-t-butyl; Z is selected from the group consisting of, R3 2Si-SiR3 2, R3 2C-CR3 2, R3 2Si-CR3 2, R3 2Si and CR3 2 radicals, wherein R3 is a hydrogen atom or a linear or branched, saturated or unsaturated C1-C20 alkyl, C6-C20 aryl or C7-C20 arylalkyl radicals, optionally containing heteroatoms belonging to groups 13-17 of the Periodic Table of the Elements; two R3 can also form a C4-C7 saturated or