A thermoplastic copolycarbonate resin having improved low temperature impact strength and a process for the preparation of the resin are disclosed. Accordingly the structure of the copolycarbonate contains(i) units of the formula ##STR1## wherein D is a divalent residue of a dihydroxy compound, and (ii) units of the formula ##STR2## wherein m is 1-90, n is 5-300, R is an alkyl or a phenyl radical p is 2-12 and q is 2-20. The preparation of the copolycarbonate entails reactive blending of polycarbonate with a suitable lactone siloxane copolymer in the presence of a transesterification catalyst.