yl and Rq and Rp are independently methyl, ethyl, n-propyl or isopropyl and Rq and Rr are a hydrogen atom or are defined in the same way as Ro or Ro and Rq are together trimethylene and Rp and Rr are a hydrogen atom, and Rs is a direct bond, --CH2 --, --C(CH3)2 --, 2,2-propylidene or --CO--, and in which the radical X' is selected from the group consisting of ##STR64## in which Ru is a direct bond