The cationic ring-opening polymerization of 3,3-bis(hydroxymethyl)oxetane (BHMO) and the copolymerization of BHMO with 3-ethyl-3-(hydroxymethyl)oxetane (EOX) were studied. Medium molecular weight polymers (number-average molecular weight approximate to 2 X 10(3)) were obtained in bulk polymerization. Poly [3,3-bis(hydroxymethyl)oxetane], as highly insoluble, was only characterized by gel permeation chromatography and NMR methods in the esterified form. Copolymers of BHMO and EOX that were slightly soluble in organic solvents were characterized in more detail. In a copolymerization from a 1:1 mixture, the comonomers were consumed at equal rates. Matrix-assisted laser desorption/ionization time-of-flight analysis confirmed that a random 1:1 copolymer was formed. C-13 NMR analysis indicated that in contrast to previously described homopolymers of EOX in which the degree of branching was limited, the homopolymers of BIIMO were highly branched. This pattern was preserved in the copolymers; EOX units were predominantly linear, whereas BHMO units were predominantly branched. The copolymerization of BHMO with EOX provides, therefore, a route to multihydroxyl branched-polyethers with various degrees of branching containing -OH groups exclusively as units.