Microencapsulation polyurea

The second general method, IMPR, for the preparation of polymer supported metal catalysts is much less popular. In spite of this, microencapsulation of palladium in a polyurea matrix, generated by interfacial polymerization of isocyanate oligomers in the presence of palladium acetate [128], proved to be very effective in the production of the EnCat catalysts (Scheme 3). In this case, the formation of the polymer matrix implies only hydrolysis-condensation processes, and is therefore much more compatible with the presence of a transition metal compound. That is why palladium(II) survives the microencapsulation reaction... 

Some of the most remarkable achievements include microencapsulation in polystyrenes such as entrapped 0s04 for olefin hydroxylation (exploiting the interaction between n-electrons of benzene rings of the polystyrenes used as polymer backbones and the vacant orbitals of the catalysts) 5 polyurea-entrapped palladium (PdEnCat)6 for a multiplicity of C C forming reactions and the use of carboxylic acid-functionalized polymer (FibreCat).7 In general, however, metal leaching cannot be avoided. The PdEnCat catalyst, for instance, leaches some 4% of palladium per catalytic reaction run. 

In a recently published work by Bourbigot et al.,63-65 microencapsulation of ammonium phosphate with PU and polyurea shells has been carried out to make an intrinsic intumescent system compatible in normal PU coating for textiles. Microencapsulation of ammonium phosphate helps in reducing the water solubility of the phosphate and hence, increases the durability of the textile back-coating. 

Giraud, S., Bourbigot, S., Rochery, M., Vroman, I., Tighzert, L., Delobel, R., and Poutch, F. 2005. Flame retarded polyurea with microencapsulated ammonium phosphate for textile coating. Polym. Deg. Stab., 88 106-113. 

An approach to immobilization which has recently become popular is microencapsulation in polymers, such as polystyrene and polyurea, developed by the groups of Kobayashi [34] and Ley [35], respectively. For example, microencapsulation of palladium salts or palladium nanoparticles in polyurea microcapsules... 

Scheme 1.1 Ring-opening hydrogenolysis of epoxides catalyzed by PdNPs (2nm) microencapsulated in polyurea. Recycling experiments can be carried out at least ten times with 97-99% yield. (Ref. [14a], Yu group, Org. Lett. 2003, 4665).

Ley et al. used Pd(OAc)2 which was stabilized by microencapsulation in polyurea in Heck reactions of various aryl bromides with butyl acrylate. Their optimized reaction conditions (supercritical CO2 as solvent, 0.4mol% Pd, 100 C, Bu4NOAcas base) also allowed the conversion of 4-chloronitrobenzene within 16h (58% yield). This system seems to be Umited to strongly activated aryl chlorides [163]. 

Ley and Malla Reddy independently reported OSO4 microencapsulated in polymer matrices such as polyurea or polysulfone, respectively. Polyurea-microencapsulated OSO4 has proven to be extremely robust and long-term storable. High chemical yield and excellent enantioselectivity was also achieved using polysulfone-microencapsulated OsOa even without the need for slow addition of olefins. ... 

Due to the high volatility and toxicity of OSO4, the same group applied OsEnCat 40 (microencapsulation of OSO4 in polyurea) to oxidative cleavage of olefins. The OsEnCat 40 catalyst was recovered and reused three times but with a sacrifice of catal)dic activity (eq 71). ... 

Microencapsulation by interfacial polycondensation is a usefiil method to microencapsulate a liquid core material. Especially, polyurea and polyurethane microcapsules have been extensively investigated in various industries [209]. For example, aliphatic hexamethylene diisocyanate (HMDI) and aliphatic ethylene diamine (EDA) have been used to prepare polyurea microcapsules containing insecticide called diazinon [210]. A urea linkage is formed immediately by the reaction between an amine and an isocyanate group (see Figure 4.31), and a polyurea is synthesized by the reaction between an amine with two or more amine groups and an isocyanate with two or more isocyanate groups. 

Core/wall ratio cannot only decide the wall thickness of a microcapsule but also the effectiveness of the microencapsulation." The work" on the microencapsulation of xylitol by poly (urethane-urea), which was performed in a water-in-oil system, indicates that the most proper core/wall ratio is 77/23 in terms of high encapsulation yield and xylitol loading content. Another work on the microencapsulation of perfume by polyurea, which was performed in a oil-in-water system, found that with the decrease of the core/wall ratio, the size of the formed microcapsule increased even the original droplet size is roughly the same. An outward diffusion mechanism was proposed to explain... 

Some examples of microencapsulated EO are those reported by Scarfato et al. (2007), in which the polyurea microcapsules containing the EO of lemon bahn, lavender, sage, and thyme with anti-germinative activity were prepared by interfacial polymerization in 0/W annlsion. 

Liang, C. Lingling, X. Honbo, S. Zhibin, Z. Microencapsulation of butyl stearate as a phase change material by interfacial polycondensation in a polyurea system. Energ Comer Manag 50 (2009) 723-729. 

Fumigants may be acaricidal, nematicidal and/or fungicidal in their action. Some insecticides with relatively high vapour pressure can be applied in soil granule form to act as soil fumigants, for example, dichlorvos and sulphotepp. Microencapsulation with polyureas can be used to prolong their action [35]. 

Hirech et al. have described preparation of polyurea microcapsules containing hquid pesticide (e.g., diazinon) suspended in concentrated disinfectant solution, by use of a two-step microencapsulation process [28], The first step is the hquid-hquid dispersion in a Sulzer static mixer (SMX) the second step is microencapsulation by inter-fadal polymerization in a stirred-tank reactor (Fig. 5.4). It has been shown that the... 

From DSC and activity measurements using glucosidase (GOD), approximately the same results have been obtained (see Fig. 3) [18]. Komori et al. measured die residual activity of the microencapsulated GOD from A. niger in polyurea by interfacial polymerization after keeping the sample at a specified temperature for 10 min. They reported that the thermal stability of GOD was improved by fixation in a capsule [19]. Despite the difierence in the heat treatment method, the trend of the data is the same and the results on the free enzyme were nearly the same (see Fig. 3). Glucosidase fixim A. niger is a protein that contains about 16% saccharide chain [20]. Similar to invertase, it is possibly stabilized by interaction of the PVA-saccharide chain-water. 

There has been considerable interest in hydrogenation and Pd-mediated C—C bond formation reactions in supercritical CO2 (scC02) [40]. Lee et al. [36] applied polyurea as a suitable encapsulation matrix to ligate and retain metal species such as Pd(OAc)2. The microcapsules were prepared by an in situ interfadal polymerization approach. A standard HPLC capillary was packed with immobilized catalyst. The combination of microencapsulated Pd(OAc)2 and nBu4NX (X = OMe and OH) was an efficient and economical catalytic system for Suzuki cross couplings at low temperatures in both organic solvents and SCCO2. 

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