there is disclosed a process for polymerizing olefinic monomers , such as vinyl halides , vinylidene halides and vinylidene monomers having at least one terminal ch 2 = c & lt ; grouping , and mixtures thereof whereby polymer build - up on the inner surfaces of the reactor is substantially eliminated . the process is carried out in a reaction vessel the inner surfaces of which are coated with a coating composition containing , as a primary ingredient , polyethyleneimine and wherein the polymerization medium in contact with said coated surfaces contains a water - soluble divalent tin salt as a water - phase polymerization inhibitor . the coating and inhibitor produce a synergistic effect .