Potassium initiation

A polymer similar to natural hevea rubber is obtained using the anionic polymerization of isoprene under these conditions. In more polar solvents employing sodium and potassium initiators the amount of cA-1,4 units decreases and trans-, A and trans- i,4 units predominate. 

Anionic ring-opening polymerization of l,2,3,4-tetramethyl-l,2,3,4-tetraphenylcyclo-tetrasilane is quite effectively initiated by butyllithium or silyl potassium initiators. The process resembles the anionic polymerization of other monomers where solvent effects play an important role. In THF, the reaction takes place very rapidly but mainly cyclic live- and six-membered oligomers are formed. Polymerization is very slow in nonpolar media (toluene, benzene) however, reactions are accelerated by the addition of small amounts of THF or crown ethers. The stereochemical control leading to the formation of syndiotactic, heterotactic or isotactic polymers is poor in all cases. In order to improve the stereoselectivity of the polymerization reaction, more sluggish initiators like silyl cuprates are very effective. A possible reaction mechanism is discussed elsewhere49,52. 

On the basis of the profound difference in copolymer composition from a free radical or cationic type polymerization, it was stated that the sodium and potassium initiated polymerizations were carbanionic in nature. This has been one of the strongest arguments in favor of the anionic nature of the sodium and potassium polymerizations. The authors also suggested that the composition of a styrene-methyl methacrylate copolymer might be used as a criterion of the type of propagation induced by a given initiator. 

Potassium and rubidium alkyls behave alike when used as polymerization initiators (5) n-amylpotassium, n-octylpotassium, n-hexadecylpotassium, and benzylpotassium all produce tactic polystyrene. However, an astonishing fact is that to achieve the same stereospecificity, the polymerization in n-hexane must be conducted at a temperature of —60° to —70° C. with potassium initiators in the case of dodecylrubidium, a lower temperature of —80° C. is necessary, whereas with n-amylsodium a temperature of only —20° C. is required. The decrease in stereospecificity runs parallel with the increase in polymerization rate as temperature rises. 

It has been proposed that the acid-base nature of catalyst and support can modify the adsorption/desorption of reactants and products [2,3,5]. In this way, and according to both the catalytic and the physicochemical properties, it can be concluded that the incorporation of potassium initially increases the selectivity to propylene as a consequence of lowering the numberof acid sites. 

The extraction of the ash with single water washes revealed that significant quantities of the potassium could be easily dissolved (generally in excess of 80%). This agrees with the identification of K2C03 and K2C03 1%H20 as the predominant potassium compounds that are extremely soluble in water. By evaporating the water wash to dryness, crystalline hydrated and anhydrous potassium carbonate were obtained. This recovered product contained about 75% of the potassium initially in the ash. 

Li " > Na " > K ". The polymerization by the potassium initiator under certain conditions gave the pdymer rich in heterotactidty as described later. With solvent having h her dielectric constant such as hexamethyiphoi horic triamide the micro-structure of the polynsr was not affected by the nature of counter ions and rather qrndiotactic. ... 

The homopolymer, prepared by polymerization in liquid ammonia with sodium initiator at-77 °C, is insoluble in acetone, but it is soluble in dimethylformamide. When it is formed with lithium in liquid ammonia, at -75 °C, the molecular weight of the product increases with monomer concentration and decreases with initiator concentration. If, however, potassium initiates the reaction rather than lithium, the molecular weight is independent of the monomer concentration. " " Polymethaciylonitrile prepared with n-butyllithium in toluene or in dioxane is crystalline and insoluble in solvents like acetone. When polymerized in petroleum ether with /i-butyllithium, methacrylonitrile forms a living polymer. Highly crystalline polymethacrylonitrile can also be prepared with beryllium and magnesium alkyls in toluene over a wide range of temperatures. 

