A solution process for polymerizing one or more .alpha.-olefins of the formula CH.sub.2.dbd.CHR, where R is H or an alkyl radical C.sub.1-C.sub.18, to produce a polymer that is soluble in the reaction medium, comprising the steps of: continuously polymerizing in a liquid phase the .alpha.-olefin in the presence of a catalyst system based on a transition metal compound to obtain a solution of polymer in the reaction medium; the polymeric solution obtained from step a) is then mixed in one or more mixing stages with an aqueous mixture comprising one or more organic compounds having at least a hydroxy or epoxy group, said aqueous mixture having a dynamic viscosity at 30.degree. C. higher than 50 cP (centiPoise).