Solid-state grafting of glycidyl methacrylate (GMA) onto medium density polyethylene (MDPE) was performed using supercritical carbon dioxide (SCCO2). The polymer powders were impregnated in the SCCO2 phase with the GMA using benzoyl peroxide (BPO) as thermal initiator. The grafting degree of the samples may be controlled by the following variables: concentration BPO, time, temperature, and pressure. A central composite design to evaluate the main and interaction effects of such factors on the grafting of the MDPE by GMA (grafting response) was elaborated from FTIR data. Better grafting response was obtained for concentration of temperature of 119 degrees C, pressure of 170 bar, time of 150 min, and concentration BPO of 0.43 phr. After grafting experiments, purification of the grafted samples by SCCO2 in optimum condition of grafting for removing of residual monomer and initiator was performed. For achieving maximum purification yield using Box-Behnken method the temperature, pressure, and time were adjusted 53 degrees C, 209 bar, and 74 min, respectively. (C) 2012 Elsevier B.V. All rights reserved.