Process for preparation of copolymers from polyesters and aliphatic polyformals

A process for preparing a copolymer having an inherent viscosity of at least 0.4 measured at 25.degree.C. using 0.50 gram of polymer per 100 ml. of a solvent composed of 60 volumes of phenol and 40 volumes of tetrachloroethane comprising contacting within a temperature range of about 220 to about 280.degree.C.

• A. a polyester comprised of
• 1. a dicarboxylic acid which is at least 80 mole percent terephthalic acid, and
• 2. a diol which is at least 80 mole percent 1,4-butanediol, and
• B. based on the weight of the copolymer, from 10 to 60 weight percent of a polyformal having an inherent viscosity of at least 0.05 measured at 25.degree.C. using 0.50 gram of polymer per 100 ml. of a solvent composed of 60 volumes of phenol and 40 volumes of tetrachloroethane corresponding to the structure
• H--O--R--O--CH.sub.2 --O--R--OH
• wherein R is ##SPC5##
• a linear aliphatic divalent radical having 2 to 10 carbon atoms, or
• --R'--O--.sub.n R'--
• where R' is a linear aliphatic divalent radical having 2 to 4 carbon atoms and n is an integer from 1 to 10.