Polymethacrylate with semiconducting side chains (P1), synthesized by free radical polymerization, was used as a donor material for polymer solar cells. P1 is of high molecular weight (Mn = 82 kg mol−1), good thermal stability, narrow band gap (1.87 eV), and low-lying HOMO energy level (−5.24 eV). P1 possesses not only the good film-forming ability of polymers but also the high purity of small organic molecules. Power conversion efficiencies (PCEs) of 0.63% and 1.22% have been obtained for solar cells with M1:PC71BM and P1:PC71BM as the active layers, respectively. With PC61BM as the acceptor, PCEs of M1 and P1 based devices decrease to 0.61% and 0.76%, respectively. To the best of our knowledge, this is the first report that free radical polymerization can be used to prepare polymer donors for photovoltaic applications.