Percent cross-linking, effect

Most investigations of polymer-supported onium ion phase transfer catalysts have used cross-linked polystyrenes. Not all of them have the same structure, even when they have the same formal degree of cross-linking with divinylbenzene. (The effect of percent cross-linking is considered in a later section). Two principal methods have been used to functionalize polystyrene for phase transfer catalysts, chloromethylation of pre-formed beads and copolymerization of chloromethylstyrene monomer with styrene and divinylbenzene. The chloromethylation route employs chloromethyl methyl ether (a cancer suspect agent), and a Lewis acid, usually stannic chloride.Substitution proceeds >90% para and is accompanied by some intrapolymer alkylation, which puts additional cross-links into the polymer... 

Under these circumstances the probability that a given structural unit is cross-linked is not entirely independent of the status of other units in the same primary molecule. If an abnormally large fraction of some of the units of a given primary molecule are found to be cross-linked, the likelihood that it was formed toward the end of the polymerization process is enhanced hence the probability that one of its other units is cross-linked will be greater than the over-all p for the system. Calculations indicate that the magnitude of the non-randomness is not excessive below about 70 percent conversion. For most purposes its effect probably may be ignored without serious error, thus obviating a more elaborate theory which would take into account non-randomness of this nature. 

Comparisons of reactions of cis- versus fran -DDP with sulfur donors and other cellular components may afford some insight into their relative biological effects, however. /rans-DDP is much more reactive than the cis isomer with GSH (6, 27). Depletion of GSH with use of the inhibitor BSO had no effect on the cisplatin sensitivity of two human ovarian carcinoma cell lines, but made them 2.7 times more sensitive to trans-DY) (4). In a related finding, it was necessary to add 14 times more trans- than ds-DDP to cells in culture to achieve the same percent inhibition of in vivo SV40 DNA replication (23). These results suggest that part of the differential activity of these isomers in biological systems may arise from the differential reactivity of fran -DDP toward cellular components other than DNA. Thus trani-DDP could be more effectively inactivated prior to encountering DNA, or be better blocked as monofunctional platinum-DNA cross-links. 

Surfactant concentration (varied after polymerization) greatly affects the viscosity of associating polymer systems. Iliopoulos et al. studied the interactions between sodium dodecyl sulfate (SDS) and hydrophobically modified polyfsodium acrylate) with 1 or 3 mole percent of octadecyl side groups [85]. A viscosity maximum occurred at a surfactant concentration close to or lower than the critical micelle concentration (CMC). Viscosity increases of up to 5 orders of magnitude were observed. Glass et al. observed similar behavior with hydrophobically modified HEC polymers. [100] The low-shear viscosity of hydrophobically modified HEC showed a maximum at the CMC of sodium oleate. HEUR thickeners showed the same type of behavior with both anionic (SDS) and nonionic surfactants. At the critical micelle concentration, the micelles can effectively cross-link the associating polymer if more than one hydrophobe from different polymer chains is incorporated into a micelle. Above the CMC, the number of micelles per polymer-bound hydrophobe increases, and the micelles can no longer effectively cross-link the polymer. As a result, viscosity diminishes. 

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