Propylene polymers and process for the preparation thereof

A process for preparing a propylene polymer, optionally containing up to 5 mol % of ethylene, comprising:

• i) a molecular weight distribution (MW/Mn) ≦4;
• ii) a number of internal vinylidene per polymer chain ≧2
• the process comprising contacting, under polymerization conditions, propylene and optionally ethylene, with a catalyst system comprising:
• a) a metallocene complex of formula (I)
• wherein:
• M is titanium, zirconium or hafnium;
• the groups X, equal to or different from each other, are monoanionic sigma ligands chosen from hydrogen, halogen, —R, —OR, —OCOR, —OSO2CF3, —SR, —NR2 and —PR2, wherein R is a linear or branched C1-C20 alkyl, C2-C20 alkenyl, C3-C20 cycloalkyl, C6-C20 aryl, C7-C20 alkylaryl or C7-C20 arylalkyl radical;
• the groups R1, R2, R3 and R4, equal to or different from each other, are chosen from hydrogen, linear or branched C1-C20 alkyl, C2-C20 alkenyl, C3-C20 cycloalkyl, C6-C20 aryl, C7-C20 alkylaryl and C7-C20 arylalkyl radicals, optionally containing heteroatoms belonging to groups 13-17 of the Periodic Table;
• two or four adjacent groups R1, R2, R3 and R4 optionally form together one or more 3-6 membered aromatic or aliphatic rings, optionally substituted with hydrocarbyl radicals optionally containing heteroatoms belonging to groups 13-17 of the Periodic Table;
• with the proviso that either R1 is different from R4 or R2 is different from R3;
• Z is a carbon or silicon atom;
• the groups R5 and R6, equal to or different from each other, are chosen from hydrogen, linear or branched, C1-C20 alkyl, C2-C20 alkenyl, C3-C20 cycloalkyl, C6-C20 aryl, C7-C20 alkylaryl and C7-C20 arylalkyl radicals;
• optionally containing heteroatoms belonging to groups 13-17 of the Periodic Table;
• R5 and R6 optionally form together a 3 to 6 membered ring;
• and
• b) a suitable activating cocatalyst;
• wherein a concentration of hydrogen formed during the polymerization reaction is reduced.