<p>In this study, we successfully prepared soluble polymers via the tris(pentafluorophenyl)borane (BCF)-catalyzed hydrosilylation of dimethylhydrosilyl-functionalized cage octasiloxane (DMHS-OS) and acetone, followed by dehydrocarbon condensation polymerization (Piers–Rubinstzajn reaction) between the unreacted hydrosilyl (Si–H) groups and converted isopropoxysilyl (Si–OiPr) groups of DMHS-OS. Notably, polymer Poly(DMHS-OS)-5, formed at a feed molar ratio of Si–H groups to acetone of 8:5, exhibited a relatively high weight-average molecular weight (<i>M</i>_w = 2.11 × 10⁴). On the basis of the ¹H NMR, ²⁹Si NMR, and gel permeation chromatography results, Poly(DMHS-OS)-5 consists of approximately 17–18 linked cage octasiloxane repeating units. Despite the presence of alkoxysilyl groups, such as Si–OiPr, in the side chains, immersion in purified water for 1 h did not affect the solubility of the polymer, indicating its good water stability. Thermogravimetric analysis revealed that the 10% weight loss temperature of Poly(DMHS-OS)-5 was 535 °C and only 21% weight loss occurred at 1000 °C, indicating exceptionally low thermal degradation. These findings highlight the remarkably high thermal stability of the soluble polymer Poly(DMHS-OS)-5.</p>