Postmodification

In order to better understand the properties of the metathesized plant oils, postmodification of these polymerized seed oils was also examined [25]. To explore the possibilities of new materials that could arise from the polymerized oils, Larock and coworkers evaluated both the hydrogenation and epoxidation of metathesized soybean oils. Normally, thermally polymerized metathesized oils yield a yellow, brittle gel. This brittle gel is often undesirable, so other processes were investigated to develop a more usable material. In order to alter the properties of the material, complete hydrogenation was accomplished with 10% palladium on carbon to produce a white/pale cream-colored, crystalline material with melting points between 53 and 60 C. This metathesized soybean oil was also treated with conditions to invoke epoxidation of the double bonds. Unfortunately, the initial attempts with m-chloroperbenzoic acid (mCPBA) were problematic for the metathesized soybean oil despite being satisfactory for unaltered soybean oil. It was found that material that had been epoxidized by mCPBA would polymerize in a few hours, most likely due to the difficulty in the complete removal of the w-chlorobenzoic acid generated during the epoxidation. To circumvent this problem, conditions 

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Phosphorus also permits the postmodification of the backbone of dendrimers. Indeed charges, and also various functional groups, can be selectively introduced into the internal voids. Such a transformation can be performed where and when required. 

Monolithic columns with the chiral anion exchange-type selectors incorporated into the polymer matrix obtained through in situ copolymerization process of a chiral monomer (in situ approach) [80-83,85] or attached to the surface of a reactive monolith in a subsequent derivatization step (postmodification strategy) [84], both turned out to be viable routes to enantioselective macroporous monolithic columns devoid of the limitations of packed columns mentioned earlier. 

The postmodification strategy, in which apoly(glycydyl methacrylate-co-ethylene dimethacrylate) monolith was activated with hydrogen sulfide to a thiol-modified monolith and subsequently derivatized with an 0-9-(tert-butylcarbamoyl)quinine selector by radical addition reaction, yielded slightly less efficient capillary columns. However, this procedure has the advantage that only minute amount of chiral selector are needed to end-up with a useful enantioselective capillary column [84]. 

Wholly aromatic polymers are thought to be one of the more promising routes to high performance PEMs because of their availability, processability, wide variety of chemical compositions, and anticipated stability in the fuel cell environment. Specifically, poly(arylene ether) materials such as poly-(arylene ether ether ketone) (PEEK), poly(arylene ether sulfone), and their derivatives are the focus of many investigations, and the synthesis of these materials has been widely reported.This family of copolymers is attractive for use in PEMs because of their well-known oxidative and hydrolytic stability under harsh conditions and because many different chemical structures, including partially fluorinated materials, are possible, as shown in Figure 8. Introduction of active proton exchange sites to poly-(arylene ether) s has been accomplished by both a polymer postmodification approach and direct co-... 

The most common way to modify aromatic polymers for application as a PEM is to employ electrophilic aromatic sulfonation. Aromatic polymers are easily sulfonated using concentrated sulfuric acid, fuming sulfuric acid, chlorosulfonic acid, or sulfur trioxide (or complexs thereof). Postmodification reactions are usually restricted due to their lack of precise control over the degree and location of functionalization, the possibility of side reactions, or degradation of the polymer backbone. Regardless, this area of PEM synthesis has received much attention and may be the source of emerging products such as sulfonated Victrex poly (ether ether ketone). 

Poly(ether ether ketone) (PEEK) is an aromatic, high performance, semicrystalline polymer with extremely good thermal stability, chemical resistance, and electrical and mechanical properties. This polymer shows little solubility in organic solvents due to the crystallinity. One of the first ways to characterize PEEK was by sulfonating the polymer. By adding sulfonic acid groups to the backbone, the crystallinity decreased and solubility increased.Commercially available Victrex appears to be one of the more interesting poly(arylene ether) s used for postmodification. 

Figure 15. Rh-containing organosilane precursor 2 incorporated into mesoporous silica through postmodification and one-pot synthesis [76].

The fourth method for the preparation of polymer stabilized Au NPs is the postmodification of pre-formed Au NPs . This method is used to avoid broad distribution of sizes of Au NPs stabilized with polymers through any of the methods described previously. As we have mentioned before, in a first step very monodisperse Au NPs are obtained by common methods, such as the citrate reduction or the Brust-Schiffrin method. In a second step, the exchange of weakly bound citrate ions with polymer or modification ofend-functionalized thiols with polymers is performed. 

The concept of postmodification of the molecule s periphery is thus not only appropriate for the introduction of simple functionalities determining the physical and chemical properties of the molecule (e.g. solubility), but also allows the introduction of complex function-bearing units (e.g. catalytically active units, anti-tumour agents), which could be too sensitive for convergent dendrimer synthesis. 

Fig. 3.5 Postmodification Dendrimer growth with protected monomeric branching units permits subsequent introduction of functionalities into the scaffold of the dendrimer precursor...

Lochmann et al. were the first to attempt postmodification of the internal molecular framework of Frechet-type dendrons [46]. Multifunctionalisation of the interior of the dendron via metallation (superbase followed by reaction with various electrophiles) proved to be of a somewhat random nature owing to a lack of regioselectivity. It is admittedly suitable for engineering the solubility properties of the dendrons, but not for producing dendritic structures having functional groups in defined positions of the internal scaffold of the molecule. 

This process involves the covalent locking in of structures formed by reversible self-assembly. The irreversible, post-assembly step switches off the equilibrium process involved in the self-assembly. As we will see in the following sections, self assembly with covalent postmodification is involved in a range of biochemistry (e.g. insulin synthesis) and elegant abiotic supramolecular synthesis as in the formation of catenanes and knots. 

The use of metal ions as kinetic synthetic templates is extremely widespread, and is an excellent way in which to bring about the organisation of a number of reacting components in order to direct the geometry of the product. Because some metal ions, such as the transition metals, often have preferred coordination geometries (e.g. tetrahedral, square planar, octahedral etc), changes in metal ion may have a profound effect on the nature of the templated product. Metal-ion-templated syntheses may be classified more generally as examples of self-assembly with covalent postmodification. For example, the synthesis of the artificial siderophore 10.2 is effected by the use of an octahedral Fe3+ template.8 In this case, the macrobicyclic product is obtained as the Fe3+ complex from which it is difficult to separate. 

Combination of 10.41 with crown ether derivative 10.48 and Ag0) gives a trimetallic sheathed rack in which a rack of Ag(I) ions is threaded through the cavities of three heterocrowns (Figure 10.42). This complex is an example of a pseudorotaxane (stricdy a [4] pseudorotaxane because there are four components - three loops and an axel) and we will return to these kinds of assemblies, which are precursors for the synthesis of complex interlocked molecules by postmodification techniques, in Section 10.7. 

Postmodification of Aromatic PMO Precursors and PMO Materials in the Solid State... 

The synthesis of mesoporous materials with metal-organoflinctional groups has attracted a great deal of interest fi-om many researchers due to their potential applicability as catalysts. MCM 41 materials, comprising hexagonally-packed arrays of one-dimensional cylindrical pores, can provide large surface areas for functionalisation with metals and/or organic species, either by postmodification or direct synthesis. ... 

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