+ (n-1) (SiX2)x where n ≥ 2). For example, SiCl4 and Si forms SinCl2n cyclic oligomers (with n = 12-16) at 900-1200°C. Under conditions of high vacuum and fast pumping, SiCl2 may be isolated by rapidly quenching the reaction products or, under less stringent vacuum conditions, (SiCl2)n polymer is deposited just beyond the hot zone while the perchlorosilanes SinCl2n+2 are trapped farther downstream. The infrared multiphoton dissociation of trichlorosilane (HSiCl3) also yields polysilicon dichloride, (SiCl2)n, along with HCl. SiBr4 and SiI4 react with Si at high temperatures to produce SiBr2 and SiI2, which polymerize on quenching. Reactivity The polysilicon dihalides are generally stable under vacuum up to about 150-200°C, after which they decompose to perhalosilanes, SinX2n+2 (where n = 1 to 14), and to polysilicon monohalides. However, they are sensitive to air and moisture. Polysilicon difluoride is more reactive than the heavier polysilicon dihalides. In stark contrast to its carbon analog, polytetrafluoroethylene, (SiF2)n ignites spontaneously in air, whereas (SiCl2)n inflames in dry air only when heated to 150°C. The halogen atoms in polysilicon dihalides can be substituted with organic groups. For example, (SiCl2)n undergoes substitution by alcohols to give poly(dialkoxysilylene)s. The polysilicon monohalides are all stable to 400°C, but are also water and air sensitive. Polysilicon monofluoride reacts more vigorously than the heavier polysilicon monohalides. For example, (SiF)n decomposes [to SiF4 and Si] above 400°C explosively. See also Polysilicon hydride References External links Inorganic Chemistry (Holleman and Wiberg) Inorganic silicon compounds Halides