The Anionic Route

It should be noted that BH4 is not a typical Lewis base (such as the O atoms of ethers or N atoms of amines are). However, it is well known that Lewis acids add BH4 via 3c2e bonds. Thus, B2H7 is formed readily by the interaction of diborane (or BH3 L) with NaBH4 in the presence of a polyether or, even better, in the presence of a large cation such as tetrabutylammonium as shown in Eqs. (15) and (16) 

The reaction proceeds according to Eq. (17). An additional equivalent of iodine reacts at ambient temperature to produce diborane. If this reaction is performed with Et4N[B2H7] in CH2CI2 then the unstable anion H3BI, and the more stable anions H2Bl2 and HBI3 can be detected by 11 B NMR spectroscopy in small amounts. 

However, even then, about 30% of the BH4 is already converted into Bn H14 . The yield of the undecahydroundecaborate(—1) reaches a maximum (75%) at 135 °C when a 26% excess of I2 as demanded by Eq. (20) is employed due to loss of HI at this temperature, and this is achieved within 2.5 h [16]. The Bn anion can be precipitated from an aqueous solution as Ph4As[BnHi4] or Bu4N[BnHi4]. Yields of up to 62% were observed by a similar reaction using NaBH4 and BF3OEt2 [17, 18]. 

Another, quite versatile, route to hydropolyborates is based on the thermal decomposition of tetraorganylammonium tetrahydroborates, R4N[BH4]. Here, the products formed depend on temperature and time. For each hydropolyborate reaction conditions have to be optimized. 

A systematic study using the thermolysis of Et4N[BH4] has shown that within the first 20 h at 155 °C the reaction proceeds principally as described by Eq. (21)  

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The double diphenylethylenes (DDPE) approach was first reported by Hocker and Latterman [172] and was later developed mainly by the Quirk (anionic) [173,174] and Faust (cationic) groups [175,176]. Representative reactions for the synthesis of the A2B2 4//-star by the anionic route are shown in Scheme 91. 

The reductive elimination step has undergone much less examination, with the majority of authors considering that the acyl species produces CH3COI and regenerates the active anionic [Irl2(CO)2] species. When a DFT study was carried out by Kinnunen and Laasonen [39], the Jac,cis-[Ir(COCH3)l3(CO)2] isomer was seen to be the dominant intermediate for the anionic route, whereas for the neutral pathway the mer,ds-[lr(COCHi)l2(CO)i] isomer allowed a faster reductive elimination reaction. 

While high polymers of /3-lactones can also be formed by cationic polymerization, most of the commercial production seems to be by the anionic route. Carboxylate salts such as sodium acetate or benzoate are commonly the initiators, but other nucleophiles, such as triethylamine, betaine, potassium f-butoxide, aluminum and zinc alkoxides, various metal oxides and tris(dimethylamino)benzylphosphonium chloride (the anion of which is the initiator), are of value. Addition of crown ethers to complex the counter cation increases the rate of reaction. When the reaction is carried out in inert but somewhat polar organic solvents, such as THF or ethyk acetate, or without solvent, chain propagation is very fast and proceeds without transfer reactions. 

Firestone and Shell started commercial production of cis-polyisoprene by the anionic process in the 1950 s but these plants are no longer in operation now. About the same time, Phillips started the manufacture of polybutadienes by the anionic route and ever since, there has been a steady growth in their use, particularly in the tire-tread as well as tire-carcass formulations. These solution polybutadienes, generally, have low vinyl contents but recently, Phillips has found some interesting applications for medium vinyl polybutadienes as well.14 Polybutadienes with 50-55 percent vinyl contents behave like emulsion polymerized SBR in tire tread formulations and exhibit very similar tread... 

In conclusion, it is evident from the above discussion that anionic polymerization has emerged from a laboratory curiosity to an important industrial process in a relatively short span of time. Currently, over a million tons of polymers are produced by the anionic route in about twenty manufacturing plants around the world. We at Phillips are quite proud of being one of the pioneers along with Firestone and Shell in harnessing this new technology to commercial applications. The fact that our polymers find such wide ranging applications from tire treads to injection molded blood filters and from lubricant additives to solid rocket binders bears ready testimony to this. 

Diblock and triblock copolymers are usually synthesized anionically, which inherently limits the possible blocks and homopolymers. However, the anionic route gives very good control over molecular weight, molecular architecture and polydispersity and is ideally suited for a systematic study of the effect of connector molecules. Furthermore, the problems frequently encountered during the anionic synthesis have been ironed out for many common polymers and the synthesis, although tedious, can be considered a routine procedure. 

