Growth versus Cyclization in the Early Stages of the Polycondensation of Metal Alkoxides

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Abstract
The early steps of the polycondensation of transition metal alkoxide have been studied from the chemical and structural points of view. Polyoxoalkoxides are described like macromolecules by the composition of the repeating unit, the degree of polymerization (N), and the radius of gyration (R). The fraction p of binding sites of the coordination sphere of the metal centers occupied by terminal ligands determines N as follows: N ∝ pdf/(dA-df), where df is the fractal dimension and dA is defined by Np ∝ RdA. This approach addresses difficulties raised by both coordinative unsaturation and cyclization in the modelization of the polycondensation of metal alkoxides. The coordinative unsaturation is accounted for by a particularly small value of dA = 1 in the very early steps, while the cyclization frequency is measured by the difference dA − df. This difference is not constant along the polycondensation process, and its dependence on the extent of reaction provides clues for understanding the high apparent kinetics order of gelation often reported in the literature.