The anionic ring-opening polymerization of heptamethyl-1,3-dioxa-5-aza-2,4,6-trisilacyclohexane, initiated with organolithium compounds, has been investigated. With the addition of dimethylformamide (DMF), the polymerization is controlled. Backbiting reactions are limited and give a unique specific cyclic compound in low proportion. Thermodynamic and kinetic studies indicate that such polymerization is equilibrated. The apparent rate constants of propagation and depolymerization along with the thermodynamic parameters (enthalpy and entropy) of the polymerization have been calculated. The active species are ion pairs externally solvated by DMF in equilibrium with unreactive aggregated ion pairs.