Fractionation molecular distributions from

For preparative purposes batch fractionation is often employed. Although fractional crystallization may be included in a list of batch fractionation methods, we shall consider only those methods based on the phase separation of polymer solutions fractional precipitation and coacervate extraction. The general principles for these methods were presented in the last section. In this section we shall develop these ideas more fully with the objective of obtaining a more narrow distribution of molecular weights from a polydisperse system. Note that the final product of fractionation still contains a distribution of chain lengths however, the ratio M /M is smaller than for the unfractionated sample. 

The GPC traces in Fig. 24 reveal a broad molecular weight distribution, MJMn = 4.42, for the dual reactor blend sample. On the other hand, the diblock OBC displays an overall MJMn of 1.67. The narrowing of the distribution indicates that the polymerization has CCTP characteristics. The theoretical molecular weight distribution from an ideal living polymerization in a series of two CSTR reactors is given by the following equation, where/j and/2 are the mass fractions of polymer comprising the two blocks [11] ... 

Table I. Yields and Molecular Weights (Relative to Polystyrene) for Both Fractions of the Bimodal Distribution from Gel Permeation Chromatography in THF of Organosilane Polymers.

Figure 9.8 Double logarithmic plot of the retardation spectrum versus the retardation time for a polystyrene fraction of molecular weight 3400 with a narrow molecular weight distribution. (From Ref. 8.)...

To study the molecular distribution of water-soluble and MeOH-soluble portions, gel permeation chromatography (GPC) (Shimadzu LC-IOA) which consisted of a high pressure pump (Shimadzu Co, Model LC-IOAT) was performed with a column of KF-803L, operated at 401 with flow rate of 1.0 mL/min of tetrahydrofuran, The molecular weight of the fractions was determined from calibration curve obtained with polystyrene as a standard. 

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