Modified polymer, method of preparing the same, and rubber composition including the modified polymer

A modified polymer comprising a unit derived from a substituted styrene-based compound represented by Formula 1 and a unit derived from a modifier represented by Formula 2:

• wherein, in Formula 1,
• R is a hydrogen atom or a monovalent hydrocarbon group having 1 to 20 carbon atoms, and
• m is an integer of 1 to 11, and
• in Formula 2,
• R1 and R2 are each independently a monovalent hydrocarbon group having 1 to 20 carbon atoms, or a monovalent hydrocarbon group having 1 to 20 carbon atoms which includes at least one heteroatom selected from the group consisting of nitrogen (N), sulfur (S), and oxygen (O),
• R3 is a divalent hydrocarbon group having 1 to 20 carbon atoms which is substituted or unsubstituted with at least one substituent selected from the group consisting of an alkyl group having 1 to 20 carbon atoms, a cycloalkyl group having 3 to 20 carbon atoms, and an aryl group having 6 to 30 carbon atoms,
• R4 and R5 are each independently a monovalent hydrocarbon group having 1 to 20 carbon atoms, and
• n is an integer of 1 to 3.