Catalyst systems for syndiospecific polymerization of styrenes

A process for producing syndiotactic polystyrene comprising contacting a feed comprising styrene, under polymerization conditions, with a catalyst composition comprising at least three components:

• (i) trimethylaluminum,
• (ii) at least one of the organotin compounds with general formulae:
• R.sub.1 R.sub.2 R.sub.3 Sn--O--SnR.sub.4 R.sub.5 R.sub.6
• or R.sub.1 R.sub.2 R.sub.3 Sn--OH
• or R.sub.1 R.sub.2 SnO
• or R.sub.1 Sn(O)OH;
• wherein R.sub.1, R.sub.2, R.sub.3, R.sub.4, R.sub.5 and R.sub.6 are the same or different C.sub.1 -C.sub.20 alkyl groups or aryl groups or alkylaryl groups,
• (iii) at least one derivative of a transition metal having the formula Cp.sub.n MA.sub.m-n, wherein
• M is a metal selected from the group of titanium, zirconium and hafnium;
• Cp is a cyclopentadienyl or a substituted cyclopentadienyl group, wherein one or several substituents are alkyl groups of 1 to 6 carbon atoms;
• A is selected from the group consisting of a halogen, hydrogen, or an oxyhydrocarbyl group OR', wherein R' is a C.sub.1 -C.sub.6 linear or branched alkyl group, a substituted or non-substituted aryl group, or an arylalkyl group;
• n is 0 or 1;
• and m is 4 or 3;
• provided that the molar ratio between said trimethylaluminum and the organotin compound is higher than 1.