Iodine, as initiator

The observation of rather similar bimodal distributions in polymers prepared using iodine as initiator (164) tends to confirm this idea, since contributions from covalent iodides are quite likely. 

Rate coefficients for propagation at 30 C, with molecular iodine as initiator [62-65]. 

Scheme 8.10 Mechanism for the hving cationic polymerization of vinyl ethers with hydrogen iodide and iodine as initiating system.

Sawamoto et al. [448] used hydrogen iodide as initiator for the living cationic polymerization of NVK in toluene and dichloromethane solution. In dichloromethane with added tetrabutylammonium iodide at —78°C, the polymerization is in a living fashion, demonstrated by the linear dependence of the polymer molecular weight from the monomer/initiator ratio. The polymers had a molecular weight distribution (A/w/Mn= 1.2) markedly narrower than that for iodine as initiator (Mw/M =1.5). S. Oh [449] used the initiator system l-iodo-l-(2-methylpropyloxy)-ethane/tetrabutyl-ammoniumperchlorate/tetrabutylammonimn iodide at —25 °C in dichloromethane and got PVK with very narrow molecular weight distribution (Mw/M < 1.15) as well. 

The above described results demonstrate that, given a suitably strong interaction between the propagating carbocation and its counteranion, a broad range of vinyl monomers yield polymers with bimodal MWD. AD these polymers have been formed by 0X0 acids and iodine as initiators the formation of polymers with a bimodal MWD is one of the important characteristics of the polymerizations by non-MX initiators. It should therefore be reasonable and useful to classify cationic initiators into two categories, MX and non-MX types, according to the structure of the counteranions... 

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). 

A unique problem arises when reducing the fissile isotope The amount of that can be reduced is limited by its critical mass. In these cases, where the charge must be kept relatively small, calcium becomes the preferred reductant, and iodine is often used as a reaction booster. This method was introduced by Baker in 1946 (54). Researchers at Los Alamos National Laboratory have recently introduced a laser-initiated modification to this reduction process that offers several advantages (55). A carbon dioxide laser is used to initiate the reaction between UF and calcium metal. This new method does not requite induction heating in a closed bomb, nor does it utilize iodine as a booster. This promising technology has been demonstrated on a 200 g scale. 

The reaction generally starts without addition of iodine as an initiator, but the use of a crystal of iodine (no stirring) may occasionally be necessary with old magnesium or insufficiently... 

The polymerization of 1,3,3-trimethyl-2,7-dioxabicyclo[2.2.1 Jheptane 35 was carried out in methylene chloride, toluene, and 1-nitropropane at temperatures between —78 and 0 °C32l Boron trifluoride etherate, triethyloxonium tetrafluoro-borate, antimony pentachloride, and iodine were used as initiators. Irrespective of the solvents and initiators employed, the products obtained at 0 °C were white powders with melting points of 50—55 °C, while those obtained at tower temperatures were sirups. The number average molecular weight of the unfractionated products ranged from 400 to 600. The molecular weight distribution of the oligomers prepared at 0 °C was broad, in contrast to the relatively narrow distribution of those obtained at -40 °C. 

This reaction undergoes conversion in one sequence of consecutive elementary reaction steps and so only one propagating front is formed in a spatially distributed system [68]. Depending on the initial ratio of reactants, iodine as colored and iodide as uncolored product, or both, are formed [145]. 

However, there are also many systems in which the evidence indicates that the propagating species cannot be a carbenium ion. Such reactions have been termed pseudo-cationic and in these polymerisations the propagating species is believed to be an ester. The most thoroughly investigated systems comprise aromatic monomers (styrene, acenaphthylene [11]) and protonic acids (HC104) or iodine [11] as initiators. The simplest representation of the propagation is as the addition of the ester (stabilised by four styrene molecules) across the double-bond of the monomer [12] ... 

This procedure is based on a report by Fessenden and Fessenden.2 Cuprous chloride3 is a more efficient initiator than iodine as specified in the original procedure. 

If the solution is titrated with standard sodium thiosulphate solution, the total concentration of the iodine, both as free I2 and combined as IJ, is obtained, since, as soon as some iodine is removed by interaction with the thiosulphate, a fresh amount of iodine is liberated from the tri-iodide in order to maintain the equilibrium. If, however, the solution is shaken with carbon tetrachloride, in which iodine alone is appreciably soluble, then the iodine in the organic layer is in equilibrium with the free iodine in the aqueous solution. By determining the concentration of the iodine in the carbon tetrachloride solution, the concentration of the free iodine in the aqueous solution can be calculated from the known distribution coefficient, and therefrom the total concentration of the free iodine present at equilibrium. Subtracting this from the total iodine, the concentration of the combined iodine (as Ij) is obtained by subtracting the latter value from the initial concentration of potassium iodine the concentration of the free KI is deduced. The equilibrium constant ... 

Thus, the terms initiator and coinitiator, as well as catalyst and cocatalyst, must be clearly distinguished. As proposed earlier [68], an initiator is consumed in the initiation process whereas a catalyst remains unchanged during the polymerization. In the polymerization of alkenes initiated directly by Lewis acids (e.g., iodine initiated polymerization of vinyl ethers) [69], the Lewis acid plays both roles. Nevertheless, Lewis acids usually act only as catalysts rather than as initiators, with protonogenic compounds such as adventitious moisture being the initiator. 

Polymerization of vinyl monomers 6.1 MOLECULAR IODINE AS AN INITIATOR... 

Silphenylenes, aromatic-containing silicon monomers, can be produced in 50 88% yields by the reaction of halo- and dihalobenzenes with chloro- and alkoxysilanes under Barbier conditions. Iodine, bromine, and cuprous chloride are used as initiators with THE, TMEDA, toluene, or xylene as solvents. Shavings or powdered magnesium can be used [Eqs. (66) and (67) 134,135]. 

The degradation of carboxylic acids to alkyl halides using mercuric oxide and halogens involves the initial formation of the mercuric salt of the acid, followed by a normal Hunsdiecker reaction of the salt with halogen. The relative insensitivity of the reaction to water is a consequence of the solubility of the mercury salts in the solvent (CC14). There are two limitations tertiary acids are not degraded, and use of iodine as the halogen frequently leads to the ester RCOOR as the major product. The yields in the modified reaction are usually lower than those obtained with the silver salt method.2... 

Ingots 15 mm in diameter and 80 mm in length (80-100 g weight) were arc-melted in a vacuum of 10 Pa. Iodine titanium, zirconium, technically pure aluminum and silicon were used as initial materials. 

Minisci has reported an interesting iodination reaction involving perfluoroalkyl iodides [103], For instance, cyclohexane is cleanly iodinated when treated with per-fluorobutyl iodide in acetic acid with /err-butylhydroperoxide as initiator (Scheme... 

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