Branched Polystyrene

Figure 9.17 Plot of log [i ]M versus retention volume for various polymers, showing how different systems are represented by a single calibration curve when data are represented in this manner. The polymers used include linear and branched polystyrene, poly(methyl methacrylate), poly(vinyl chloride), poly(phenyl siloxane), polybutadiene, and branched, block, and graft copolymers of styrene and methyl methacrylate. [From Z. Grubisec, P. Rempp, and H. Benoit, Polym. Lett. 5 753 (1967), used with permission of Wiley.]...

Beaucage [83] showed that it could be possible to get branching information for polymers using this approach. In Figure 14, where neutron scattering data for branched polystyrene is fit to the unified equation [83,107,110-112], it was shown that it is possible to calculate the parameters dmin and c, from such a fit [83]. These model branched polystyrene samples were synthesized by using divinyl benzene (10%) as a comonomer, to obtain controlled levels of branching but where the placement is random. 

Figure 14 Neutron scattering data from branched polystyrene fit to the unified equation [83]. Reprinted with permission from Beaucage [83]. Copyright 2004, American Physical Society. (Both the branched PS fractions were synthesized using divinyl benzene as a comonomer. Fraction F2 has a weight average molar mass of 18 x 108 g/mol and fraction F5 has a weight average molar mass of 2.0 x 108 g/mol, see Ref. 147.)...

The polymer types shown are linear polystyrene, two types of branched polystyrene (methyl methacrylate), poly (vinyl chloride), poly butadiene poly (phenyl soiloxane), and two types of copolymer. 

The new branched polystyrene chain may eventually terminate, crosslinked by reacting with another polystyrene chain or an immobilised divinyl benzene radical. Grafting is thus enhanced mainly... 

Application to Randomly End Linked Star-Branched Polystyrenes. 169... 

Fig. 22. KMHS relationships for the fractions of end-linked 3-arm star-branched polystyrene molecules and of linear polystyrene fractions. The data refer to three samples of different in the pregel state and one from the sol fraction of a gel. The curves for the branched macromolecules coincide within experimental error in the high molar mass region. The deviations at lowM result from a different amount of non-reacted end-functionalized stars. The exponents of the end-linked and linear PS chains are a =0.42 0.02 while that of linear chains is 0.70 0.01 [95,120,123,124]. Reprinted with permission from [95]. Copyright [1997] American Society...

Like SEC/LALLS, the viscosity detector is sensitive to high molecular weight fractions as shown in Fig, 7. A shoulder at 3,000,000 molecular weight detected by the DRI becomes a peak when detected by the viscometer detector. The usefulness of the SEC/Viscometer method is exemplified by the study of branched polymers. Fig. 8 shows a log [ ] vs. log M plot for a randomly branched polystyrene obtained from the SEC/ iscometer technique. 

The randomly branched polystyrene and two star-shaped polystyrenes were obtained from the Polymer Science Department at the University of Akron, Akron, OH. 

Randomly Branched Polystyrene. Branched molecules in solution are more compact than linear molecules and therefore the overall size of a branched polymer molecule in solution is smaller than the... 

Figure 16 shows the viscometer and DRI traces of another star-branched polystyrene. This sample contained about 12% of the starting linear arm precursor which eluted at retention volume ca. 52 ml. The kinetic molecular weight of the linear precursor was 260,000. The results obtained for the individual peak through the SEC/Viscosity methodology are summarized in Table 7. It is seen that the measured of the linear arm is very closed to the kinetic value. The average functionality of this star polymer is calculated to be f = 10. 

Figure 15. Plot of Branching Index as a Function of Molecular Weight for a Randomly Branched Polystyrene.

In 1965, Milkovich (. ) reported that divinylbenzene could be utilized for the formation of star-branched macromolecules. Later, Rempp and coworkers (2, 3, 4) successfully applied this method for the synthesis of star-branched polystyrenes. Moreover, Fetters and coworkers (54 ) used this procedure for the synthesis of multi-arm star-branched polyisoprene homopolymers and poly-... 

Another approach to the TLC separation of linear and branched polystyrenes was reported by Belenkii etalThe adsorbent used was silica gel with a mean pore diameter of 80 A (notice that no molecular sieving mechanism should be operative in this pore diameter), and the developer was composed of cyclohexane, benzene, and acetone in a composition of 12 4 y, where 7 is the relative amount of acetone added. The TLC experiments were performed for different 7 values ranging from 0.4 to 1.5. With increase in the 7 value, the Rf values increased both for the linear and branched samples but the dependence for the linear samples was remarkably stronger than for the branched ones. 

When a tetra-chain, star-shaped polystyrene is prepared by a coupling reaction between living polymer and coupler (e.g 1,2,4,5-tetrachloromethyl benzene), the reaction is often carried out with the polystyryl anion in slight excess in order to avoid by-production of types of branched polystyrene other than the tetra-chain. 

Experimental data on branched polystyrenes were used to test the theory. There are some discrepancies in particular the parameter A varies from one polymer to another, especially for comb-shaped polymers y> also varies. It is suggested that the reason for the discrepancies in A may be the failure of the assumption of a Gaussian distribution of segment density. 

If A is small, so that < 0Ai < 0, a will exceed unity at T= 0Ai, which is qualitatively consistent with results obtained on model branched polystyrenes (Section 7). 

Pannell (38) has correlated the intrinsic viscosities of comb-branched polystyrenes with calculated rather than measured values of . He finds that... 

Pannell has also studied the behaviour of branched polystyrenes in GPC (45). He found that the product now widely accepted as a parameter charac-... 

It is now generally accepted that the GPC retention volume is a function of the product M tf, independent of the nature or structure of the polymer 46, 47) though Pannell 45) found that it failed to correlate the elution behaviour of his highly branched polystyrenes, it may be accepted that M rf will be determinable from GPC retention volumes for moderately branched polymers. To estimate branching, it is necessary to separate this product so that M and [rf are both known and the relation between them can then be used, subject to the uncertainties mentioned in Subsection 9.2.2, for this purpose. It is usual to measure rf rather than M in order to make the separation, as it is easier. The combination of GPC and intrinsic viscosity measurements is now the most usual method for studying long branching. 

Since a branched polymer has more than two chain ends, the determination of both the thickness of the adsorbed polymer and the adsorbance is of particular interest. Kawaguchi and Takahashi106) investigated well-characterized comb-branched polystyrene adsorbed on a chromium plate from cyclohexane solution at the theta point by ellip-sometry and compared the results with those for the corresponding linear polystyrene. 

Galin, M. Rupprecht, M. C., "Study by Gas-Liquid Chromatography of the Interactions between Linear and Branched Polystyrenes and Solvents in the Temperature Range 60-200C," Polymer, 19, 506 (1978). 

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