A new diblock copolymer of isoprene (I) and methacrylic acid (MAA) was prepared by combination of an anionic mechanism with a charge transfer complex mechanism. In the first step, the polyisoprene (PI) macroanion formed by initiation with butyllithium was capped by p-dimethylaminobenzaldehyde (capped polyisoprene = PId); a dimeric coupling product was not detected. Then the binary system constituting of PId and benzophenone was used to initiate the polymerization of MAA under UV irradiation. The resulting diblock copolymer (PI-b-PMAA) was characterized by IR, NMR and gel permeation chromatography (GPC) in detail.