The cationic polymerization of dimethylketene is achieved in dichloromethane at -30 degrees C, using a stoichiometric mixture of aluminum bromide (AlBr3) and tetra-n-butylammonium bromide (n-Bu4N+Br-) as initiator. Characterizations by H-1 and C-13 NMR show that the resulting polymers have a perfect polyketonic microstructure. Capillary viscosity, DSC, and SEC analysis show that for a constant monomer/initiator ratio, polymers synthesized in the presence of tetra-n-butylammonium bromide are more crystalline and have better properties than those produced only with AlBr3. Melting temperatures, inherent viscosities and average molecular weights are systematically higher. A good linearity is observed between ln (inherent viscosity) versus ln (Mn) over bar (theory) for the system with n-Bu4N+Br-, showing a good control of the molecular weight by the initial feed ratio. The effect of this compound suggests a reversible equilibrium between active and dormant species, which limits the transfer and/or termination reactions, and enables a better control of the cationic polymerization. (c) 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014, 52, 1493-1499