A process for the preparation of a high molecular weight polymer by a condensation process of a liquid reaction mixture wherein at least one volatile by-product is eliminated, characterized in that said condensation process is carried out in apparatus of such form and so operated that said reaction mixture being subjected to the condensation process has no free surface and removal of volatile product or products from said reaction mixture is effected by diffusion through at least a proportion of the wall of said apparatus which is permeable to said volatile product or products but not permeable to said reaction mixture or said polymer, said proportion of the wall which is permeable serving to separate said reaction mixture from a chemically inert gaseous fluid in which the partial pressure of the volatile byproduct, or of the volatile by-products, is continually maintained below the equilibrium partial pressure for the reaction mixture under the conditions of reaction.