Catalysts for preparing ultra high molecular weight polyethylene (UHMWPE)

A metallic complex of formula VII

• wherein R1, R3, R4, R5, R6 and R7 are each independently selected from hydrogen, alkyl, cycloalkyl or aryl group comprising from 1 to 12 carbon atoms, a halogen, or a silyl group, wherein two or more of said groups can be linked together to form one or more rings;
• wherein Q is an atom selected from Group 16;
• wherein A is an atom selected from Group 15;
• wherein R2 is independently selected from an unsubstituted or substituted aryl or cycloalkyl group having from 1 to 12 carbon atoms, or from Z(R9)3 wherein Z is an atom selected from Group 14 of the Periodic Table and each R9 is independently selected from a hydrogen, substituted or unsubstituted aryl, alkyl or cycloalkyl group having from 1 to 12 carbon atoms, with the restriction that R2 is a bulky group, at least as sterically bulky as tert-butyl;
• wherein R8 is an unsubstituted or substituted aryl, alkyl or cycloalkyl group having from 1 to 12 carbon atoms;
• wherein M is a metal Group 4 of the Periodic Table;
• and
• wherein each X is the same or different and is a halogen atom, a substituted or unsubstituted alkyl, cycloalkyl or aryl group comprising from 1 to 12 carbon atoms, an amido group, or an alkoxide group.