Functionalized copolymers of isoolefins and conjugated diolefins, methods of preparing the copolymers, and their use as compatibilizers are disclosed. The diolefin monomer units of the co-polymer are modified at the C—C double bond along the backbone of the copolymer to include an oxygen containing functional group such as epoxide, ester or alcohol. The functionalized copolymers improve the wettability of a non-hydrophilic surface towards hydrophilic polymer and allows for the formation of homogenous layers of the hydrophilic polymers. In particular, the spreading of a hydrophilic polymer on a non-hydrophilic substrate is facilitated by applying the co-polymers as an interfacial layer between the two incompatible materials. The resulting coated substrates exhibit resistance to protein adsorption and cell growth after grafting. The co-polymers are especially suited in the coating of biomedical devices where a high degree of uniformity of the coated surface is required.