The Piers–Rubinsztajn (PR) reaction catalyzed by a metal-free B­(C6F5)3 catalyst was reported as efficient in synthesizing novel polysiloxanes through polycondensation of dialkoxylsilanes and dihydrosiloxane monomers at room temperature. This study is aimed at developing new cyclosiloxane polymers having no hydrocarbon linker in the main chain via an equimolar PR reaction between tetrafunctional eight-membered cyclosiloxanes (TMCS) having the four reactive silyl hydrides (Si–H) at the Si vertices and bifunctional dialkoxylsilanes. The final polymers were targeted with the remaining Si–H group at the Si vertices of the TMCS repeating units for post-functionalization, good solubility in typical organic solvents for post-processing, and a stable ring structure in TMCS for outstanding macromolecular flexibility. However, hydride transfer ring-opening polymerization (HTRP) of TMCS was reported in the B­(C6F5)3-catalyzed reaction system, which invariably causes uncontrollable gelation. Suppression of HTRP succeeded via control of total monomer concentration and reaction time reduction, thereby obtaining post-processable and post-functionalizable liquid cyclosiloxane prepolymers. Thermal curing of the prepolymers through self-cross-linking of the remaining reactive Si–H groups gave rise to free-standing films with excellent thermal stability exceeding 600 °C and a low dielectric constant. The functional versatility of the Si–H groups produces promising prepolymers for creating various functional materials.