Cyclization in hyperbranched polymer synthesis was first controlled on the basis of a steric effect using a Lewis acid with a bulky ligand. Only phenylazomethine oligomers having a cyclic structure were formed during the polymerization of 4,4'-diaminobenzophenone in the presence of TiCl4(THF)(2) as a Lewis acid with bulky ligands. The structure of a cavity in an isolated cyclic oligomer was determined by X-ray crystal analysis. Controlled cyclization is applied for hyperbranched polymer synthesis, and a novel dendrimer with a cyclic structure was quantitatively obtained via controlled cyclization on the basis of a steric effect using a monomer with a bulky dendron.