No abstract

No abstract

The interaction of zirconocene dichloride (Cp,ZrCl,) with methylalumoxane (MAO), the two components of homogeneous catalyst for olefin polymerization, was studied by 'H NMR spectroscopy. Its was found that Cp2ZrC12 is monoalkylated by Al,(CH,), present in MAO. The trimethylaluminium contained in MA0 appears to be the active species in the alkylation of the zirconocene dichloride. The Cp,ZrCH,CI likely forms, with an excess of MAO, highly polarized species. These are decomposed by a complexing agent of MA0 such as KCl.

SUMh4ARYCharacterization of methylalumoxane (MAO) by gel permeation chromatography reveals the presence of different oligomeric products. Using suitable series of columns and alkylaluminium compounds with known molecular weight, it is possible to obtain the average molecular weights of most species which are present in a sample of MAO. The study of the chromatograms reveals modifications in composition of the MA0 oligomers in relation to the time of residence in solution at given temperatures.CCC 0025-1 16X/90/$03.00 _''

The results of this paper indicate that the presence of poly-L-aspartic acid (PASP) strongly inhibits the synthesis of octacalcium phosphate (OCP) in aqueous solution and provokes remarkable morphological and structural modifications. Under the experimental conditions employed, in the absence of the polyelectrolyte, OCP precipitates as individual platelets, elongated along the c-axis direction, of mean dimensions 22.0 × 1.3 × 0.2 µm 3 . Low PASP concentrations (up to 22 µM) induce a reduction of the mean dimensions of the platelets and the formation of aggregates, which in the TEM-ED images appear mostly constituted of fibers aggregated parallel to their length, and exhibit unidirectional disorder in the direction orthogonal to the c-axis. Relatively high PASP concentrations (up to 110 µM) greatly reduce the total amount of product, which precipitates as almost spherical aggregates with increasing structural disorder. The modification of the X-ray diffraction pattern on increasing PASP concentration indicates an increasing disorder and an increasing contribution of the apatitic layer of the OCP structure, which could be related to the observed reduction of thermal stability, and suggests a possible interaction of PASP with the hydrated layer of the OCP structure.