A process for the production of lower molecular weight polymers by free radical polymerization, characterized in that there is added to the polymerization system a compound of the general formula I ##STR1## wherein R.sup.1 is hydrogen, chlorine, an alkyl group, or a group capable of activating the vinylic carbon towards free radical addition; R.sup.2 is hydrogen or an optionally substituted alkyl, alkenyl, aryl, cycloalkenyl or cycloalkyl group or the group --COZ, where Z is R.sup.3 or OR.sup.3, where R.sup.3 is hydrogen or an optionally substituted alkyl, alkenyl or aryl group. X is a hydrogen atom, an alkyl or aryl group, or a halogen, and the two X groups may be the same or different. The invention also includes polymers or oligomers made by the process and novel compounds of the general formula I as defined above.