A pore plugging material, for pH dependent membrane diffusion, in which cyclic olefins having phosphazene-functional moieties provide predictable erosion properties when used to plug pores is separation barriers and other porous membranes. Specific properties of the polymers are dependent on several factors, including molecular weight and identity of side groups attached to the phosphazene moiety. However, as a class, phosphazene-functional cyclic olefins provide both predictable erodibility and uniformly benign hydrolysis products and are, therefore, uniquely suitable as pore plugging polymers for separation barriers and membranes of all kinds. The invention, therefore, embraces the provision of a pH-sensitive erodible pore plugging material to pores in separation barriers and membranes of all kinds.