A thermoplastic polyester-carbonate resin containing in the polymer chain, divalent moieties of the formula: ##STR1## wherein R represents aliphatic hydrocarbyl of from 1 to 9 carbon atoms, inclusive, halogen substituted aliphatic hydrocarbyl of 1 to 9 carbon atoms, inclusive, aralkyl of 7 to 10 carbon atoms, inclusive and halogen substituted aralkyl of 7 to 10 carbon atoms, inclusive; a and b are each whole number integers of from 0 to 1; and the sum of a+b is 1 and wherein there are sufficient linkages of formula I to lower the glass transition temperature of the polymer.