Calcium Initiators

Early studies using calcium oxide, carbonate, and carboxylates reported low activities for the polymerization of LA, even in bulk at 120-180 °C.827,828 PolyGA and copolymers of GA with CL and L-LA have been prepared using Ca(acac)2, but again high temperatures (150-200 °C) are required.829 Under these conditions transesterification occurs, although to a lesser extent than in analogous Sn(Oct)2-initiated polymerizations. 

Higher activities may be obtained if the bis(alkoxide) is generated in situ. For example, addition of 100 equivalents L-LA to a 1 2 mixture of Ca(NTMS2)2 and MeOH in THF generates PLA to 97% conversion in 18 min at room temperature (c.f. 66% conversion after 90 min in bulk at 120 °C using commercial Ca(OMe)2).830 Furthermore, no epimerization is observed. Polydispersities are narrow (1.1), with Mn values slightly higher than those predicted from the monomer initiator 

If mono-hydroxyl functionalized polyethylene glycol), HO-PEG, is added to Ca(NTMS2)2.THF2, then addition of LA affords the diblock PEG-b-PLA (Mn= 15,500, Mn calc = 15,500, Mw/Mn = 1.03).832 Using a similar strategy the reaction of CaFI2 with telechelic diol HO-(PEG)-OH, followed by polymerization of L-LA results in a triblock structure, PLA-b-PEG-b-PLA of narrow polydispersity (1.02-1.08).835 836 Triblock copolymers of morpholine-2,5-diones with PEO have also been prepared in this manner.837 

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Finally, an increase in volume or pressure within a tube or hollow organ causes stretch or distortion of the smooth muscle in the organ wall. This may cause activation of stretch-activated Ca++ channels. The subsequent influx of calcium initiates contraction of the smooth muscle. This process is referred to as myogenic contraction and is common in blood vessels. 

Similarly, a turn-over frequency (TON) of 227 of the polymerization process was distinctly low for 77d with [M]/[I] = 350, at 110 °C for 6 h, using in the melt polymerization conditions. Biocompatible calcium complex 77a used as catalyst at 110 °C produced in 30 min PLAs with high molecular weight (65,000-110,600) and narrow polydispersities (1.02-1.05) using [M]/[I] = 350-700. It is worthy of note that complex 77a displayed a notable heteroselectivity (probability of racemic linkages between monomers, = 0.73, see Sect. 4.2) in polymerization of rac-lactide in THF at 33 °C. Data on the aforementioned calcium initiators and their lactide polymerization are listed in Table 4. 

Calcium transfer observed in chick embryo may also be controlled by a similar mechanism since the newly hatched chick has six times the amount of calcium initially present in the egg yolk. In this case the egg shell is the bound Ca2+ pool. It is not clear to what extent calcium-activated ATPase or a calcium-binding protein carrier is involved in active transcellular transport of calcium277,278. ... 

Many studies show that divalent cations promote membrane fusion (27, 28, 29) and thereby may initiate attachment of granules to the insides of plasma membranes during secretion. Actually these ideas (i.e. enzyme activation and fusion of lipoid membranes by calcium), are not mutually exclusive since it is possible that calcium initiates more than one intracellular change to trigger the secretory process. 

At low tin contents, the calcium initially precipitates rapidly as PbsCa in the same manner as in binary lead-calcium alloys. These alloys have a fine grain structure and quickly reach high hardness as discontinuous precipitation predominates. At higher tin contents, the mode of precipitation changes to a mixed discontinuous precipitation of PbsCa followed by a continuous precipitation reaction of Pb cSn Ca. These reactions lead to overageing of the precipitates and a decrease in mechanical properties. 

It was shown that the calcium chloride solutions decreases with an increase in the excess in the sedimenter and the temperature of the reaction mixture. In the solution with the higho calcium initial concentration, the pH value of the suspension is accordingly greater. 

Chisholm, M. H. Gallucci, J. C. Phomphrai, K. Well-defined calcium initiators for lactide polymerization. Inorg. Chem. 2004, 43, 6717-6725. 

