Method of preparing graft polymer

A method of preparing a graft polymer, comprising:

• adding a diene-based rubber polymer, a first monomer mixture including a (meth)acrylate-based monomer and an aromatic vinyl-based monomer, and a reactive ultraviolet (UV) stabilizer to a reactor and carrying out polymerization to prepare a composite rubber polymer;
• and
• graft-polymerizing a second monomer mixture including a (meth)acrylate-based monomer, an aromatic vinyl-based monomer, and a vinyl cyanide-based monomer to the composite rubber polymer to prepare the graft polymer,
• wherein the first monomer mixture includes the (meth)acrylate-based monomer and the aromatic vinyl-based monomer in a weight ratio of 70:30 to 50:50, and
• wherein, based on 100 parts by weight of the sum of the diene-based rubber polymer, the first monomer mixture, and the second monomer mixture:
• the reactive ultraviolet (UV) stabilizer is added in an amount of 0.1 to 2 parts by weight, the diene-based rubber polymer is added in an amount of 15 to 25 parts by weight, and the second monomer mixture is added in an amount of 35 to 45 parts by weight.