Process for producing end-group-functionalized oligomers based on styrene by means of free-radical polymerization, starting from an amount of monomer comprising at least 90 mol% styrene, wherein - the polymerization is started by self-initiation - and is controlled by means of a difunctional regulator comprising the functional groups R F1 and R F2, where the group R F1 is an unsubstituted sulfanyl group and where the group R F2 of the regulator is selected from the group consisting of hydroxyl groups (-OH), carboxyl groups (-COOH), ester groups (-COOR), primary amino groups (-NH 2), secondary amines (-NHR), - wherein a regulator reference amount is defined such that the quantitative ratio of monomers in the amount of monomer to regulator molecules in the regulator reference amount is selected in the range from 100: 20 to 100: 0.5, characterized in that - the total amount of monomer is initially charged, - the batch with the monomer comprises not more than 10% by weight of a solvent, - up to 5% by weight of the regulator reference amount is added to the amount of monomer prior to the start of the reaction, - the temperature of the amount of monomer is controlled such that the polymerization starts by self-initiation, - wherein the further addition of regulator to the polymerization reaction is carried out continuously or in discrete steps, until a polymerization conversion of at least 85% is achieved, and in such a manner that at all times the following condition is met: C t − 0.05 ≤ X reg t ≤ C t + 0.05 wherein C(t) = polymerization conversion up to the time point t C(t) = n mon (t)/n mon,tot n mon (t) = total amount of reacted monomers in the course of the polymerization reaction up to the time point t n mon,tot = amount of all monomers used n mon,tot (amount converted) X reg (t) = fractional amount of regulator X reg (t) = R (t) /R B R(t) = total amount of regulator added up to the time point (t) R B = regulator reference amount. - the polymerization is started by self-initiation - and is controlled by means of a difunctional regulator comprising the functional groups R F1 and R F2, where the group R F1 is an unsubstituted sulfanyl group and where the group R F2 of the regulator is selected from the group consisting of hydroxyl groups (-OH), carboxyl groups (-COOH), ester groups (-COOR), primary amino groups (-NH 2), secondary amines (-NHR), - wherein a regulator reference amount is defined such that the quantitative ratio of monomers in the amount of monomer to regulator molecules in the regulator reference amount is selected in the range from 100: 20 to 100: 0.5, - the total amount of monomer is initially charged, - the batch with the monomer comprises not more than 10% by weight of a solvent, - up to 5% by weight of the regulator reference amount is added to the amount of monomer prior to the start of the reaction, - the temperature of the amount of monomer is controlled such that the polymerization starts by self-initiation, - wherein the further addition of regulator to the polymerization reaction is carried out continuously or in discrete steps, until a polymerization conversion of at least 85% is achieved, and in such a manner that at all times the following condition is met: C t − 0.05 ≤ X reg t ≤ C t + 0.05 wherein C(t) = polymerization conversion up to the time point t C(t) = n mon (t)/n mon,tot n mon (t) = total amount of reacted monomers in the course of the polymerization reaction up to the time point t n mon,tot = amount of all monomers used n mon,tot (amount converted) X reg (t) = fractional amount of regulator X reg (t) = R (t) /R B R(t) = total amount of regulator added up to the time point (t) R B = regulator reference amount. C(t) = polymerization conversion up to the time point t C(t) = n mon (t)/n mon,tot n mon (t) = total amount of reacted monomers in the course of the polymerization reaction up to the time point t n mon,tot = amount of all monomers used n mon,tot (amount converted) X reg (t) = fractional amount of regulator X reg (t) = R (t) /R B R(t) = total amount of regulator added up to the time point (t) R B = regulator reference amount.