Bulk polymerization of rac-lactide initiated by guanidinate alkoxide complexes of rare earth metals. The molecular structure of the cluster [{(Me3Si)2NC(NPri)2}Nd]4(μ3-OPri)8Li7(μ2-Cl)3(μ3-Cl)2(μ4-Cl)2

The complexes {(Me3Si)(2)NC(NPri)(2)}(2)LnOBu(t) (Ln = Y (1), Lu (2)) initiate the bulk polymerization of racemic lactide (LA) at 130 A degrees C. At the monomer: initiator molar ratio ([LA]: 1) equal to 1000: 1, the quantitative conversion of the monomer is achieved within 6 h. The resulting polymers are characterized by a rather narrow monomodal molecular weight distribution (M (w)/M (n) = 1.46-1.82) and molecular weights up to 33400 g mol(-1). The molecular weights of the resulting polylactides measured by gel permeation chromatography are 3-11 times lower than the values calculated from the monomer: initiator ratio on the assumption of one growing polymer chain per catalytic center. The reaction of the in-situ prepared complex [(Me3Si)(2)NC(NPri)(2)]NdCl2 with 2 equiv. of (PrOLi)-O-i produced the 11-nuclear cluster [{(Me3Si)(2)NC(NPri)(2)}Nd](4)(mu(3)-OPri)(8)Li-7(mu(2)-Cl)(3)(mu(3)-Cl)(2)(mu(4)-Cl)(2) (3), which was isolated in 62% yield. The structure of compound 3 was established by X-ray diffraction. Complex 3 initiates both the bulk and solution polymerization of rac-lactide. In the bulk polymerization at the molar ratio [LA]: [Nd] = 500: 1, the 89% conversion of the monomer was achieved within one hour. The polylactide thus synthesized has the molecular weight M (n) = 19720 g mol(-1) and a rather narrow polydispersity M (w)/M (n) = 1.54.