A poly(ester-carbonate) copolymer, comprising carbonate units of the formula and ester units of the formula wherein: T is a C 2-20 alkylene, a C 6-20 cycloalkylene, or a C 6-20 arylene; and R 1 and J are each independently (a) a bisphenol A divalent group of the formula and (b) a phthalimidine divalent group of the formula wherein R a and R b are each independently a C 1-12 alkyl, C 2-12 alkenyl, C 3-8 cycloalkyl, or C 1-12 alkoxy, each R 3 is independently a C 1-6 alkyl, R 4 is hydrogen, C 1-6 alkyl, or phenyl optionally substituted with 1 to 5 C 1-6 alkyl groups, preferably wherein R 4 is hydrogen, methyl, or phenyl, p, q, and j are each independently 0 to 4, provided that the phthalimidine group (b) is present in an amount of 40 mol% to 50 mol% based on the total moles of the bisphenol A divalent groups (a) and the phthalimidine groups (b); the ester units are present in an amount of 40 mol% to 60 mol% based on the sum of the moles of the carbonate units and the ester units; and wherein the poly(ester-carbonate) copolymer has a weight average molecular weight of 18,000 Daltons to 24,000 Daltons, as measured by gel permeation chromatography, using a crosslinked styrene-divinylbenzene column and calibrated to bisphenol A homopolycarbonate references; and a molded sample of the composition has a glass transition temperature of 210 °C to 235 °C as determined by differential scanning calorimetry as per ASTM D3418 with a 20 °C/min heating rate; and a melt viscosity of less than 1050 Pa·s at 644 sec-1 and 350°C, determined according to ISO11443. T is a C 2-20 alkylene, a C 6-20 cycloalkylene, or a C 6-20 arylene; and R 1 and J are each independently (a) a bisphenol A divalent group of the formula and (b) a phthalimidine divalent group of the formula wherein R a and R b are each independently a C 1-12 alkyl, C 2-12 alkenyl, C 3-8 cycloalkyl, or C 1-12 alkoxy, each R 3 is independently a C 1-6 alkyl, R 4 is hydrogen, C 1-6 alkyl, or phenyl optionally substituted with 1 to 5 C 1-6 alkyl groups, preferably wherein R 4 is hydrogen, methyl, or phenyl, p, q, and j are each independently 0 to 4, (a) a bisphenol A divalent group of the formula and (b) a phthalimidine divalent group of the formula wherein R a and R b are each independently a C 1-12 alkyl, C 2-12 alkenyl, C 3-8 cycloalkyl, or C 1-12 alkoxy, each R 3 is independently a C 1-6 alkyl, R 4 is hydrogen, C 1-6 alkyl, or phenyl optionally substituted with 1 to 5 C 1-6 alkyl groups, preferably wherein R 4 is hydrogen, methyl, or phenyl, p, q, and j are each independently 0 to 4, R a and R b are each independently a C 1-12 alkyl, C 2-12 alkenyl, C 3-8 cycloalkyl, or C 1-12 alkoxy, each R 3 is independently a C 1-6 alkyl, R 4 is hydrogen, C 1-6 alkyl, or phenyl optionally substituted with 1 to 5 C 1-6 alkyl groups, preferably wherein R 4 is hydrogen, methyl, or phenyl, p, q, and j are each independently 0 to 4, provided that the phthalimidine group (b) is present in an amount of 40 mol% to 50 mol% based on the total moles of the bisphenol A divalent groups (a) and the phthalimidine groups (b); the ester units are present in an amount of 40 mol% to 60 mol% based on the sum of the moles of the carbonate units and the ester units; and the phthalimidine group (b) is present in an amount of 40 mol% to 50 mol% based on the total moles of the bisphenol A divalent groups (a) and the phthalimidine groups (b); the ester units are present in an amount of 40 mol% to 60 mol% based on the sum of the moles of the carbonate units and the ester units; and the poly(ester-carbonate) copolymer has a weight average molecular weight of 18,000 Daltons to 24,000 Daltons, as measured by gel permeation chromatography, using a crosslinked styrene-divinylbenzene column and calibrated to bisphenol A homopolycarbonate references; and a molded sample of the composition has a glass transition temperature of 210 °C to 235 °C as determined by differential scanning calorimetry as per ASTM D3418 with a 20 °C/min heating rate; and a melt viscosity of less than 1050 Pa·s at 644 sec-1 and 350°C, determined according to ISO11443. a glass transition temperature of 210 °C to 235 °C as determined by differential scanning calorimetry as per ASTM D3418 with a 20 °C/min heating rate; and a melt viscosity of less than 1050 Pa·s at 644 sec-1 and 350°C, determined according to ISO11443.