Rubbery polymer, method of preparing the same, graft copolymer, and thermoplastic resin composition

A method of preparing a rubbery polymer, wherein a conjugated diene monomer is polymerized in the presence of a crosslinking agent, a molecular weight regulator, and an emulsifier,

• wherein the crosslinking agent is a compound represented by Formula 1 below and is added before polymerization initiation or when a polymerization conversion rate is 30% or less:
• [CH2═CR1CO2(R2O)nR3]mC(R4)l [Formula 1]
• (wherein R1 is hydrogen or an alkyl group selected from C1 to C3 alkyl groups, R2 is an alkylene group selected from C1 to C4 alkylene groups, R3 is an alkylene group selected independently from C1 to C3 alkylene groups, R4 is an alkyl group selected from C1 to C3 alkyl groups, 1 is an integer of 0 to 2, n is an integer of 4 to 15, m is an integer of 3 or 4, and the product of m and n is 12 to 60),
• the molecular weight regulator is added at a polymerization conversion rate of 40% to 75%, and
• the emulsifier is an emulsifier having a critical micelle concentration (CMC) of 150 mg/L or less and is added at a polymerization conversion rate of 50% to 85%.