Star-block copolymers (173) have been synthesized by independently preparing living polystyryl using a 1-phenylethyl-potassium initiator and poly(ethylene oxide) with the same initiator. A four-arm star-block is prepared by first reacting polystyrylpotassium with silicon tetrachloride at a polystyrylpotassium-to-silicon tetrachloride mole ratio of 2 1. That product is then sequentially reacted with living poly(ethylene oxide) again at a 2 1 mole ratio. A three-arm star can be prepared by a similar procedure with trichloromethylsilane in place of silicon tetrachloride. 

Figure A3.10.19 Variation of the initial sticking codFicient of N2 with increasing potassium surface concentration on Fe(lOO) at 430 K [50],...

Transfer the quinoline chlorozincate to a beaker, add a small quantity of water, and then add 10% sodium hydroxide solution until the initial precipitate of zinc hydroxide completely redissolves, and the free quinoline separates. Transfer the mixture to a separating-funnel, wash out the beaker with ether, adding the washings also to the solution in the funnel, and then extract the quinoline twice with ether. Dry the united ethereal extracts by adding an ample quantity of powdered potassium hydroxide and... 

Acetamido-4-methylselenazole can react with mercuric acetate to yield 5-mercuriacetate derivatives that can be converted to the chloro derivatives by the action of sodium chloride. Treatment with potassium iodide leads to reduction regenerating the initial compound with loss of mercury (Scheme 16) (4). 

Bromophenol blue 2, 7 -DichIorofluorescein Eosin, tetrabromofluorescein Fluorescein Potassium rhodizonate, C404(0K)2 Rhodamine 6G Sodium 3-aIizarinsuIfonate Thorin Dissolve 0.1 g of the acid in 200 mL 95% ethanol. Dissolve 0.1 g of the acid in 100 mL 70% ethanol. Use 1 mL for 100 mL of initial solution. See Dichlorofluorescein. Dissolve 0.4 g of the acid in 200 mL 70% ethanol. Use 10 drops. Prepare fresh as required by dissolving 15 mg in 5 mL of water. Use 10 drops for each titration. Dissolve 0.1 g in 200 mL 70% ethanol. Prepare a 0.2% aqueous solution. Use 5 drops per 120 mL endpoint volume. Prepare a 0.025% aqueous solution. Use 5 drops. 

Potassium bifluoride, KE HE, is used as a raw material to charge the cells initially and for makeup when cells are rebuilt. A newly charged cell requires about 1400 kg KE HE. Overall consumption of KE HE per kilogram of fluorine generated is small. Gommercial-grade flake potassium bifluoride is acceptable. Its specifications are... 

Difluoropyridines. 2,4-Difluoropyridine can be prepared (26% yield) from 2,4-dichloropyridine and potassium fluoride in sulfolane and ethylene glycol initiator (403). The 4-fluorine is preferentially replaced by oxygen nucleophiles to give 2-fluoro-4-hydroxypyridine derivatives for herbicidal apphcations (404). 

Conduction furnaces utilize a Hquid at the operating temperature to transfer the heat from the heating elements to the work being processed. Some furnaces have a pot filled with a low melting metal, eg, lead, or a salt mixture, eg, sodium chloride and potassium chloride, with a radiation-type furnace surrounding the pot. Although final heat transfer to the work is by conduction from the hot lead or salt to the work, the initial transfer of heat from the resistors to the pot is by radiation. 

Other Borohydrides. Potassium borohydride was formerly used in color reversal development of photographic film and was preferred over sodium borohydride because of its much lower hygroscopicity. Because other borohydrides are made from sodium borohydride, they are correspondingly more expensive. Generally their reducing properties are not sufficiently different to warrant the added cost. Zinc borohydride [17611-70-0] Zn(BH 2> however, has found many appHcations in stereoselective reductions. It is less basic than NaBH, but is not commercially available owing to poor thermal stabihty. It is usually prepared on site in an ether solvent. Zinc borohydride was initially appHed to stereoselective ketone reductions, especially in prostaglandin syntheses (36), and later to aldehydes, acid haHdes, and esters (37). 

