Zwitterionic titanoxanes {Cp[eta(5)-C5H4)(C6F5)(3)]Ti}(2)O(I) and {(eta(5)-(PrC5H4)-Pr-i)[eta(5)-1,3-(PrC5H3B)-Pr-i(C6F5)(3)]Ti}(2)O(II), which contain two positively charged Ti(IV) centres in the molecule, are able to catalyse the ring-opening polymerization of epsilon-caprolactone (epsilon-CL) in toluene solution and in bulk. The process proceeds with a noticeable rate even at room temperature and accelerates strongly on raising the temperature to 60degreesC. The best results have been obtained on carrying out the reaction in bulk. Under these conditions, the use of I as a catalyst (epsilon-CL:1 = 1000:1) gives at 60degreesC close to quantitative yield of poly-epsilon-CL with the molecular mass of 197 000. An increase in the epsilon-CL:I ratio to 6000:1 increases the molecular mass of poly-epsilon-CL to 530 000. Tetrahydrofuran (THF) is also polymerized under the action of I albeit with a lesser rate. However, the molecular mass of the resulting poly-THF can reach rather big values under optimal conditions (up to 217 000 at, 20degreesC and the THF:I ratio of 770:1). A rise in the reaction temperature from 20 to 60degreesC results here to a decrease in the efficiency of the process. Titanoxane II is close to I in its catalytic activity in the epsilon-CL polymerization but it is much less active in the polymerization of THF. Propylene oxide (PO), in contrast to epsilon-CL and THF, gives with I only liquid oligomers in wide temperature and PO:I molar ratio ranges (-30 to +20degreesC, PO:I = 500-2000:1). gamma-Butyrolactone and 1-methyl-2-pyrrolidone are not polymerized under the action of I at room temperature. The reactions found are the first examples of catalysis of the cationic ring-opening polymerization by zwitterionic metallocenes of the group IVB metals. (C) 2003 Elsevier Science B.V. All rights reserved.