Patent ID: 8143328

Claim:
A method for preparing a graft copolymer comprising: (a) preparing a seed by polymerizing a monomer mixture including 50 to 100 percent by weight of alkyl methacrylate, 0 to 50 percent by weight of one or more compounds selected from the group consisting of alkyl acrylate, an aromatic vinyl compound, and a conjugated diene compound, and 0 to 5 percent by weight of one or more crosslinking agents and a graft agent, based on the total weight of the seed, wherein the seed is 1 to 30 percent by weight based on the total weight of the graft copolymer; (b) preparing a rubber core surrounding the seed by polymerizing 60 to 95 percent by weight of alkyl acrylate, 5 to 40 percent by weight of one or more compounds selected from the group consisting of alkyl methacrylate, an aromatic vinyl compound, and a conjugated diene compound, and 0 to 5 percent by weight of one or more crosslinking agents and a graft agent based on the total weight of the rubber core, wherein the rubber core is 30 to 69 percent by weight based on the total weight of the graft copolymer; (c) preparing a pre-formed graft base polymer surrounding the rubber core by polymerizing a monomer mixture including 60 to 100 percent by weight of alkyl methacrylate, and 0 to 40 percent by weight of one or more compounds selected from the group consisting of an aromatic vinyl compound and an alkyl acrylate compound, based on the total weight of the pre-formed graft base polymer, to perform graft polymerization and adding acid, salt, or polymer coagulant when the monomer conversion of the polymerization is 70% to 95% while continuously performing the polymerization to have an increased average particle size of 100 to 500 nm, wherein the pre-formed graft base polymer is 1 to 35 percent by weight based on the total weight of the graft copolymer; and (d) preparing a graft base polymer surrounding the preformed graft base polymer by polymerizing a monomer mixture including 60 to 100 percent by weight of alkyl methacrylate, and 0 to 40 percent by weight of one or more compounds selected from the group consisting of an aromatic vinyl compound and an alkyl acrylate compound based on the total weight of the graft base polymer, wherein the graft base polymer is 1 to 35 percent by weight of the total weight of the graft copolymer.