Use of Strong Base Alone

Strong bases such as alkali metals, metal hydrides, metal amides, metal alkoxides, and organometallic compounds initiate the polymerization of a lactam by forming the lactam anion XXXIV [Hashimoto, 2000 Sebenda, 1989 Sekiguchi, 1984], for example, for e-caprolactam with a metal 

The lactam anion reacts with monomer in the second step of the initiation process by a ring-opening transamidation to form the primary amine anion XXXV. Species XXXV, unlike 

The anionic polymerization of lactams proceeds by a mechanism analogous to the activated monomer mechanism for anionic polymerization of acrylamide (Sec. 5-7b) and some cationic polymerizations of epoxides (Sec. 7-2b-3-b). The propagating center is the cyclic amide linkage of the IV-acyllactam. Monomer does not add to the propagating chain it is the monomer anion (lactam anion), often referred to as activated monomer, which adds to the propagating chain [Szwarc, 1965, 1966]. The propagation rate depends on the concentrations of lactam anion and W-acy I lactam, both of which are determined by the concentrations of lactam and base. 

Initiation consists of the reaction of the A -acyllactam with activated monomer followed by fast proton exchange with monomer 

Species XL and XLI correspond to species XXXV and XXXVI for polymerization in the absence of an acylating agent. The acylating agent achieves facile polymerization of many 

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The use of strong base alone for aninic polymerization of lactam is limiting. As noted previously, the polymerization is characterized by induction periods and, moreover, only the more reactive lactams, such as e-caprolactam and 7-heptanolactam ( -enantholactam), readily undergo polymerization, while the less reactive lactams, 2-pyrrolidinone, and 2-piperidinone, are... 

The use of strong bases alone is a limiting factor in the anionic lactam polymerization since high polymerization temperatures and relatively slow reaction rates are necessarily implied side reactions are, therefore, unavoidable. Moreover, only the more reactive lactams, such as CL and ((-enantholactam, readily polymerize in nonactivated reaction conditions. The less reactive lactams, such as 2-pyrrolidone and 2-piperidone, are much harder to polymerize because the formation of the imide dimer is more difficult. These limitations can be overcome if the imide is generated by reaction of... 

This process, based on strong reducing agents, can be avoided by the use of disperse dyes that are removed by aqueous alkaH alone. Two types of dye are used dyes containing diesters of carboxyHc acid and dyes destroyed by mild alkaH. The reaction of diester dyes is shown in equation 5. 

FIGURE 13.57 NOTE The importance of Solvent/column interaction using Jordi DVB columns cannot be over emphasized. We have found that a SOySO mbc of MeOH/ACN for the strong solvent Is adequate for many reverse phase separations and is better than either alone. We have now observed that the use of THF/ACN as strong solvent is often better than MeOH/ACN. In general Lewis bases (electron donor solvents) deactivate the aromatic rings and often dramatically increase column efficiencies. 

Soil Pb is an important pathway of human Pb exposure (Mielke Reagan 1998). Empirical evaluation between soil Pb and BPb indicated a strong positive and nonlinear association between soil Pb and BPb (Mielke et al., 2007a). The soil Pb footprint of New Orleans indicates that in the case of public and private properties an enormous disparity exists between the inner- city and outlying areas of New Orleans that cannot be attributed to older Pb-based paint alone dust Pb from the previous use of lead additives to gasoline provide a better explanation between... 

Recent work by Ford et al. demonstrates that a variety of metal carbonyl clusters are active catalysts for the water-gas shift under the same reaction conditions used with the ruthenium cluster (104a). In particular, the mixed metal compound H2FeRu3(CO)13 forms a catalyst system much more active than would be expected from the activities of the iron or ruthenium systems alone. The source of the synergetic behavior of the iron/ruthenium mixtures is under investigation. The ruthenium and ruthenium/iron systems are also active when piperidine is used as the base, and in solutions made acidic with H2S04 as well. Whether there are strong mechanistic similarities between the acidic and basic systems remains to be determined. 

A tertiary alkyl halide when treated with sodium methoxide forms an ether or an alkene (Above fig.). A protic solvent is used here and this favours both the SN1 and El mechanisms. However, a strong base is also being used and this favours the E2 mechanism. Therefore, the alkene would be expected to be the major product with only a very small amount of substitution product arising from the SN2 reaction. Heating the same alkyl halide in methanol alone means that the reaction is being done in a protic solvent with a non-basic nucleophile (MeOH). These conditions would yield a mixture of substitution and elimination products arising from the SN1 and El mechanism. The substitution product would be favoured over the elimination product. 

Benzene cannot be nitrated using nitric acid alone, which lacks a strong electrophilic centre, but it is readily achieved using a mixture of concentrated nitric acid and concentrated sulfuric acid, the so-called mixed acid . The product is nitrobenzene. The interaction of nitric acid and sulfuric acid produces the electrophile, the nitronium ion N02, according to Scheme 2.3. The sulfuric acid is also the source of the base HSO4" that removes the proton in the second step. 

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