AbstractThe quality of polymer fractionation depends on the choice of the mixed solvent as well as on the particular conditions of operating the continuous countercurrent extraction. With a polyisobutylene (PIB) sample of medium molecular weightMw= 98,400 g/mol andU= (Mw/Mn) − 1 = 1.4 plus the mixed solvents toluene/2‐butanone (TOL/MEK) andn‐heptane/2‐butanone (HEP/MEK) (both giving comparably good fractionation in equilibrium experiments), possible ways to optimize the CPF were tested. The mixed solvent HEP/MEK turns out to be superior to TOL/MEK for kinetic reasons. Due to the larger gap between its density and that of the pure polymer, the coexisting phases can still move through the column, even if the working point of the CPF is chosen close to the consolute point of the ternary system. Under these conditions the mass transfer becomes more rapid, and up to 80 of the (mixed) solvent can be economized as compared with TOL/MEK. Four CPF runs with TOL/MEK (each dividing the polymer material into portions approximately equal in weight, and rejecting the first low molecular fraction) yielded four 200 g samples ofMwranging from 80,600 to 257,000 withUvalues of
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