Communication: Ring-opening polymerization of a seven-membered cyclic carbonate, 1,3-dioxepan-2-one, was investigated with a novel initiator system, BCl3-HCl . Et2O. Addition of HCl . Et2O promoted the polymerization even at 0 degrees C to produce the corresponding polycarbonate with controlled molecular weight and narrow polydispersity ratio (<1.2).