Functionalization covalent

The covalent binding of a metal complex to a solid support is the most commonly applied technique of functionalizing a microporous or mesoporous material. In essence, this technique can be further categorized into two subsections (1) grafting, this is the direct attachment of a metal complex to the silica framework of the material and (2) tethering, whereby a spacer ( tether ) is used between the wall of the material and the metal complex. 

In comparison to the mesoporous materials, less research has been published on the functionalization of microporous materials by direct grafting (excluding various types of ion exchange). It has been stated that some of the newly modified mesoporous materials suffer from the adsorption of products and by-products onto the amorphous walls of the support structure.11031 Microporous zeolitic 

Chemical functionalization can produce strong interfacial bonds of the CNTs with many polymers, allowing nanocomposites with embedded CNTs to detain higher mechanical and functional properties. 

However, chemical functionalization can have essentially two major drawbacks 1) during functionalization reaction, especially those that use ultrasonication process, can cause large defects on the sidewalls of CNTs and can cause fragmentation of the CNTs into smaller pieces and 2) normally these processes used concentrated acids or strong oxidants, which are environmentally rmfriendly. 

This strategy is based on the covalent attachment of functional groups to the C=C double bonds of the polyaromatic network of CNTs. By adjusting the reaction conditions (i.e., catalysts and reactants concentration, temperature and solvent), this strategy has allowed the selective and controlled modification of CNTs and, due to the nature of the bonds, has provided long-term stability to the dispersion. Nevertheless, the two main drawbacks of covalent surface modification are (i) the inevitable loss of their electrical and/or electronic properties [49] and (ii) the low degree of functionalization. 

The covalent chemistry of CNTs maybe classified into two categories, depending on the different reactivities of the bare nanotubes. The first involves the semifuUerene tips and the defects over the sidewalls and the second one, the sp hybridized sidewalls. 

Addition chemistry has developed into a promising tool for the modification and derivatization of the surface of nanotubes [24, 26], However, it is difficult to achieve chemoselectivity and regioselectivity control of addition reactions, requiring hot addends such as arynes, carbenes, radicals, nitrenes or halogens under drastic reaction conditions. 

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The polyelectrolyte covalently functionalized with reactive groups may be viewed as an enzyme-like functional polymer or as a molecular reaction system in the sense that it has both reactive centers and reaction rate-controlling microenvironments bound together on the same macromolecule. 

Bae et al. [114] performed a study to modify Ni-DOBDC with pyridine molecules. The study showed that pyridine molecule made the normally hydrophilic internal surface more hydro-phobic as a result, water absorption was reduced, while substantial C02 capture capacity was retained to a certain level. Fracaroli et al. [132] improved the interior of IRMOF-74TII by covalently functionalizing it with a primary amine, and used a MOF, IRMOF-74-IIICH2NFI2, for the selective capture of C02 in 65% relative humidity. 

Several studies concerning the biodistribution and clearance of CNTs have been reported [119-126] most of them investigated the biokinetics of covalently functionalized CNTs, whereas only a few evaluated the distribution of noncovalently modified CNTs in the body. 

S.S. Wong, E. Joselevich, A.T. Woolley, C.L. Cheung, and C.M. Lieber, Covalently functionalized nanotubes as nanometre-sized probes in chemistry and biology. Nature 394, 52—55 (1998). 

A non-covalently functionalized dendrimer was also applied in a continuous allylic amination reaction.[33] PPI dendrimers functionalized with urea adamantyl groups can act as host molecules for phosphorus ligands equipped with acetyl urea groups (Figure 4.18). The so formed supramolecular complex was reacted with a palladium precursor... 

Both the acid and ester were applied in continuous allylic amination. The maximum conversion (ca. 80%) was reached after 1 h in both experiments. Using the acid derivative of the guest, a slight drop in activity was observed ((a) in Figure 4.19), which is probably caused by a slow deactivation of the catalyst and has also been observed for covalently functionalized dendrimers (described above). When using the ester-functionalized guest, the activity dropped faster ((b) in Figure 4.19). This decrease in activity is caused by lack of retention (99.4% for the acid vs. 97% for the ester) as well as by deactivation. 

Figure 4.19. Continuous catalysis with non-covalently functionalized dendrimers a) acid-, b) ester-functionalized guest. (Reprinted with permission from ref. 33. Copyright 2001 American Chemical Society)...

In the VB picture, each (oab)2 bond pair is described by a HeitlerLondon covalent function V ab(cov), whose spatial dependence is of the form... 

The sensors discussed so far are based on ligands covalently bound to the polymer backbone. Other methods of detection - often referred to as mix and detect methods - work by simple noncovalent incorporation of the polymer with the ligand of interest. Reichert et al. generated liposomes of polydiacetylene with sialic acid for the same purpose of detection as Charych s surface-bound polymers, but realized that covalent functionalization of the polymer was not necessary [17]. Through simple mixing of the lipid-bound sialic acid with the polymer before sonication and liposome formation, they were able to form a functional colorimetric recognition system (Fig. 8). 

