In the polymerization of c-caprolactone (tCL) initiated with (CH,CH,),AlOCH,, (CH,CH,),-A10CH2CH=CH2, or [(CH,),CHCH,],AlOCH, and carried out in THF solvent at 20 or 25 O C linear poly(cCL) with 1.03 < Mw/Mn < 1.13 and free of macrocyclics was obtained. Comparison of Mn, calculated and measured, indicated that each initiator molecule starts one macromolecule and that during polymerization transfer reactions, including cyclization, are practically absent. 'H and 13C NMR studies of the end-group structures revealed that only alkoxy groups (OR') from initiator (R2A10R) were directly involved in initiation. The described polymerization of tCL can be considered as the living process, yielding linear polyesters free from cyclics within the polymerization time required to reach 99% of monomer conversion. Cyclics are produced after much longer time from the linear polymer already formed. The ratios of propagation rate constant (k,(l)) to the rate constants of back-biting, leading to the cyclic dimer (kb(2)), have been used as a measure of kinetic enhancement in linear polymer. For cCL polymerizations carried out in THF at 20 O C and initiated with (CH,CH2),A10CH, and [(CH,)2CHCH2]2A10CH, kp(1)/kb(2) = 4.6 X lo4 and 7.7 X lo4 Lsmol-', respectively. These values are substantially higher than for the anionic polymerization of cCL with Na+ counterion (kp(1)/kb(2) = 1.55 X lo3 Lsmol-').

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The study of the structure of the end-groups of poly(8-propiolactone) and poly(e-caprolactone) by 'H, ,IP[lH] NMR, and IR spectroscopy revealed that the mode of ring scission in the monomer in the first propagation step depends on the initiator used. In the initiation of the 8-propiolactone and e-caprolactone polymerization with halonium salts ((CH, )2Br@SbFp or (CH,),I@SbFp ) the exocyclic oxygen atoms in the monomer molecules are attacked and oxonium ions are formed. The propagation steps proceed with alkyl-oxygen bond scission in the active center and with the attack at.the exocyclic oxygen atom in the subsequent monomer molecule. Thus, the tertiary oxonium ions are the exclusive active species from the very beginning of the polymerization. Acylium cations (CH,CO@ SbFg or CH,CH,CO@SbFp ) as initiators attack both exo-and endocyclic oxygen atoms in the monomers and the initiation proceeds with almost equal proportions of alkyl-oxygen and acyl-oxygen bond scission. Thus, in the polymerization of /3-propiolactone the concentration of the acylium growing species decreases with increasing number of propagation steps and finally the tertiary oxonium ions are becoming the exclusive growing species. In the polymerization of E-caprolactone the active acylium centers apart from propagation participate also in side reactions leading to the cleavage of a proton.