Polymerization of epsilon-caprolactone (epsilon CL) initiated with aluminum isopropoxide (Al((OPr)-Pr-i)(3)) trimer (A(3)), tetramer (A(4)), or both has shown that these two aggregates react with epsilon CL with different rates (k(i3)/k(i4) approximate to 10(3), where k(i3) and k(i4) denote the rate constants of initiation with A(3) and A(4), respectively). Moreover, the H-1 NMR measurements revealed relatively low rates of A(3) reversible arrow A(4) interconversion in their diluted solutions : the initial rates of A(4) --> A(3) and A(3) --> A(4) conversions were found to be equal to 0.015 and 0.04 mol %. h(-1) at 25 degrees C, and to 1.3 and 0.1 mol %. h(-1) at 70 degrees C, respectively ([Al((OPr)-Pr-i)(3)](0) = 0.1 mol . L(-1), benzene solutions). Therefore, when an A(3)/A(4) mixture is used in epsilon CL polymerization, A(3) is consumed completely whereas A(4) remains unreacted, at least within the time required for the complete epsilon Cl polymerization. In the initiator reacted with epsilon CL, all three (OPr)-Pr-i groups from Al((OPr)-Pr-i)(3) are transferred into the poly(epsilon CL) macromolecules as the end groups. Molecular weights (($) over bar M(n)) calculated with an assumption that in the involved active centers three poly(ECL) chains grow from one Al atom agree well with ($) over bar M(n) determined by gel permeation chromatography and the end group analysis (H-1 NMR) for the polymers deactivated with HClaq. ($) over bar M(w) of the living poly(epsilon CL), determined by multiangle laser light scattering revealed, after comparison with ($) over bar M(n) and ($) over bar M(w)/($) over bar M(n) data for the deactivated poly(epsilon CL), that the growing species assumes the nonaggregated (monomeric) three-armed structure : Al{[-O(CH2)(5)C(O)](n)-OCH(CH3)(2)}(3) (P-n*). Al-27 NMR spectra of the living poly(epsilon CL) suggest that, at least in a certain fraction of P-n*, the additional coordination of the Al atoms by the acyl oxygen atoms of the polymer repeating units takes place. Thus, the tetra-, penta-, and hexacoordinated Al atoms coexist during polymerization. Studies of the polymerization kinetics allowed the epsilon CL propagation rate constants on the >Al-OCH2-... active species k(p) = 0.62 mol(-1) L . s(-1) to be determined (THF, 25 degrees C). This rate constant refers to the total concentration of the active species of various coordination numbers.