Copoly(hyperbranched macromonomer)s, i.e., poly(phenylacetylene)s having many branches were synthesized by one-pot simultaneous copolymerization of two kinds of dimethylsilyl-phenylacetylenes. Membranes from the copoly(hyperbranched macromonomer)s having several kinds of branches showed higher permselectivities for oxygen and carbon dioxide over nitrogen than those of the corresponding copoly(hyperbranched macromonomer)s without any branches. In particular, the copoly(hyperbranched macromonomer)s having long and rigid branches had both the advantages of poly(phenylacetylene)s and poly(dendron)s. In summary, copoly(hyperbranched macromonomer)s showing good performance as gas-separation membranes were synthesized very easily by the one-pot simultaneous polymerization method. To our knowledge, this is the first synthesis of such complex copolymers showing good performance as gas-separation membranes.