We have discovered that pentamethylcyclopentasiloxane (D,H) can be readily polymerized into poly(pentmethylcyclopentasiloxane) (PD5) with a Pt (Karstedt) catalyst in the presence of water in bulk or in solution at 100 degreesC and that the product is a solid with extraordinary properties. The polymerization starts with the oxidation of the SiH groups by water into an intermediate containing SiOH groups (SiH + H2O -->SiOH + H-2), which is followed immediately by the condensation (2SiO --> Si-O-Si) of D5H rings into complex aggregates of cyclosiloxane moieties. According to Raman spectroscopy, an average of three of the five SiH functionalities are converted, and the final product contains only a negligible number of SiOH groups. The melting and glass-transition temperatures of the monomer are exceptionally low: T-m.D5H = -137.6 +/- 1 and T-g,T-D5H = -152 +/- 2 degreesC. The polymer exhibits an unprecedented combination of properties: it is a stiff and brittle solid, is insoluble in common solvents, does not exhibit a melting endotherm but has an extremely low glass transition (T-g.PD5 = -151 +/- 0.5 degreesC), and is thermally stable up to at least 700 degreesC. Brillouin scattering indicates very slow variation of the relaxation time with temperature, a property characteristic of strong glass-forming systems such as silica glass. This characteristic may account for the unique combination of properties of the new polymer: an extremely low glass-transition temperature combined with solidlike properties even at ambient temperature (more than twice its glass-transition temperature).