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lecture 06: Polythiazyl polymer by Dr. Salma Amir

polythiazyl polymer synthesis, chain growth, and its properties

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lecture 06: Polythiazyl polymer by Dr. Salma Amir

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  1. Lecture No. 06Course title:Inorganic polymerTopic: Polythiazyl Polymer Course instructor: Dr. Salma Amir GFCW Peshawar

  2. Polythiazyl Polymer • Polythiazyl [or poly(sulfur nitride)] is another interesting inorganic polymer • It was the first conductive inorganic polymer discovered • It is gold- or bronze-colored polymer with metallic luster • Polythiazylis shown schematically

  3. Synthesis of Polythiazyl polymers • Polythiazylis synthesized via a ROP reaction. • Polythiazyl can be obtained by spontaneous polymerization of alternating four-member ring dimer, disulfurdinitride (S2N2). The dimer (Disulfurdinitride) is obtained by thermal cracking of cyclic alternating sulfur nitride tetramer, tetrasulfurtetranitride (S4N4) on silver wool. n/4(SNSNSNSN)ring 200°C/0.01torrn/2(SNSN)ring 25°C (–S=N–)n The tetramer itself can be prepared by the reaction of ammonia with elemental sulfur, sulfur fluorides, or sulfur chloride S2Cl2 + NH3 or SF4 + NH3 or S8 + NH3

  4. Mechanism • The mechanism is thought to involve ring cleavage, with the two resulting radicals combining with the nearest neighbors bracketing them in the crystal structure • Because asublimation step is involved in the polymerization process, there has been considerable interest in epitaxial chain growth (where the polymerization occurs on surfaces of known and controlled structure).The dimer is also very unstable (potentially explosive). It is condensed on a liquid nitrogen-cooled cold finger. In practice, the polymer depolymerizes at approximately 150°C to a gaseous S4N4 isomer. When this isomer in the vapor state comes into contact with the desired surface at a lower temperature, it spontaneously repolymerizes to give the (SN)xfilm.

  5. S4N4 + 8 Ag → 4 Ag2S + 2 N2 • S4N4 (w/ Ag2S catalyst) → 2 S2N2 (w/ 77K cold finger) → S2N2 • S2N2 (0°C, sublimes to surface) → thermal polymerization → (SN)x However, since S2N2 and S4N4 are explosive and the polymerization is slow (6-8 weeks), recent synthetic work has concentrated on devising new routes.

  6. In the first preparation from solution, Passmore and coworkers prepared the powdered polymer (rather than the crystalline material obtained using the classical method) by addition of excess trimethylsilylazide to (NSCl)3 in acetonitrile at -15°C • (NSCl)3 + 3Me3SiN3 9/2N2+ 3Me3SiCl + 3/x(SN)x They also obtained this material in a similar manner by addition of: i) Triazyltrichloride(NSCl)3to a slurry of sodium azide and ii) Trimethylsilylazide to slurries of Chlorothiazyl chloride S3N2Cl2 or (S3N2Cl)2

  7. Using a different approach, Banister and co-workers prepared microcrystalline layers of polymer on platinum electrodes by electrolysis of [S5N5]+ salts in liquid sulfur dioxide or acetonitrile; the mechanism was thought to involve S5N5. • The most satisfactory results were obtained in acetonitrile since liquid sulfur dioxide is not electrochemically inert • In summary, the classical route using S2N2 is still the method of choice for the preparation of high-purity polymer suitable for electronic applications

  8. Properties of Polythiazyl polymers • They are soft and malleable, and have a golden metallic luster. • They conduct electricity like a metal down to very low temperatures, with highly anisotropic conductivity in the case of the oriented chains. The conductivity increases by several orders of magnitude as the temperature is decreased from room temperature to a few degrees Kelvin. At 0.26 K, (SN)x becomes a superconductor. The electrical conductivity is thought to require both the charge delocalization that is possible along the chains, and the highly ordered chain arrangements present in the crystalline state • It is diamagneticpolymer

  9. The chains were found to be stable in an inert atmosphere, but decomposed above approximately 140°C, into sulfur, nitrogen and low-molecular-weight sulfur-nitrogen molecules • A final difficulty is the explosive nature of many sulfur-nitrogen compounds. The situation could possibly be simplified if sulfur nitrogen chains could be prepared with substituents linked to the S atoms. These and other problems merit additional investigation. • Because of the inorganic nature inherent in the strings of sulfur atoms in their backbones, these polymers have excellent resistance to hydrocarbon solvents that can swell common hydrocarbon elastomers • They also have good resistance to moisture, oxygen, and ozone, good weatherability, and better-than-average low temperature properties

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