A BF3 complex has been grafted onto silica and used as initiator to polymerize 1,3-dioxolane and 1,3-dioxepane. Grafting the active centers leads to a decrease of the reaction rate. The molecular weight increases steadily, parallel to the monomer conversion and the polydispersity is rapidly stabilized at a low value. MALDI-TOF mass spectrometry analysis shows that most of the polymer chains are initiated by a proton and lost a CH2O unit. An exchange process between polymer chains bound to the silica and those remaining in the reactional medium was proposed. Finally, a semicontinuous process was investigated by adding continuously dioxolane during a 100 hours period.