All the chemical reagents were used as received without further purification except for BPO and styrene. BPO was purified with recrystallization method by using chloroform as dissolvent and methanol as precipitant. In order to remove the inhibitor in the monomer, styrene was placed in a separating funnel, and then washed by 5% aqueous sodium hydroxide. After being washed three times, styrene was dried by anhydrous Na2SO4 for 3 h. Purified BPO and styrene were kept in different brown reagent bottles.