In organosilicon chemistry, polysilazanes are polymers in which silicon and nitrogen atoms alternate to form the basic backbone (···−Si−N−Si−N−···). Since each silicon atom is bound to two separate nitrogen atoms and each nitrogen atom to two silicon atoms, both chains and rings of the formula [R2Si−NR]n occur. R can be hydrogen atoms or organic substituents. If all substituents R are hydrogen atoms, the polymer is designated as perhydropolysilazane, polyperhydridosilazane, or inorganic polysilazane ([H2Si−NH]n). If hydrocarbon substituents are bound to the silicon atoms, the polymers are designated as Organopolysilazanes. Molecularly, polysilazanes [R2Si−NH]n are isoelectronic with and close relatives to polysiloxanes [R2Si−O]n (silicones).
History The synthesis of polyorganosilazanes was first described in 1964 by Krüger and Rochow. By reacting ammonia with chlorosilanes (ammonolysis), trimeric or tetrameric cyclosilazanes were formed initially and further reacted at high temperatures with a catalyst to yield higher molecular weight polymers. Ammonolysis of chlorosilanes still represents the most important synthetic pathway to polysilazanes. The industrial manufacture of chlorosilanes using the Müller-Rochow process, first reported in the 1940s, served as the cornerstone for the development of silazane chemistry. In the 1960s, the first attempts to transform organosilicon polymers into quasi-ceramic materials were described. At this time, suitable (“pre-ceramic”) polymers heated to 1000 °C or higher were shown to split off organic groups and hydrogen and, in the process, the molecular network is rearranged to form amorphous inorganic materials that show both unique chemical and physical properties. Using polymer derived ceramics (PDCs), new application areas can be opened, especially in the area of high performance materials. The most important pre-ceramic polymers are polysilanes [R2Si−SiR2]n, polycarbosilanes [R2Si−CH2]n, polysiloxanes [R2Si−O]n and polysilazanes [R2Si−NR]n.
Structure Like all polymers, polysilazanes are built from one or several basic monomer units. Linking together of these basic units can result in polymeric chains, rings or three-dimensionally crosslinked macromolecules with a wide range of molecular mass. While the monomer unit describes the chemical composition and the connectivity of the atoms (coordination sphere) it does not illustrate the macro-molecular structure.
In polysilazanes, each silicon atom is bound to two nitrogen atoms and each nitrogen atom to at least two silicon atoms (three bonds to silicon atoms are also possible). If all remaining bonds are with hydrogen atoms, perhydropolysilazane [H2Si−NH]n results (proposed structure is shown to the right). In organopolysilazanes, at least one organic substituent is bound to the silicon atom. The amount and type of organic substituents have a predominant influence on the macro-molecular structure of polysilazanes. Silazane copolymers are normally produced from ammonolyses of chlorosilane mixtures. In this chemical reaction, different chlorosilanes react at similar rates so that the monomers are statistically distributed in the copolymer.
Preparation Ammonia and chlorosilanes, both readily available and low-priced, are used as starting materials in the synthesis of polysilazanes. In the ammonolysis reaction, large quantities of ammonium chloride are produced and must be removed from the reaction mixture.
R 2 SiCl 2 + 3 NH 3 ⟶ 1 n [ R 2 Si − NH ] n + 2 NH 4 Cl {\displaystyle {\ce {R2SiCl2 + 3 NH3}}\longrightarrow {\tfrac {1}{n}}\,{\ce {[R2Si-NH]}}_{n}+{\ce {2 NH4Cl}}}
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