Synthesis of block copolysaccharides by ring-opening polymerization of anhydro sugar derivatives

Ring-opening polymerization of anhydro sugar derivatives was carried out with Lewis acid catalysts to produce block copolysaccharides. Polymerization of 1,4-anhydro-2,3-di-O-tert-butyldimethylsilyl-alpha-D-ribopyranose (ADSR) with boron trifluoride etherate proceeded linearly and rapidly within 1 h to give silylated 1,5-alpha-D-ribofuranan in almost quantitative yield. Although the molecular weight distribution of the polymers was not narrow ((M) over bar(w)/(M) over bar(n)=1.61 similar to 1.73), the relationship between monomer conversion and number-average molecular weight was linear, indicating livingness of the ring-opening polymerization. Thus, an AB block copolysaccharide composed of two different polysaccharide chains was synthesized for the first time in the two-stage polymerization of ADSR and 1,4-anhydro-2,3-di-O-benzyl-alpha-D-ribopyranose (ADBR) at -40 degrees C. flock copolysaccharides having different stereoregular monomeric units (i.e., ribopyranosidic and ribofuranosidic units) were obtained by block copolymerization of 1,4-anhydro-2,3-O-benzylidene-alpha-D-ribopyranose (ABRP) and ADBR. In addition, a block heteropolysaccharide consisting of(1-->5)-alpha-D-xylofuranan and (1-->5)-alpha-D-ribofuranan was prepared from 1,4-anhydro-2,3-di-O-tert-butyldimethylsilyl-alpha-D-xylopyranose (ADSX) and ADBR by making use of this polymerization technique. The GPC curves of block copolymers shifted toward the higher molecular weight region, maintaining the molecular weight distributions of the corresponding homopolymers in the :first stage of polymerization. The number-average degree of polymerization of the polyADSR-block-polyADBR was estimated to be 113 for ADSR and 221 for ADBR unit. The C-13 NMR spectra of the resulting block copolymers were different from those of the random copolymers.