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. 

Tertiary nitrogen and iodine initially form a /i-o-complex, from which a strongly reactive iodine cation is produced this cation can bring about electrophilic substitutions on aromatic systems or cause oxidations [2]. 

Some Lewis acid-initiated polymerizations have been proposed to proceed by direct addition of the Lewis acid to the monomer s double bond. However, this is usually an exception, and has been clearly proven only for iodine [69,135] and boron halide [136,137] initiated systems. Iodi-nation and haloboration are reversible processes which produce deactivated alkyl halides due to the electron-withdrawing substituent at the neighboring carbon [Eq. (30)]. 

An example of the use of the tandem Staudinger/aza-Wittig reaction to form a seven-membered ring comes from the total synthesis of the alkaloid (-)-stemospironine (76).34 In this example, azidoaldehyde 74 was treated with triphenylphosphine to form a seven-membered cyclic imine which was then followed by an in situ sodium borohydride reduction to afford 75. Treatment of the resulting amine with iodine initiated the formation of the pyrrolidino butyrolactone system of the final target. 

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. 

In a procedure described in Ref. [474], sodium orthosilicate was added to the reaction medium to raise the polymer yield to 95%. The initiator was ammonium persulfate with a special emulsifier that is believed to be perfluorinated carboxylated emulsifier from 3M Corp [458]. Other initiators used are redox systems such as potassium persulfate-sodium metabisulfite or ammonium persulfate-sodium sulfite [475]. Different initiators are also of some interest because they affect the particle diameter of the emulsion. Polymer particles formed with a diameter between 0.36 and 18 pm, depending on the initiator system and the polymerization procedures, are reported [458]. In a patent of Dynamit Nobel [476], iodine-containing compounds such as ammonium iodide or isopropyl iodide are used. These compounds give rise to polymers of improved thermal stability and resistance to color deterioration. The procedure did not require the use of an emulsifier. After 150 min the internal pressure of the autoclave dropped from 2 x 10 to 3 X 10 Pa and a conversion to polymer of 86% was found. More recently Uschold [477] describes also an emulsion polymerization of PVF in high yields and having excellent color. 

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