A poly(amide-imide) copolymer that is a reaction product of a diamine represented by Chemical Formula 1, a diamine represented by Chemical Formula 2, a dicarbonyl compound represented by Chemical Formula 3, and a tetracarboxylic acid dianhydride represented by Chemical Formula 4: wherein in Chemical Formula 1, R 1 and R 2 are each independently an halogen atom, L 1 is a substituted or unsubstituted C1 to C20 alkylene group, a substituted or unsubstituted C3 to C30 cycloalkylene group, -O-, -S-, -C(=O)-, -CH -, -S(=O) 2 -,-Si(CH 3) 2 -, -(CF 2) q - wherein, 1≤q≤10, -C(CH 3) 2 -, -C(CF 3) 2 -, -C(=O)NH-, or a combination thereof, a and b are each independently an integer ranging from 0 to 2, provided that 1≤a+b≤4, c and d are each independently an integer ranging from 0 to 2; Chemical Formula 2 NH 2 -A-NH 2 wherein in Chemical Formula 2, A is a ring system comprising two or more C6 to C30 aromatic rings linked by a single bond, wherein each of the two or more aromatic rings is independently unsubstituted or substituted by an electron-withdrawing group; wherein, in Chemical Formula 3, R 3 is a substituted or unsubstituted phenylene or biphenylene group, and each X is an identical or different halogen atom; wherein, in Chemical Formula 4, R 10 is a single bond, -O-, -S-, -C(=O)-, -CH -, -C(=O)NH-, -S(=O) 2 -,-Si(CH 3) 2 -, -(CH 2) p -, -(CF 2) q -, -C(C n H 2n+1) 2 -, -C(C n F 2n+1) 2 -, -(CH 2) p C(C n H 2n+1) 2 (CH 2) q -, or-(CH 2) p C(C n F 2n+1) 2 (CH 2) q - wherein 1≤n≤10, 1≤p≤10, and 1≤q≤10, R 12 and R 13 are each independently a halogen, a hydroxy group, a substituted or unsubstituted C1 to C10 aliphatic organic group, a substituted or unsubstituted C6 to C20 aromatic organic group, an alkoxy group of formula -OR 201, wherein R 201 is a C1 to C10 aliphatic organic group, or a silyl group of formula -SiR 210 R 211 R 212, wherein R 210, R 211, and R 212 are each independently hydrogen or a C1 to C10 aliphatic organic group, and n7 and n8 are each independently an integer ranging from 0 to 3. R 1 and R 2 are each independently an halogen atom, L 1 is a substituted or unsubstituted C1 to C20 alkylene group, a substituted or unsubstituted C3 to C30 cycloalkylene group, -O-, -S-, -C(=O)-, -CH -, -S(=O) 2 -,-Si(CH 3) 2 -, -(CF 2) q - wherein, 1≤q≤10, -C(CH 3) 2 -, -C(CF 3) 2 -, -C(=O)NH-, or a combination thereof, a and b are each independently an integer ranging from 0 to 2, provided that 1≤a+b≤4, c and d are each independently an integer ranging from 0 to 2; R 3 is a substituted or unsubstituted phenylene or biphenylene group, and each X is an identical or different halogen atom; R 10 is a single bond, -O-, -S-, -C(=O)-, -CH -, -C(=O)NH-, -S(=O) 2 -,-Si(CH 3) 2 -, -(CH 2) p -, -(CF 2) q -, -C(C n H 2n+1) 2 -, -C(C n F 2n+1) 2 -, -(CH 2) p C(C n H 2n+1) 2 (CH 2) q -, or-(CH 2) p C(C n F 2n+1) 2 (CH 2) q - wherein 1≤n≤10, 1≤p≤10, and 1≤q≤10, R 12 and R 13 are each independently a halogen, a hydroxy group, a substituted or unsubstituted C1 to C10 aliphatic organic group, a substituted or unsubstituted C6 to C20 aromatic organic group, an alkoxy group of formula -OR 201, wherein R 201 is a C1 to C10 aliphatic organic group, or a silyl group of formula -SiR 210 R 211 R 212, wherein R 210, R 211, and R 212 are each independently hydrogen or a C1 to C10 aliphatic organic group, and n7 and n8 are each independently an integer ranging from 0 to 3.