Vaudry et al. [66] described the employment of long-chain carboxylic acids, the anionic route, for the synthesis of organized mesoporous aluminas. These syntheses are carried out in alcohols, formamide, chloroform or diethyl ether. While aluminas prepared by Vaudry et al. exhibited surface areas between 500 and 700 m2/g and pore sizes around 2.0 nm, we have reported [74] the effect of the hydrocarbon chain length on the pore size diameter of organized mesoporous aluminas prepared using long-chain carboxylic acids of different chain... 

This structural variation notwithstanding, only a few cationic transition-metal ions react efficiently with molecular oxygen under gas-phase conditions (see below). In contrast, many anionic metal complexes and clusters are readily oxidized by O2 to afford various metal-oxide anions [19]. From a conceptual point of view, however, anionic species appear to be inadequate reagents for the activation of hydrocarbons, because they generally require electrophilic attack. At present, only a few oxidations by transition-metal oxide anions have been reported to occur in the gas phase, and they are mostly limited to relatively polar substrates, such as the CH3OH CH2O conversion [20]. Because of the lower reactivity of hydrocarbons, their C-H bond activation by metal-oxide anions is likely to be limited to radical pathways driven purely thermodynamically, i.e., when Z)(0-H) exceeds Z)(C-H) of the substrate [21]. As radical-type pathways are prone to create selectivity problems, and over-oxidation is particularly difficult to control, the anionic route appears less attractive as far as partial oxidation of alkanes is concerned. 

In another paper, as already observed by Zhang et al. [25], liao demonstrated that the experiments carried out with a toluene- or xylene-OC suspension instead of a cyclohexane one, resulted in the formation of more stable emulsions, so that aromatic solvents could be successfully used to synthesize PB/day nanocomposites by in-situ polymerization through the anionic route [22],... 

Because of the high activation energy (30 kCal/ mol) associated with this reaction and the dependence of the reaction rate on temperature, quantity of radicals, etc., the anionic route is strongly favored. In general, free radical induced polymerization yields high molecular weight polymer. 

In contrast to CL hydrolytic polymerization, in which a thermodynamic equilibrium is established in most cases among linear maciomolectilar chains, monomer, and higher (cydic and linear) oligomers, the anionic route is very often under stringent kinetic control. In this respect, the residual monomer content, which is 7-8 wt.% in hydrolytic polymerization, can be far bdow the value predicted on the basis of thermodynamic considerations ( 3 wt.%) and, depending on the reaction conditions chosen, found to be as low as 1 wt.% or even less, ° as shown in Figure 22. 

Saverio Russo is a senior professor of industrial chemistry at Genoa University, Italy. He has been, and still is, project leader of several research programs supported by the European Union, Italian Ministry of University, and Chemical Companies. He has been working for more than 40 yeare in the field of macromolecular science and technology, mainly on advanced polymeric materials synthesis, characterization and applications. Polyamide 6 by the anionic routes has been one of the major topics of his research. He is author of more than 250 scientific publications, mostly in international journals, and six patents. Prof Russo has been member of the Scientific Committee of INSTM (Interuniversity Consortium of Materiab Science and Technology) and director of its S Hon on Functional and Structural Polymeric Materials. He was the co- itor of four volumes of Comprehensive Polymer Science, Peigamon, 1989 and two supplement volumes (1992 and 1996). He was the organizer and co-chairman of two lUPAC Symposia of Free Radical Polymerization Kinetics and Mechanism, in 1987 and 1996. 

SKF 525-A did, however, depress the rate of biliary excretion of indocyanine green. This dye has a complex chemical structure, with one eat ionic and two anionic centers (see structure XXXIII). The liver seems to have independent mechanisms for the secretion of organic cations and anions (Schanker and Solomon, 1963). Thus it i.s possible that indocyanine green is excreted by both pathways, while succinylsulfathiazole and phenolphthalein glucuronide can be secreted only by the anionic route. SKF 525-A does not affect the biliarv excretion of these two anions,... 

In PMMA samples made by the anionic route, in which all the chain ends are saturated, degradation is initiated only by random scission and so the early TVA peak, with maximum at about 290-300 °C (Figure 4a) is not observed (Figure 4b). 

Thus a plot of a vs. t should be linear through the origin. This has been found to be the case for PS made by the anionic route (Figure 5). Samples prepared by free radical polymerization, however, give linear plots which do not pass through the origin. This indicates a relationship... 

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