Copolymerizations of 1,4-anhydro-2,3-O-benzylidene- and 1,4-anhydro-2,3-bis-O-(tert-bu-tyldimethylsilyl)-alpha-D-ribopyranoses in various feeds were carried out with antimony pentachloride as catalyst at 0-degrees-C to give, 1,4-beta-linked ribopyranan copolymers. Next the copolymers were subjected to a branching reaction by D- and L-glucose ethyl orthoacetates followed by the selective removal of silyl groups to afford branched ribopyranans. These were sulfated with piperidine-N-sulfonic acid to form branched ribopyranan sulfates having potent anti-AIDS virus activities, EC50 = 0.3-0.9 mug/mL (a standard curdlan sulfate, EC50 = 0.43 mug/mL). Stereoregular ribopyranan sulfates and sulfated poly(ribose)s composed of 1,4-beta-pyranosidic and 1,5-alpha-furanoside structures were also synthesized to examine the relationship between polymer structure and anti-AIDS virus activity. The structural analysis of the ribopyranans was performed by high resolution C-13 NMR spectroscopy.