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 or tetramethylene and Rp and Rr are a hydrogen atom, and Rs is a direct bond, --CH2 --, --C(CF3)2 --, 2,2-propylidene or --CO--, and in which the radical X' is defined in the same way as X or is selected from the group consisting of ##STR12## where these radica