A molecular brush with block copolymer side chains, i.e., a crystalline poly(epsilon-caprolactone) (PCL) core with an amorphous poly(n- butyl acrylate) (PBA) shell, was synthesized by the "grafting from" approach using a combination of atom transfer radical polymerization (ATRP) and ring-opening polymerization (ROP). The brush was prepared in the following steps. First, a well-defined poly(2-hydroxyethyl methacrylate) (PHEMA) was synthesized by ATRP in a methanol/anisole cosolvent. In the second step, PCL was grafted from the PHEMA macroinitiators by ring-opening polymerization (ROP) in the presence of tin(II) 2-ethylhexanoate (Sn(EH) 2) catalyst, and then the pendent hydroxyl groups in the resulting PCL brush were transformed to bromopropionyl ATRP initiating groups. Finally, the PBA chains were grafted from the PCL brush macroinitiators by ATRP. Gel permeation chromatography (GPC) analysis provided evidence for formation of a well-defined block copolymer side chain brush. The melting point and the heat of fusion of the homo-PCL brush and the PCL-PBA block copolymer side chain brush were measured by differential scanning calorimetry (DSC). According to the DSC results, the melting point of the PCL segments in the PCL-PBA block copolymer side chain brush was depressed when compared with a homo- PCL brush. This was due to the PCL crystalline domain being disturbed by the presence of the second PBA block. The crystalline core-amorphous shell structure of the brush was directly visualized by atomic force microscopy (AFM). On the basis of the AFM results, the polymer molecules obtained from the combined methods of ATRP and ROP were indeed well-defined brush macromolecules, indicating successful polymerization at each step.