Cross-linked polymers isomers

High selectivity can be obtained by imprinting polymers with neutral molecules.80 In this process, a cross-linked polymer is prepared in the presence of a template. Then the template is removed by solvent extraction. The extracted polymer is then used to pick up the template molecules from other sources. Among the examples in the literature are some that deal with atrazine (an herbicide), cholesterol, other sterols, dipeptides, TV-acetyltryptophane resolution (L-isomer favored by a factor of 6), adenine, and barbiturates.81 The polymerizations in the first two examples, are shown in (7.16) (The cross-linking comonomer with the cholesterol-containing monomer was ethylenebis-methacrylate. The cholesterol was cleaved from the polymer with sodium hydroxide in methanol.)... 

The curing mechanism is rather complex [337-339]. From 200 to 300 °C, isomerization of the as-prepared endo norbornenyl isomer takes place, then a retro Diels-Alder process occurs with evolution of cyclopentadiene, and then polymerization and copolymerization of a number of unsaturated species (cyclopentadiene, maleimide, nadimide and nadimide-cyclopentadiene adducts) proceeds, to give a final material of highly complex composition. The possible structures present in the cross-linked polymer are shown in Scheme (66). 

For all three diallyl phthalate isomers, gelation occurs at nearly the same conversion DAP prepolymer contains fewer reactive allyl groups than the other isomeric prepolymers (36). More double bonds are lost by cyclisation in DAP polymerisation, but this does not affect gelation. The heat-distortion temperature of cross-linked DAP polymer is influenced by the initiator chosen and its concentration (37). Heat resistance is increased by electron beam irradiation. 

The yield of cross-linking depends on the microstructure of polybutadiene and purity of the polymer as well as on whether it is irradiated in air or in vacuum. The cross-link yield, G(X), has been calculated to be lowest for trans and highest for vinyl isomer [339]. The introduction of styrene into the butadiene chain leads to a greater reduction in the yield of cross-linking, than the physical blends of polybutadiene and polystyrene [340]. This is due to the intra- and probably also intermolecular energy transfer from the butadiene to the styrene constituent and to the radiation stability of the latter unit. 

The reaction of dienophiles with polyacetylene (33) was investigated for limiting the decomposition of the polyene. Previously, intermolecular Diels-Alder reactions involving ds bonds were believed to form cross-links and render polyacetylene intractable, and that their elimination would lead to soluble products (34). No reaction of maleic anhydride or benzoquinone with polyacetylene was observed. In contrast, a second study (35) described the successful formation of the Diels-Alder adduct of maleic anhydride with remnant cis bonds in thermally isomerized samples of fran -polyacetylene. These data were used to support the notion that the thermal isomerization process used to convert cw-polyacetylene to the trans isomer does not go to completion. Unfortunately, the effect of this transformation on the tract-ability or stability of the polymer was not noted. 

Table V compiles the ZIE product isomer ratios of reactions from Table I for which such information has been reported. With 0.5-2% cross-linked polystyrene supptxis, the isomer ratios are similar to those obtained from Wittig reactions in solution. The exceptions appear to be the reactions on more highly cross-linked supports shown in Table IV. There is a clear trend toward greater E selectivity as the degree of cross-linking of the polymer increases. This probably should be explained as an environmental effect, but comparison with solvent effects on stereochemistry of Wittig reactions in the literature reveals no tendency for aromatic solvents, structurally similar to polystyrene, to increase E selectivity.

Notation. Throughout this chapter the degree of functionalization of polystyrene is reported as DF, the fraction of rings substituted. The % yield of a transformation on a polymer is 1(X) x DF(product)/DF(reactant). The % cross-linking of a polystyrene is reported as wt % divinylbenzene (DVB) in the monom mix at the start of copolymerization. Technical DVB typically contains 55% active DVB meta and para) and 45% ethylvinylbenzenes. Thus a 2% cross-linked polystyrene also contains 1.6% ethylvinylbenzene. Circled P is used for polystyrene, either all para or mixed meta and para isomers. 

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