Cationic ring-opening polymerization of anhydrodisaccharide monomer (1,6-anhydro-3-O-benzyl-2 -deoxy-4-O-(2＇,3＇,4＇,6＇-tetra-O-benzyl-alpha-D-glucopyranosyl)-beta-D-arabino-hexopyranose), which was prepared by glycosylation of an anhydroglucose derivative, was carried out under high vacuum in dichloromethane with phosphorus pentafluoride as initiator. The Lewis acid catalyzed polymerization gave a cyclic oligosaccharide (thermodynamically preferred) and a (1-->6)-alpha-linked comb-shaped polysaccharide (kinetically preferred) having an alpha-glucopyranosyl branching unit per sugar residue in the backbone chain. The branched polysaccharide has a number-average molecular weight of about 21 300 (DPn similar to 28) and a specific rotation of +84.1 degrees (cl, CHCl3). Variations in reaction conditions significantly affected the course of polymerization. Copolymerization of the anhydrodisaccharide monomer with an anhydromonosaccharide monomer (1,6-anhydro-3,4-di-O-benzyl-2-deoxy-beta-D-arabino-hexopyranose) was also carried out under high vacuum in the presence of PF5 initiator at -60 degrees C. The homopolymer and copolymer structures were determined by optical rotation and NMR spectroscopy. The mechanism of ring-opening polymerization of the anhydrodisaccharide monomer is discussed.