The soluble catalyst system methylaluminoxane (MA0)-Ni(acac), (acac: acetylacetonate) gives polystyrene consisting of an amorphous (aPS) and a crystalline isotactic (ips) fraction. Al(CH,),, which is always present in commercial samples of MAO, decreases both polymer yield and stereospecificity. The polymer yield increases with increasing the MAO/Ni ratio but, at the same time, the iPS/aPS ratio decreases. Addition of N(C2H5), (mole ratio N/Ni = 1) increases the proportion of the isotactic fraction, while it decreases the polymer yield. A tentative interpretation of the stereospecificity is reported. Gel permeation chromatography: Mw = 33 -lo3; M, = 18 -1W3. dl Mw = 33 * lo3; M,, = 18lo'. Mw = 110 * lo3; M,, = 53 * 10'. e, M, = 42 -lo3; M,, = 24 -lo3.4

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