COMPOUNDS OF FORMULA (I) WHEREIN: R₁ REPRESENTS A HYDROGEN ATOM OR A – COOR GROUP, R₂ REPRESENTS A GROUP G OR A LINEAR OR BRANCHED (C₁-C₆) ALKYL GROUP SUBSTITUTED BY A GROUP G, WHEREIN G REPRESENTS A–(CH₂)n-A-(CH₂)m-B-(CR₄R₅)p-(CH₂)o-R₆ GROUP AS DEFINED IN THE DESCRIPTION, R₃ REPRESENTS A HYDROGEN ATOM, AN ALKYL GROUP OR AN NO₂ GROUP.