Two examples of molecular calcium initiators supported by dianionic multi-dentate ONNO ligands were reported. In 2006, Bochmann and coworkers prepared the dimeric [ ONNO Ca]2 (73) (Fig. 23) [95], They used it as a singlecomponent initiator to promote the sluggish but pseudo-living ROP of CL, reaching half-conversion of 200 equiv of monomer after 12 h at 60 °C in toluene (Mw/M 1.1-1.2) the nature of the mechanism was not discussed. The related Ca homoleptic complex ON 2Ca(THF)2 (74) containing two imino-phenolate ligands NO was inactive towards CL under the same conditions. 

Calcium initiators are much weaker than the aluminum or zinc systems in forming oxirane complexes. As a result, they are particularly effective in polymerizing EO, which forms complexes more readily in comparison with the substituted oxiranes. Calcium initiators form weaker complexes with the substituted oxiranes, PO in particular, and initiate a low-rate polymerization. 

Qualitative examples abound. Perfect crystals of sodium carbonate, sulfate, or phosphate may be kept for years without efflorescing, although if scratched, they begin to do so immediately. Too strongly heated or burned lime or plaster of Paris takes up the first traces of water only with difficulty. Reactions of this type tend to be autocat-alytic. The initial rate is slow, due to the absence of the necessary linear interface, but the rate accelerates as more and more product is formed. See Refs. 147-153 for other examples. Ruckenstein [154] has discussed a kinetic model based on nucleation theory. There is certainly evidence that patches of product may be present, as in the oxidation of Mo(lOO) surfaces [155], and that surface defects are important [156]. There may be catalysis thus reaction VII-27 is catalyzed by water vapor [157]. A topotactic reaction is one where the product or products retain the external crystalline shape of the reactant crystal [158]. More often, however, there is a complicated morphology with pitting, cracking, and pore formation, as with calcium carbonate [159]. 

Both forms sublime very readily, even at room temperature a small sample on exposure to the air will completely volatilise in a short time, particularly on a warm day or if the sample is exposed to a gentle current of air. Hence the above method for rapid drying. A sample confined in an atmospheric desiccator over calcium chloride rapidly disappears as the vapour is adsorbed by the calcium chloride. A sample of the hexahydrate similarly confined over sodium hydroxide undergoes steady dehydration with initial liquefaction, for the m.p. of the hydrated-anhydrous mixture is below room temperature as the dehydration proceeds to completion, complete resolidification occurs. 

Assemble in a fume-cupboard the apparatus shown in Fig. 67(A). Place 15 g. of 3,5-dinitrobenzoic acid and 17 g. of phosphorus pentachloride in the flask C, and heat the mixture in an oil-bath for hours. Then reverse the condenser as shown in Fig. 67(B), but replace the calcium chloride tube by a tube leading to a water-pump, the neck of the reaction-flask C being closed with a rubber stopper. Now distil off the phosphorus oxychloride under reduced pressure by heating the flask C in an oil-bath initially at 25-30, increasing this temperature ultimately to 110°. Then cool the flask, when the crude 3,5-dinitro-benzoyl chloride will solidify to a brown crystalline mass. Yield, 16 g., i.e,y almost theoretical. Recrystallise from caibon tetrachloride. The chloride is obtained as colourless crystals, m.p. 66-68°, Yield, 13 g Further recrystallisation of small quantities can be performed using petrol (b.p. 40-60°). The chloride is stable almost indefinitely if kept in a calcium chloride desiccator. 

Refractionation of the low-boiling impurities gives a further quantity of the acetoacetate, but if the initial distillation has been carefully conducted, the amount recovered is less than i g., and the refractionation is not worth while. If possible, complete the preparation in one day. If this is not possible, it is best to allow the cold crude sodium derivative (before acidification) to stand overnight, the flask being closed by a cork carrying a calcium chloride tube the yield will now fall to about 38 g. Alternatively, the crude ester may be allowed to remain overnight in contact with the sodium sulphate, but in this case the yield will fall to about 30 g. 