These reactions are usehil for the preparation of homogeneous difunctional initiators from a-methylstyrene in polar solvents such as tetrahydrofuran. Because of the low ceiling temperature of a-methylstyrene (T = 61° C) (26), dimers or tetramers can be formed depending on the alkaU metal system, temperature, and concentration. Thus the reduction of a-methylstyrene by sodium potassium alloy produces the dimeric dianionic initiators in THF (27), while the reduction with sodium metal forms the tetrameric dianions as the main products (28). The stmctures of the dimer and tetramer correspond to initial tail-to-tail addition to form the most stable dianion as shown in equations 6 and 7 (28). 

GopolymeriZation Initiators. The copolymerization of styrene and dienes in hydrocarbon solution with alkyUithium initiators produces a tapered block copolymer stmcture because of the large differences in monomer reactivity ratios for styrene (r < 0.1) and dienes (r > 10) (1,33,34). In order to obtain random copolymers of styrene and dienes, it is necessary to either add small amounts of a Lewis base such as tetrahydrofuran or an alkaU metal alkoxide (MtOR, where Mt = Na, K, Rb, or Cs). In contrast to Lewis bases which promote formation of undesirable vinyl microstmcture in diene polymerizations (57), the addition of small amounts of an alkaU metal alkoxide such as potassium amyloxide ([ROK]/[Li] = 0.08) is sufficient to promote random copolymerization of styrene and diene without producing significant increases in the amount of vinyl microstmcture (58,59). 

Seaweeds. The eadiest successful manufacture of iodine started in 1817 using certain varieties of seaweeds. The seaweed was dried, burned, and the ash lixiviated to obtain iodine and potassium and sodium salts. The first process used was known as the kelp, or native, process. The name kelp, initially apphed to the ash of the seaweed, has been extended to include the seaweed itself. About 20 t of fresh seaweed was used to produce 5 t of air-dried product containing a mean of 0.38 wt % iodine in the form of iodides of alkah metals. The ash obtained after burning the dried seaweed contains about 1.5 wt % iodine. Chemical separation of the iodine was performed by lixiviation of the burned kelp, followed by soHd-Hquid separation and water evaporation. After separating sodium and potassium chloride, and sodium carbonate, the mother Hquor containing iodine as iodide was treated with sulfuric acid and manganese dioxide to oxidize the iodide to free iodine, which was sublimed and condensed in earthenware pipes (57). 

The most common water-soluble initiators are ammonium persulfate, potassium persulfate, and hydrogen peroxide. These can be made to decompose by high temperature or through redox reactions. The latter method offers versatility in choosing the temperature of polymerization with —50 to 70°C possible. A typical redox system combines a persulfate with ferrous ion ... 

The ionic nature of the radicals generated, by whatever technique, can contribute to the stabilisation of latex particles. Soapless emulsion polymerisations can be carried out usiag potassium persulfate as initiator (62). It is often important to control pH with buffets dutiag soapless emulsion p olymerisation. 

The polyalkylene glycol polymer employs a starter that consists of a relatively reactive alcohol and a smaller amount of its potassium or sodium salt. With propylene oxide, for instance, initiation of the polymeri2ation then involves the starter in the following steps ... 

Manganate(VI) formed in the initial oxidation process must first be dissolved in a dilute solution of potassium hydroxide. The concentrations depend on the type of electrolytic cell employed. For example, the continuous Cams cell uses 120 150 g/L KOH and 50 60 g/L K MnO the batch-operated Bitterfeld cell starts out with KOH concentrations of 150 160 g/L KOH and 200 220 g/L K MnO. These concentration parameters minimize the disproportionation of the K MnO and control the solubiUty of the KMnO formed in the course of electrolysis. 

Other early match-like devices were based on the property of various combustible substances mixed with potassium chlorate to ignite when moistened with strong acid. More important was the property of chlorates to form mixtures with combustibles of low ignition point which were ignited by friction (John Walker, 1827). However, such matches containing essentially potassium chlorate, antimony sulfide, and later sulfur (lucifers), mbbed within a fold of glass powder-coated paper, were hard to initiate and unreHable. 

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