Matrix-assisted laser desorption/ionization (MALDI)-time-of-flight (TOF) mass spectrometry allowed direct determination of the extent of macrocycle formation that occurred during the polymerization and copolymerization of A2B and A4B monomers. Cyclization in hyperbranched pentafluorophenyl- terminated poly(benzyl ether)s was indicated by the presence of ions 20 Da less than the masses of acyclic species, owing to the loss of the HF chain ends during polymerization. This loss occurs by intramolecular nucleophilic aromatic substitution of the benzyl oxide focal point functionality upon a pentafluorophenyl chain end. Homopolymerizations and copolymerization of A2B and A4B monomers gave cyclic products in all cases, and the extent of cyclization depended on counterion, reaction time, and reaction temperature. In the copolymerization, product distributions revealed that larger proportions of A2B repeat units in the product led to increased amounts of cyclic products.