Selectivity of Cross-Linking Agents

A variety of cross-linking techniques may be used to prepare elastomeric polymer networks, and the cross-linking agents employed can vary tremendously in their reactivity or specificity Although such differences in reactivity are well known, there is very little relevant quantitative information most of the conclusions reached are only qualitative or 

Vali of themolecalarwei t Mc(f) between cross-links, as rabxdated from the stress-strain isotherms, for some of the (tetrafunctional) networks prepared from PDMS chains with adjacent vinyl groups The results are shown as a function of the fraction F, of chains having these cross-linking sites segregated at the ends of the chains. The three types of networks were prepared by benzoyl peroxide (O), Si[OSi(CH3)jH]4 ( ), and 2,5-di-methyl-2,5-di(t-butylperoxy) hexane (DBPH) (C) 

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Fig. 4.2. Selectivity of polymers as a function of the type and amount (X) of cross-linking agent [17]. The polymers were prepared in the presence of 1 with various proportions of the cross-linking agents (a) EDMA, (b) tetramethylene dimethacrylate, and (c) DVB. After removal of the template la, the separation factor a = Xd/Xl was determined for the resolution of D,L-la in a batch process.

In the field of water purification technology by membrane filtration, many researchers have studied the effect of cross-linking agents on membrane performance by varying the cross-linking conditions. The desired permeation and selectivity properties of the top thin layer vary with the concentration of polymer and cross-linking conditions like temperature and concentration of the cross-linker. Miao et al. [24] prepared... 

The selectivity is mainly influenced by the kind and amount of cross-linking agent used in the synthesis of the imprinted polymer [16,17]. Figure 2 shows the selectivity dependence for racemic resolution of the racemate of 2 on the structure of polymers of the type described in Scheme 2. 

The direct injection of liquid additives, such as polyisobutylene (PIB), to produce stretched film prevents difficulties in extruding and offers a processor a wider range of materials from which to select. It provides cost reductions due to the use of more economical formulations. This method is suitable for the injection of cross-linking agents, liquid colors, and the like, via the extruder or a gear pump. 

Depending on the chemistry adopted by the column manufacturer, eluant selection may be limited with respect to pH and type and level of organic solvent that can be tolerated. For example, the choice and level of cross-linking agent in polyvinyl alcohol (PVA)-based packings (e.g., Asahipak) influences both the pH stability and the organic solvent compatibility. In all cases the manufacturer s literature should specify eluant compatibility. 

In this section the use of polystyrene and copolymers of styrene with various cross-linking agents as supports for solid-phase organic synthesis is discussed. Copolymers of styrene with divinylbenzene are the most common supports for solid-phase synthesis. Depending on the kind of additives used during the polymerization and on the styrene/divinylbenzene ratio, various different types of polystyrene can be prepared. However, non-cross-linked polystyrene has also been used as a support for organic synthesis [10,16-22], Linear, non-cross-linked polystyrene is soluble in organic solvents such as toluene, pyridine, ethyl acetate, THF, chloroform, or DCM, even at low temperatures, but can be selectively precipitated by the addition of methanol or water. 

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