Starting from solutions of unsubstituted cellulose (Avicel PH101, Mw = 30.1 kg/mol and Mw/Mn = 3 or Solucell 500, Mw = 230 kg/mol, Mw/Mn = 2.8) in either Ni‐tren (0.8 M aqueous solution of the dihydroxotris(2‐aminoethly)amine nickel(II) complex) or in a mixed solvent DMAc+LiCl (consisting of N,N‐dimethylacetamide plus lithium chloride) it was investigated whether the segregation of a second phase caused by the addition of suitable precipitants leads to polymer fractionation. With Ni‐tren the long chains accumulate in the precipitate formed upon the addition of sulfuric acid; as the pH falls below ≈ 9, the solution is free of cellulose. Nevertheless this route option for fractionation must be ruled out because of a pronounced chain scission taking place in that solvent. For (DMAc + LiCl) the best low molecular weight precipitant that could be found was acetone; it allows the fractionation of the lower molecular weight Avicel but fails for Solucell as a result of the high viscosities of its solutions in the presence of acetone. It was therefore investigated whether that deficiency could be overcome by the use of high molecular precipitants. In these experiments poly‐(methyl methacrylate), incompatible with cellulose, was used to cause phase separation. The results demonstrate the suitability of this system for discontinuous experiments even in the case of the higher molecular weight Solucell.
Summary: Access to sufficiently large amounts of material with adequate molecular and chemical uniformity from polydisperse natural products or synthetic materials has been a long‐standing challenge to polymer scientists. We have developed a broadly applicable preparative fractionation method consisting of a special kind of continuous extraction removing the easier soluble components from the initial product. It is rendered possible by the use of spinning nozzles through which a concentrated polymer solution is pressed into a liquid of tailored thermodynamic quality. The initially produced jets of the source phase disintegrate rapidly into minute droplets of typically 50 μm diameter. This efficient subdivision provides the large surfaces and short routs of transport required for successful fractionation. Thus the pronounced kinetic hindrances resulting from the high viscosities of reasonable concentrated polymer solutions can be overcome. We portray the principal features of continuous spin fractionation and present two examples of practical importance.
The possibilities to fractionate copolymers with respect to their chemical composition on a preparative scale by means of the establishment of liquid/liquid phase equilibria were studied for random copolymers of styrene and acrylonitrile (SAN). Experiments with solutions of SAN in toluene have shown that fractionation does in this quasibinary system, where demixing results from marginal solvent quality, take place with respect to the chain length of the polymer only. On the other hand, if phase separation is induced by a second, chemically different polymer one can find conditions under which fractionation with respect to composition becomes dominant. This opportunity is documented for the quasi-ternary system DMAc/SAN/polystyrene, where the solvent dimethyl acetamide is completely miscible with both polymers. The theoretical reasons for the different fractionation mechanisms are discussed.
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