In this report, highly active metallocene initiators are used for the polymerization of a ketene monomer, dimethylketene, which typically contains two adjacent double bonds (R CCO). By using the methylzirconocene methyltriarylborate complex (Cp ZrMe MeB(C F ) ) as the activation system, associated with the possible cleavage of CC and CO bonds in ketene monomers, a structure-specific and high-molecular-weight polyester ( > 300 000 g mol ) can be afforded. The resulting polyester structure, comprehensively characterized by NMR, indicates a significant reactive selectivity of the "bent-sandwich" cationic site Cp ZrMe . It reveals that the positively charged zirconium (Zr ) prefers to coordinate with a negatively charged oxygen (O ) when it is already bonded to the carbon, while a negatively charged carbon (C ) will be assigned in priority if the oxygen-zirconium bond exists. This report allows for broadening the application field of metallocene initiators in non-olefin reactions and deepening our insight into the mechanism of the living insertion/Ziegler-Natta polymerization.