Encapsulation reactions

By carrying out the encapsulation reaction in the presence of a varying concentration of 3-carboxy-PROXYL, the number of entrapped radicals could be varied from 0.3 to 6.0 molecules per dendritic box as determined by electron spin resonance (ESR) spectroscopy [36]. Tbe number of 3-carboxy-PROXYL radicals in the dendritic box does not increase above six (Figure 9), clearly demonstrating that... 

Figure 9 Number of 3-carboxy-PROXYL radicals trapped in the dendritic box, as determined by ESR spectroscopy, versus the molar ratio of radical and dendrimer in the initial solution prior to the encapsulation reaction...

Insect cellular defense reactions involve phagocytosis of bacteria or fungi (2,3) and encapsulation of foreign objects by a layer of blood cells to seal the invader from the hemolymph (4,5). Humoral immunity in some Diptera involves a phenoloxidase-based humoral encapsulation reaction (6,7), and in many types of... 

The Zn-porphyrin coordination reactions (Scheme 9.1) were studied in 100 pm by 50 pm microchannels (Figure 9.7a), while the p-cyclodextrin ( -CD) encapsulation reactions (Scheme 9.2) were studied using 50 pm by 20 pm reaction channels (Figure 9.7b). 

Substrate size- and shape-selective reactions can also be explained in terms of effective concentration and transition state stabilization. 1) In a mixture of substrates a higher concentration can be achieved for those having a complementary size and shape to the nanoreactor portals and cavity (and thus can enter the nanoreactor) compared to those substrates that can not easily enter or do not fit within the cavity. In some cases, when the substrate encapsulation is a slow process (slow diffusion), substrate encapsulation can become the rate-determining step. 2) One can also imagine that substrates of identical size and shape can both enter the cavity, but the transition state of one of the reaction pathways is stabilized to a greater extent than the other. Importantly, for all encapsulated reactions one should keep in mind that the substrate residence time within the nanoreactor and the kinetic rates of the encapsulated reaction should at least have a comparable magnitude. ... 

Rebek and co-workers have used softball B as a nanoreactor for bimolecular Diels-Alder reactions. The Diels-Alder reaction between p-benzoquinone (3) and cyclohexa-diene 4a within nanoreactor B, present in stoichiometric amounts, has been accelerated 170-fold compared to the bulk and resulted in the encapsulated adduct [B zd 5a] (Scheme 6.2a). Even though nanoreactor B enhances the rate of the encapsulated reaction, no true catalytic behaviour was observed because of product inhibition by 5a. After encapsulating two quinones 3 within B, one molecule is occasionally replaced by a thiophene dioxide 4b, which leads to the encapsulated Diels-Alder product 5b. Product inhibition is partly suppressed if thiophene dioxide derivative 4b is used as the diene (Scheme 6.2b). This is because two molecules of the p-benzoquinone (3), i.e. [B r> (3)2] have a higher aflinity for nanoreactor B than derivative 5b, the corresponding Diels-Alder product of the thiophene dioxide. After each turnover the encapsulated product is released and replaced by two quinone reactants and true catalysis can take place. When catalytic amounts of B (10 mol%) are used, a tenfold rate enhancement (at 10 mM substrate concenlration) compared to the background reaction was observed, which is still lower than can be expected on the basis of the effective concentration. 

H. H. Zepik, S. Rajamani, M.-C. Maurel, D.W. Deamer, Oligomerization of thioglutamic add encapsulated reactions and lipid catalysis. Orig. Life Evol. Biosph, 2007, 37, 495-505. 

Polymerization on the surface is in competition with the process of new particle formation. Depending on the amount of surfactant, the type of monomer and the monomer droplet size any of the three nucleation mechanisms shown in Fig. 4 (micellar, homogeneous or droplet nucleation) can occur. In order to prevent micellar nucleation, the net surfactant concentration, after correction for the adsorbed amount on the surfaces, should be below the CMC. The presence of conventional surfactants in encapsulation reactions introduces the problem that a delicate balance between the stabilization of polymer particles and inorganic particles and the formation of new particles has to be maintained. 

Table 4.2 Common pigments and fillers involved in encapsulation reactions. Isolectric pH values, particles dimensions and fields of application.

A full description of these systems will not be attempted here, as they are more fully discussed in the following section. Tables 4.4 and 4.5 provide a nonexhaustive list of monomers, initiators, surfmers or macromonomers which have been involved in encapsulation reactions. 

Always in an attempt to compatibilize the core and shell materials, Yoshinaga and co-workers described the synthesis of a series of oHgomeric silane molecules and their use in encapsulation reactions [95]. However, as the polymerizations were performed in the absence of surfactant, the resulting composite particles were not colloidally stable. 

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