The invention relates to a method of preparing a composition of living polymers comprising a non-photoinitiated radical polymerization step using a composition consisting of: at least one ethylenically unsaturated monomer, preferably a methacrylic monomer, at least one disulphide compound having formula (II), i.e. Z.sup.1--C(.dbd.S)--S--S--C(.dbd.S)--Z.sup.2; and at least one thermally-activate free radical source which is different from the formula (II) compound.