Components and catalysts for the polymerization of olefins

#The present invention relates to catalyst components for the polymerization of olefins, to the catalysts obtained therefrom and to the use of said catalysts in the polymerization of olefins CH.sub.2 .dbd.CHR in which R is hydrogen or a hydrocarbyl radical with 1-12 carbon atoms. In particular the present invention relates to catalyst components, suitable for the stereospecific polymerization of olefins, comprising titanium, Mg, halogen and an electron donor compound selected from cyanoesters having a particular formula. Said catalyst components when used in the polymerization of olefins, and in particular of propylene, are characterized by an excellent response to hydrogen and are also capable to give polymers with very broad molecular weight distribution.

#Many patent applications have been filed in the field of the so-called high-yield polymerization catalysts, in particular regarding the catalysts for the polymerization of propylene. Tipically, said catalysts comprise a solid catalyst component, constituted by a magnesium dichloride on which are supported a titanium compound and an internal electron donor compound (usually an ester of a phthalic acid), an Al-alkyl compound and an external donor (usually a silicon compound). The above catalyst systems, although capable of providing propylene polymers with high stereoregularity and in high yields , are also characterized by a poor capability of response to hydrogen. In other words, very high concentrations of hydrogen, used as molecular weight regulator, are necessary in order to obtain polymers with low molecular weight. The low molecular weight polymers are requested in applications where high fluidity in the molten state and good mechanical properties are necessary. The production of webs for non-woven fabrics, which is carried out by melt blown or spun bonded processes, is an example of such applications. In some cases, the molecular weights required for the propylene polymers are so low that, to produce them, the prior art catalyst systems would require a pressure of hydrogen well above the operative limit of conventional bulk polymerization plants. In these cases, such low molecular weight polymers are obtained by visbreaking of the high molecular weight polymers at high temperatures (200.degree.-300.degree. C.) and in the presence of free radical generators such as organic peroxides.

#Moreover, the propylene polymers obtained with the above mentioned catalyst systems usually have a narrow molecular weight distribution (MWD) as compared for example with polyolefins prepared by using the conventional catalysts comprising a titanium trichloride based catalyst component. The narrow MWD causes a worsening of the processability of the polymers which involves a decrease of the quality of the products in applications such as molding or thermoforming. One of the approaches that have been tried to broaden the MWD comprises carrying out the polymerization in at least two steps under different polymerization conditions. However, these multisteps processes require a more complicated and sophisticated polymerization operation which increases the cost of the polymerization process.

#Preparation of Solid Catalyst Components

#Into a 500 ml four-necked round flask, purged with nitrogen, 225 ml of TiCl.sub.4 were introduced at 0.degree. C. While stirring, 10.3 g of microspheroidal MgCl.sub.2.2.1C.sub.2 H.sub.5 OH (obtained by partial thermal dealcoholation of an adduct prepared as described in Ex. 2 of U.S. Pat. No. 4,399,054 but operating at 3,000 rpm instead of 10,000) were added. The flask was heated to 40.degree. C. and 9 mmoles of cyanoester were thereupon added. The temperature was raised to 100.degree. C. and maintained for two hours, then the stirring was discontinued, the solid product was allowed to settle and the supernatant liquid was siphoned off.

#200 ml of fresh TiCl.sub.4 were added, the mixture was reacted at 120.degree. C. for one hour and then the supernatant liquid was siphoned off. The solid was washed six times with anhydrous hexane (6.times.100 ml) at 60.degree. C. and then dried under vacuum: the cyanoesters used, the amount of Ti (wt %) and of cyanoesters (wt %) contained in the solid catalyst component are reported in table 1. The polymerization results are reported in table 2.