Abstract: These compounds are denoted by the following formula (I): ##STR1## in which: Z denotes H or a linear or branched alkyl radical,X denotes O or S,each of A and B independently denotes an alkylene radical, it being possible for this radical to be substituted,R denotes H or an alkali metal, or else a linear or branched alkyl radical,x has the value of 0 or 1, andy has the value of 0, 1 or 2,the radical A not existing if x and y are both equal to 0, X being incapable of denoting O when R=alkyl and when R=H with x=y=0, and B being incapable of denoting optionally substituted methylene when X=S, x=y=0 and R=alkyl.According to the invention, different simple and efficient processes are proposed for preparing these new compounds, which can be applied to the preparation of new polymers and copolymers.