather resistance.
• Flexibility at low temperature (low glass transition temperature) because of easy rotation about Si–O or P–N bonds; many are elastomers.
• Good electrical insulation (some, like (SN)ₓ, conduct).
• Usually higher cost, and some are sensitive to hydrolysis.

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".

        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.