Solvent-assisted methods

In order to obtain polyurethanes with highly regular chains, the polymerization should be carried out in solution. Dimethylformamide (DMF), dimethylacet-amide, dimethylsulfoxide, etc., may serve as solvents. Tien and Wei [26] studied the effect of benzidine-modified montmorillonite (MMT) on the hydrogen bonding in the hard segments of TPU. The latter was produced in random one-shot synthesis in DMF, It was established that the H-bonding in the hard 

Wang et aL [31] used the solution intercalation of an organoclay to get pecuhar optical properties. In this case, the clay (saponite) was intercalated with a suitable compound bearing chromophore moieties. Ni et at [32] tried to use an amine-functionalized attapulgite as chain extender (instead of a diamine) in the polymerization reaction. The best mechanical performance was achieved by dispersing 2-5 wt% organophilic attapulgite in the polyurethane. 

Property As cast Aged (70°C, 7 days) As cast Aged (70°C, 7 days) 

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Fig. 7 Scheme of the preparation of LDH/polymer nanocomposites (a) in situ polymerization, (b) direct incorporation of polymer, (c) restacking or reconstruction, and (d) guest displacement or solvent-assisted method... 

There are several ways to produce nanocomposites e.g., [10]). The related strategies are so versatile that they cannot be easily grouped. In this chapter, the production methods will be discussed on the example of TPU nanocomposites containing layered silicates. The related production methods are grouped as follows (i) in situ techniques (polymerization, sol-gel chemistry), (ii) solvent-assisted methods (solution, latex), and (iii) melt compounding. 

The use of microwave irradiation for this reaction, compared to conventional thermal heating, was investigated. Chloroform used as solvent under the conventional heating did only allow a temperature of 60 °C and a direct comparison between the two methods is therefore somewhat unfair imder these circumstances. Nevertheless, the microwave-assisted method is attractive and proved useful for both primary and secondary amines resulting in highly substituted pyrazolo ring-fused pyridones 40 in 68-86% yields within only 10 min. 

Procedures used vary from trial-and-error methods to more sophisticated approaches including the window diagram, the simplex method, the PRISMA method, chemometric method, or computer-assisted methods. Many of these procedures were originally developed for HPLC and were apphed to TLC with appropriate changes in methodology. In the majority of the procedures, a set of solvents is selected as components of the mobile phase and one of the mentioned procedures is then used to optimize their relative proportions. Chemometric methods make possible to choose the minimum number of chromatographic systems needed to perform the best separation. 

In the context of preparing potential inhibitors of dihydrofolate reductase (DHFR), the group of Organ has developed a rapid microwave-assisted method for the preparation of biguanide libraries (Scheme 6.174) [330]. Initial optimization work was centered around the acid-catalyzed addition of amines to dicyandiamide. It was discovered that 150 °C was the optimum temperature for reaction rate and product recovery, as heating beyond this point led to decomposition. While the use of hydrochloric acid as catalyst led to varying yields of product, evaluation of trimethylsilyl chloride in acetonitrile as solvent led to improved results. As compared to the protic... 

The Forster cycle method is quite simple, which explains why it has been extensively used. One of the important features of this cycle is that it can be used even in cases where the equilibrium is not established within the excited-state lifetime. However, use of the Forster cycle is difficult or questionable when (i) two absorption bands overlap (ii) the electronic levels invert during the excited-state lifetime (usually in a solvent-assisted relaxation process) (iii) the excited acidic and basic forms are of different orbital origins (electronic configuration or state symmetry) and (iv) the changes in dipole moment upon excitation are different for the acidic and basic forms. 