From an atomic configuration point of view, a nanotube can be divided into two parts that are generated by curvatures the end caps and sidewall. The end caps are close to the hemispherical fullerene and are curved in 2D, and the sidewall contains less-distorted carbon atoms and is curved in ID (Polizu et al., 2006). Owing to their specific curvatures, the chemical reactivity at the sidewall is significantly lower than that at the end caps The sidewall is thought to be inert and highly reactive agents are required for the covalent functionalization of CNT sidewalls (Wei et al., 2007). 

Covalent functionalization may also be subdivided into two groups, depending on whether the modification is carried out on previously oxidized CNTs or not. In the former case, the various functionalities react with the carboxyl and carbonyl groups introduced on the tube during oxidation protocols. The latter case involves direct re-... 

In order to overcome this drawback, there are two main approaches for the surface modification of carbon nanostructures that reoccur in the literature. The first one is covalent functionalization, mainly by chemical bonding of functional groups and the second one is noncovalent functionalization, mainly by physical interactions with other molecules or particles. Both strategies have been used to provide different physical and chemical properties to the carbon nanostructures. Those that will be presented here are only a few examples of the modifications that can be achieved in carbon nanostructure surfaces and composite fabrication. 

When the formation of covalent bonds is established between functional groups and a surface, a covalent or chemical functionalization is reached. The main characteristic of this type of functionalization is the change in the carbon hybridization from sp2 to sp3 [104]. Although this covalent functionalization provides the possibility to obtain a... 

Covalent functionalization of fullerenes has also been used to obtain surface-modified fullerenes that are more compatible to polymer matrices in order to fabricate composites. In this context, four basic strategies were developed. The first one allows the fullerenes to react during the monomer polymerization, so that the fullerene can be attached to the polymer chain [111, 112]. Second, an already synthesized polymer is treated using specific conditions that allow the chemical reaction with fullerenes [113,114]. Third, the fullerenes are chemically bonded to a monomer which is polymerized or co-polymerized to obtain the modified monomer [115,116]. Fourth, a dendrimer can be synthesized around a fullerene which then acts as a nucleus [117,118]. 

Although there have been great advances in covalent functionalization of fullerenes to obtain surface-modified fullerene derivatives or fullerene polymers, the application of these compounds in composites still remains unexplored, basically because of the low availability of these compounds [132]. However, until now, modified fullerene derivatives have been used to prepare composites with different polymers, including acrylic [133,134] or vinyl polymers [135], polystyrene [136], polyethylene [137], and polyimide [138,139], amongst others. These composite materials have found applications especially in the field of optoelectronics [140] in which the most important applications of the fullerene-polymer composites have been in the field of photovoltaic and optical-limiting materials [141]. The methods to covalently functionalize fullerenes and their application for composites or hybrid materials are very well established and they have set the foundations that later were applied to the covalent functionalization of other carbon nanostructures including CNTs and graphene. 

The use of CNTs in composites for optical, mechanical, electronic, biological and medical applications, etc., requires the chemical modification of their surface in order to meet specific requirements depending on the application [140]. While searching for how to perform the covalent functionalization of CNTs, it was found that the tips of CNTs were more reactive than their sidewalls [142,143]. 

It has also been demonstrated that CNT sidewalls can be covalently fluorinated [148 150], or they can be derivatized with certain highly reactive chemicals such as dichlorocarbene [142], In this context, Chen et al. applied derivatization chemistry with thionychloride and octadecylamine in order to obtain organic soluble SWCNTs and later they performed a reaction with dichlorocarbene that led to the covalent functionalization of the nanotube walls. 

More recently, microwave chemistry has also been used to achieve covalent functionalization. In particular, these treatments can functionalize CNTs with sulfonated and carboxylic groups using a mixture of nitric and sulfuric acid under microwave radiation for 3 min, thus resulting in highly dispersible CNTs in ethanol and water... 

There are several chemical reactions that can be used as an alternative to achieve covalent functionalization of CNTs. Two of them are amidation and/or esterification reactions. Both reactions take advantage of the carboxylic groups sitting on the side-walls and tips of CNTs. In particular, they are converted to acyl chloride groups (-C0-C1) via a reaction with thionyl (SO) or oxalyl chloride before adding an alcohol or an amine. This procedure is very versatile and allows the functionalization of CNTs with different entities such as biomolecules [154-156], polymers [157], and organic compounds [158,159] among others. 

H. F. Yang, C. S. Shan, F.H. Li, D.X. Han, Q. X. Zhang, L. Niu, Covalent functionalization of polydisperse chemically-converted graphene sheets with amine-terminated ionic liquid, Chemical Communications, vol. 26, pp. 3880-3882, 2009. 

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