Method of preparing vinyl cyanide compound-conjugated diene rubber-aromatic vinyl compound graft copolymer, method of preparing thermoplastic resin composition including the same, and thermoplastic resin composition including the same

A method of preparing a vinyl cyanide compound-conjugated diene rubber-aromatic vinyl compound graft copolymer, comprising:

• step (a) comprising adding 0.1 to 2 parts by weight of an emulsifier, 0.1 to 0.4 parts by weight of an electrolyte, and a molecular weight regulator to 20 to 40 parts by weight of 100 parts by weight in total of a conjugated diene compound and performing polymerization;
• step (b) comprising adding, after step (a), the remaining conjugated diene compound and a molecular weight regulator and performing polymerization to obtain conjugated diene rubber latex;
• step (c) comprising preparing enlarged conjugated diene rubber latex by adding 0.3 to 1.1 parts by weight of an acid enlarging agent to 100 parts by weight of the conjugated diene rubber latex based on solids to enlarge the conjugated diene rubber latex;
• and
• step (d) comprising graft-polymerizing 40 to 70% by weight of the enlarged conjugated diene rubber latex based on solids, 15 to 45% by weight of an aromatic vinyl compound, and 5 to 25% by weight of a vinyl cyanide compound to obtain a vinyl cyanide compound-conjugated diene rubber-aromatic vinyl compound graft copolymer,
• wherein the enlarged conjugated diene rubber latex obtained in step (c) has an average particle diameter of 2,800 to 3,800 Å and a full width at half maximum of 1,400 to 2,700 Å in particle size distribution.