The cyclopolymerizations of 1,2:5,6-dianhydro-D-mannitols were carried out using cationic initiators. For the polymerization of 1,2:5,6-dianhydro-3,4-di-O-ethyl-D-manni with BF3 . OEt(2), the n-hexane-insoluble polymer consisted mainly of 2,5-anhydro-3,4-di-O-ethyl-D-glucitol as the cyclic constitutional unit and a large amount of 1,6:2,5-dianhydro-3,4-di-O-ethyl-D-glucitol was obtained as the n-hexane-soluble reaction product. The degree of polymerization for the n-hexane-insoluble polymer was 14-30, and the specific rotation ([alpha](22)(546)) was 42.7-54.3 degrees (c = 1.0 in CHCl3). On the other hand, 1,2 : 5,6-dianhydro-3,4-O-isopropylidene-D-mannitol polymerized with BF3 . OEt(2-) and SnCl4-produced polymers with cyclic and acyclic units due to the restricted free rotation of the bond between the carbons at the 3,4-positions in which the extent of cyclization was about 0.5.