As for solvents, liquid ammonia or dimethylsulfoxide are most often used. There are some cases when tert-butanol is used as a solvent. In principle, ion-radical reactions need aprotic solvents of expressed polarity. This facilitates the formation of such polar forms as ion-radicals are. Meanwhile, the polarity of the solvent assists ion-pair dissociation. This enhances reactivity of organic ions and sometimes enhances it to an unnecessary degree. Certainly, a decrease in the permissible limit of the solvent s polarity widens the possibilities for ion-radical synthesis. Interphase catalysis is a useful method to circumvent the solvent restriction. Thus, 18-crown-6-ether assists anion-radical formation in the reaction between benzoquinone and potassium triethylgermyl in benzene (Bravo-Zhivotovskii et al. 1980). In the presence of tri(dodecyl)methylammonium chloride, fluorenylpi-nacoline forms the anion-radical on the action of calcium hydroxide octahydrate in benzene. The cation of the onium salts stabilizes the anion-radical (Cazianis and Screttas 1983). Surprisingly, the fluorenylpinacoline anion-radicals are stable even in the presence of water. 

Solid samples are extracted with low-boiling solvents. As the polarity of the volatiles is different, a two-step extraction procedure is recommended, e.g. methylene chloride as the first solvent and diethyl ether as the second solvent [13]. The yield of the odorants is enhanced when the dry sample is soaked in water before the extraction procedure [14]. After filtration and drying, the extract is concentrated to approximately 50 mb and is then freed from the non-volatile material by using the solvent-assisted flavour evaporation (SAFE) method (Sect. 16.2.2.2). 

It is very common to combine methods in obtaining aroma isolates. The simultaneous distillation/extraction method previously described is an example. Another popular combination method initially involves the solvent extraction of volatiles from a food and then high-vacuum distillation of the solvent/aroma extract to provide a fat-free aroma isolate. This technique is broadly used today to provide high-quality aroma extracts for numerous purposes. The apparatus used in solvent removal has been improved upon to reduce analysis time and efficiency the modified method is termed solvent-assisted flavour extraction (SAFE) [16]. 

Danks reported a microwave-assisted variant of the classical Paal-Knoor pyrrole synthesis (Scheme 3.3)5. This solvent-free method provided a considerable rate advantage (reactions complete within 2 min compared to over 12 h in conventional thermal heating) over classical procedures and even non-nucleophilic amines were condensed smoothly in the absence of Lewis acid promoters. Purification consisted of a simple silica gel filtration. The use of an early-dedicated laboratory instrument in this work is also noteworthy however, information of how the reaction temperature was controlled is not provided. 

Use of other methods has contributed further to the emerging picture of solvolysis of most secondary systems as being solvent-assisted. For example, the solvolysis rate acceleration on substituting a-hydrogen by CH3 in 2-adamantyl bromide is 107 5, much larger than that found for other secondary—tertiary pairs such as isopropyl-/-butyl. In molecules less hindered than 2-adamantyl, the secondary substrate is accelerated by nucleophilic attack of solvent.100 Rate accelerations and product distributions found on adding azide ion to solvolysis mixtures (Problem 4) also provide confirmatory evidence for these conclu-... 

An independent method for estimating nucleophilic solvent assistance has been applied to solvolyses of bromides with similar results (Fry et al., 1970b). In this approach, secondary solvolyses are compared with solvolyses of the corresponding tertiary substrates (e.g. 2-propyl compared with t-butyl). Again the reference substrate was 2-adamantyl (and 2-methyl-2-adamantyl [8]). It was found that... 

Oufnac, D.S. Xu, Z. Sun, T. Sahliov, C. Prinyawiwatkul, W. Godber, J.S. 2007. Extraction of antioxidants from wheat bran using conventional solvent and microwave-assisted methods. Cereal Chem. 84 125-129. 

In method (a), an alkyl halide (or tosylate) reacts with [Fe(CO)4] " in N-melhyl-2-pyrrolidone to give an anionic iron alkyl (1), some of which have been isolated and characterized. Further reaction with another alkyl halide gives a complex tentatively formulated as (2), which may decompose to the ketone via the complex (3) formed by solvent-assisted migratory insertion. No alkyl coupling products are detected. 

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