SELECTIVE RING-OPENING POLYMERIZATION OF 1, 4-ANHYDRO-2, 3-O-CYCLOHEXYLIDENE-α-D-RIBOPYRANOSE AND COPOLYMERIZATION OF 1, 4-ANHYDRO-2, 3-O-CYCLOHEXYLIDENE-α-D-RIBOPYRANOSE WITH 1, 4-ANHYDRO-2, 3-O-BENZYLIDENE-α-D-RIBOPYRANOSE