Polyisoprene molecular weight variation

Molecular Weight Variation. Five jamples of carboxylato-polyisoprene 100% neutralized with Mg (PIP-Mg), of Mn ranging from 6,000 to 37,000 and low polydispersity, were investigated. ... 

Fig. 10. The variation of 6, with E for three different molecular weight polyisoprene tethered chains in a cross-linked polyisoprene lens [23].

Use of Ziegler-Natta catalysts, as seen from Table 5.8, can yield an almost all-cw-l,4-polyiso-prene or an almost all-franj- 1,4-polyisoprene. The microstructure depends upon the ratio of titanium to aluminum. Ratios of Ti A1 between 0.5 1 and 1.5 1 yield the cis isomer. A1 1 ratio is the optimum. At the same time, ratios of fi Al between 1.5 1 to 3 1 yield the trans structures. The titanium-to-aluminum ratios affect the yields of the polymers as well as the microstructures. There also is some influence on the molecular weight of the product. Variations in catalyst compositions, however, do not affect the relative amounts of 1,4 to 3,4 or to 1,2 placements. Only cis and trans arrangements are affected. In addition, the molecular weights of the polymers and the microstructures are relatively insensitive to the catalyst concentrations. The temperatures of the reactions, however, do affect the rates, the molecular weights, and the microstructures. 

Reported data on g and g for r lar star polystyrenes and polyisoprenes are collected in Table 2. In examination of the results it ould be noted that substantial variation exists in reference parameters for linear polymers. For example the used for normal polystyrene ranges from 7.45 x 10" to 8.4 x 10. Much of this variation probably reflects differences in calibration standards in the determination of light scattering molecular weights. In the cases where no data are reported on linear polymers it must be assumed that the value chosen reflects the laboratories previous... 

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