The present invention is directed to a process for the cleavage of one or more starting polymers having thiocarbonylthio groups of the formula —S—(C═S)— into derived polymers having thiol end groups of the formula —SH. The process includes contacting the aforedescribed starting polymers, with a reagent having the formula Xa Yb, wherein Xa is a nucleophilic group and Yb is an extracting group that results in the starting polymer and a byproduct, which is then separated from the derived polymer by conventional separation processes. The derived polymer is free from any odor or color that is sometimes associated with the starting polymer and it can be used in making optical lenses, such as contact lenses.