The graft copolymer poly(butoxymethylene norbornene-co-norbornenemethylene bromoisobutyrylate) [P(BN/NB)]-graft-poly(hydroxyethyl methacrylate) (PHEMA) was synthesized by the atom transfer radical polymerization of 2-hydroxyethyl methacrylate from a copolymer prepared by two functional norbornene monomers via a vinyl addition mechanism. The graft copolymer P(BN/NB)-g-PHEMA was further crosslinked with 4,5-imidazole dicarboxylic acid (IDA) and then doped with phosphoric acid (H(3)PO(4)) to form imidazole-H(3)PO-4 complexes. The results show that the polynorbornene backbone and crosslinked micromorphology produced low methanol permeability in the membranes from 1.5 x 10(-7) to 3.8 x 10(-6) cm(2)/s) and endowed the membranes with good mechanical properties (with elastic modulus values of 692.7 to 159.7 MPa, elongation at break values from 2.7 to 22.7%, and tensile strength at break values from 14.4 to 5.5 MPa) and excellent thermal stability (up to 280 degrees C). Furthermore, the proton conductivities of the membranes increased with increasing temperature and increasing content of IDA/H(3)PO(4) in the membranes. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci 121: 1166-1175, 2011