Conduct the preparation in the fume cupboard. Dissolve 250 g. of redistilled chloroacetic acid (Section 111,125) in 350 ml. of water contained in a 2 -5 litre round-bottomed flask. Warm the solution to about 50°, neutralise it by the cautious addition of 145 g. of anhydrous sodium carbonate in small portions cool the resulting solution to the laboratory temperature. Dissolve 150 g. of sodium cyanide powder (97-98 per cent. NaCN) in 375 ml. of water at 50-55°, cool to room temperature and add it to the sodium chloroacetate solution mix the solutions rapidly and cool in running water to prevent an appreciable rise in temperature. When all the sodium cyanide solution has been introduced, allow the temperature to rise when it reaches 95°, add 100 ml. of ice water and repeat the addition, if necessary, until the temperature no longer rises (1). Heat the solution on a water bath for an hour in order to complete the reaction. Cool the solution again to room temperature and slowly dis solve 120 g. of solid sodium hydroxide in it. Heat the solution on a water bath for 4 hours. Evolution of ammonia commences at 60-70° and becomes more vigorous as the temperature rises (2). Slowly add a solution of 300 g. of anhydrous calcium chloride in 900 ml. of water at 40° to the hot sodium malonate solution mix the solutions well after each addition. Allow the mixture to stand for 24 hours in order to convert the initial cheese-Uke precipitate of calcium malonate into a coarsely crystalline form. Decant the supernatant solution and wash the solid by decantation four times with 250 ml. portions of cold water. Filter at the pump. 

Initial Run. - Into each of seven stoppered bottles was placed a mixture of ethyl sulphate [Et0-S02-0Et] (120 g.) and sodium nitrite [NaNOJ solution (120 g. in 160 c.c. of water.) The bottles were shaken mechanically for 20 hours, the pressure being released at intervals. The contents were then poured into a separating funnel, and the upper layer separated, dried over calcium chloride and distilled at 14mm., the distillate up to 60° being col-... 

Other Plastics Uses. The plasticizer range alcohols have a number of other uses in plastics hexanol and 2-ethylhexanol are used as part of the catalyst system in the polymerization of acrylates, ethylene, and propylene (55) the peroxydicarbonate of 2-ethylhexanol is utilized as a polymerization initiator for vinyl chloride various trialkyl phosphites find usage as heat and light stabHizers for plastics organotin derivatives are used as heat stabHizers for PVC octanol improves the compatibHity of calcium carbonate filler in various plastics 2-ethylhexanol is used to make expanded polystyrene beads (56) and acrylate esters serve as pressure sensitive adhesives. 

The most common catalysts are sodium hydroxide and calcium hydroxide, generally used at a modest excess over the nominal stoichiometric amount to avoid formaldehyde-only addition reactions. Calcium hydroxide is cheaper than NaOH, but the latter yields a more facile reaction and separation of the product does not require initial precipitation and filtration of the metal formate (57). 

There are numerous variations of the wet process, but all involve an initial step in which the ore is solubilized in sulfuric acid, or, in a few special instances, in some other acid. Because of this requirement for sulfuric acid, it is obvious that sulfur is a raw material of considerable importance to the fertilizer industry. The acid—rock reaction results in formation of phosphoric acid and the precipitation of calcium sulfate. The second principal step in the wet processes is filtration to separate the phosphoric acid from the precipitated calcium sulfate. Wet-process phosphoric acid (WPA) is much less pure than electric furnace acid, but for most fertilizer production the impurities, such as iron, aluminum, and magnesium, are not objectionable and actually contribute to improved physical condition of the finished fertilizer (35). Impurities also furnish some micronutrient fertilizer elements. 

Contraction of muscle follows an increase of Ca " in the muscle cell as a result of nerve stimulation. This initiates processes which cause the proteins myosin and actin to be drawn together making the cell shorter and thicker. The return of the Ca " to its storage site, the sarcoplasmic reticulum, by an active pump mechanism allows the contracted muscle to relax (27). Calcium ion, also a factor in the release of acetylcholine on stimulation of nerve cells, influences the permeabiUty of cell membranes activates enzymes, such as adenosine triphosphatase (ATPase), Hpase, and some proteolytic enzymes and facihtates intestinal absorption of vitamin B 2 [68-19-9] (28). 

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