A Novel Polymeric Carbohydrate. Synthesis of (1→6)-2,5-Anhydro- d -glucitol by Regio- and Stereoselective Anionic Cyclopolymerization of 1,2:5,6-Dianhydro- d -mannitol

The anionic cyclopolymerizations of 1,2:5,6-dianhydro-3,4-di- O -methyl- d -mannitol ( 1 ) and 1,2:5,6-dianhydro-3,4- O -isopropylidene- d -mannitol ( 2 ) have been carried out using potassium tert -butoxide ( t -BuOK) and potassium hydroxide (KOH). For the polymerization of 1, a well-defined carbohydrate polymer consisting of (1→6) linked 2,5-anhydro-3,4-di- O -methyl- d -glucitol units was synthesized through a regio- and stereoselective mechanism. When a [ 1 ]/[ t -BuOK] molar ratio of 20 was used in toluene for 48 h, the yields and number-average molecular weights ( M n ) of the polymers gradually increased with polymerization time. The M n of the polymer varied with the molar ratio of monomer to initiator, and a linear relationship between them was found. The degree of polymerization was larger than that estimated from the molar ratio, resulting in an initiator efficiency of about 55%. KOH was also effective for converting monomer 1 to a gel-free polymer but was not as active as t -BuOK. The rate of polymerization was rather slow, and the polymerization was not complete, even after 100 h. The presence of a crown ether, 18-crown-6, in the cyclopolymerization allowed the M n of the polymer to approach the value estimated from the [ 1 ]/[ t -BuOK] molar ratio. On the other hand, monomer 2 tended to form a gel in the polymerization process, so soluble polymers were isolated only at early stages of the polymerization.