times with 50 ml of 1n hcl and three times with distilled water and precipitated into methanol the structure was conforms to str9 was verified by ir specifically to the carbonyl band at 1725 cm 1 the molecular weight m n of the product was 26 000 the epsu of this example contains about 20 mol ester groups preparation of block copolymer in a haake mixer the ester containing polyethersulfone prepared in accordance with example 3 and thermoplastic polyester pet which is sold under the trademark ektar by eastman kodak at a weight ratio between them of about 53 47 were placed in a haake mixer equipped with kneading elements the polymers were melt blended at 340 u00b0 c for 40 min the product thus prepared was then used for the preparation of test specimens by hot pressing at 340 u00b0 c for 2 minutes under a pressure of 2 mpa films of the as kneaded sample of block copolymers were viewed by transmission electron microscopy ruo 4 stained a comparison was conducted by blending corresponding amounts of pet with psu which contained no ester groups fig1 is an electron micrograph magnification of 7500 1 of the structure of a blend of pet and psu fig2 is an electron micrograph magnification of 7500 1 of the structure of a reactive blend of pet and epsu these micrographs demonstrate the advantage represented by the reactive blending of the present invention over the mere blending of closely corresponding components the former is much more homogeneous than the later the table below summarizes the results of the evaluation of a blend of pet psu set in comparison to the block copolymer of the invention.