Polymeric materials find industrial applications that are comparable to those of metals and ceramics.~1 In addition to the great variability via the synthesis of various monomers and the choice of the degree of polymerization (N). alloying of polymers finds increasing attention for combining favorable materials properties.(~1.2) But polymeric (binary) alloys (A,B) of lexible polymers with chain lengths N_A, N_B are also most interesting for testing theoretical concepts: changing N_A, N_B one controls the entropy of mixing, keeping intermolecular forces invariant. Variation of these control parameters thus allows stringent tests of the theories on miscibility, unmixing etc. Furthermore, the large size of the random-walk-like polymer coils (gyration radius R_(gyr) approxapprox asq rootN/6 when a is the size of an effective statisstical segment~3) yields several simplfying features: (i) R_(gyr) enters as a prefactor in the correlation length zeta of concentration fluctuations, the width w of interfaces between coexisting phases near the critical point, the wavelength gamma_(max) of maximum growth during spinodal decomposition in unstable mixtures,~(4,5) etc.
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