Hydrogen iodide/iodine initiating system

M. Miyamoto, M. Sawamoto, and T. Higashimura, Living polymerization of isobutyl vinyl ether with hydrogen iodide/iodine initiating system. Macromolecules 1984, 77(3), 265-268. 

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 mechanism for the reaction is believed to be as shown in Eq. 15.170 (start with CH3OH, lower right, and end with CHjCOOH, lower left).180 The reaction can be initiated with any rhodium salt, e.g., RhCl3, and a source of iodine, the two combining with CO to produce the active catalyst, IRItfCO y. The methyl iodide arises from the reaction of methanol and hydrogen iodide. Note that the catalytic loop involves oxidative addition, insertion, and reductive elimination, with a net production of acetic acid from the insertion of carbon monoxide into methanol. The rhodium shuttles between the +1 and +3 oxidation states. The cataylst is so efficient that the reaction will proceed at atmospheric pressure, although in practice the system is... 

Kinetics in the irradiated system HI-NO have been studied by Holmes and Sundaram . They used 3130-3660 A radiation and a reaction cell temperature of 25 or 45 °C. Uranyl oxalate actinometry was employed. The photolysis of HI in this wavelength region produces hydrogen and iodine atoms which in turn react with either HI or NO. Holmes and Sundaram found that at 25 °C additions of NO significantly reduced the initial quantum yield of Hj. As the NO/HI ratio increased, the quantum yield fell to a limiting value. Additions of nitrogen to pure HI had no effect on the quantum yield. At 45 °C the reaction products were the same but the actinometry was irreproducible due to formation of ammonium iodide on the cell windows which reduced incident light intensities. 

This initiation route is more efficiently utilized by directly adding a mixture of hydrogen iodide and either iodine or a metal halide such as ZnX2 or SnX2 to the reaction system. 

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

One of the examples illustrating the living cationic polymerization process is the polymerization of alkyl-vinyl ethers initiated by the mixture of hydrogen iodide with iodine. In this process the system is stabilized by a suitably strong interaction of carbocation with counterion ... 

With an equimolar mixture of hydrogen iodide and iodine, IBVE was polymerized in n-hexane at —15 °C For the sake of comparison, polymerizations were also carried out using either hydrogen iodide or iodine alone otherwise the same conditions were employed. The reaction by hydrogen iodide is very slow (ca. 70% conversion in 30 days), but it is considerably accelerated in the presence of equimolar iodine (ca. 70% conversion in 3.5 hr). While the time-conversion curve with iodine alone shows an induction phase, that with the HI/I2 system does not, indicating a faster initiation by the latter initiator. 

Our recent study has indicated that adducts between vinyl ethers and hydrogen iodide [CHjCHfOR)—I R = alkyl], coupled with equimolar iodine, can induce living polymerization of vinyl ethers that is virtually identical to those initiated by the HI/I2 system. Therefore, the initiation mechanism for this initiator (Eq. (27) and (28)) needs investigating further... 

As a unique reaction with the Fenton system, the alkylation of heteroaromatics with alkyl iodide, hydrogen peroxide, and dimethyl sulfoxide in the presence of FeS04 can be carried out. This reaction comprises of the initial formation of reactive HO by the reaction of FeS04 and hydrogen peroxide, reaction of HO on the sulfur atom of dimethyl sulfoxide to form CH 3 and methanesulfinic acid (SH2 reaction), reaction of CH3 on the iodine atom of alkyl iodide via SH2 pathway to form more stable R and methyl iodide, and then addition of R to the a-position of y-picoline (1) to form an addition-intermediate radical which is rearomatized under oxidative conditions to 2-alkyl-4-methylpyridine (2) [15, 16]. 

Obviously small differences in initial concentrations of hydrogen peroxide (Figure 8.5) and temperatures (Figure 8.6) can perturb the previously established dynamic state significantly. The situation is very similar if control parameters are the concentrations of hydrogen ion, iodate, iodine, iodide, or some other species that interact with the reaction system although they are not intrinsic ones [4,8,17-22,33,34,44,45,51,53,54,70,77]. 

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