Initiators in cationic polymerization

The mechanism of initiation in cationic polymerization using Friedel-Crafts acids appeared to be clarified by the discovery that most Friedel-Crafts acids, particularly haUdes of boron, titanium, and tin, require an additional cation source to initiate polymerization. Evidence has been accumulating, however, that in many systems Friedel-Crafts acids alone are able to initiate cationic polymerization. The polymerization of isobutylene for instance can be initiated, reportedly even in the absence of an added initiator, by AlBr or AlCl (19), TiCl ( )- Three fundamentally different... 

Preformed or in situ-prepared carbocation salts (tropylium, trityl, etc.) are also active in transforming alkenes to carbocations.119,138,140 Preformed carbocation salts are the simplest initiators in cationic polymerization and ideal if the cation is identical to the one derived from the momomer (e.g., fert-butyl cation in the polymerization of isobutylene). 

Uncomplexed acrylates do not copolymerize with ethylene only polyacryla-les (e.g. polyethyl acrylate) are formed. BF2 is a strong Lewis acid used as an initiator in cationic polymerizations. With acrylic monomers it forms a complex which can be relatively easily copolymerized with ethylene and propene. This process is similar to radical reactions it requires an initiator [96],... 

Mechanism and kinetics of cationic poiymerization initiation. Unlike free-radical and anionic polymerization, initiation in cationic polymerization employs a true catalyst that is restored at the end of the polymerization and does not become incorporated into the terminated polymer chain. Initiation of cationic polymerization is brought about by addition of an electrophile to a monomer molecule. TVpical compounds used for cationic polymerization include protonic acids (e.g., H2SO4, H3PO4), Lewis acids (e.g., AICI3, BF3, TiCl4, SnCl4), and stable carbenium-ion salts (e.g., triphenylmethyl halides, tropylium halides) ... 

Cationic Polymerization. The initiators in cationic polymerization are acidic among them are listed aluminum chloride, boron trifluoride, certmh activated clays, and sulphuric acid. These compounds are all strong electron acceptors and are particularly effective when the monomer is an electron donor. The polymerizations proceed rapidly at low temperatures for instance, isobutylene polymerizes iminediat at —100°C in the presence of aluminum chloride. 

Table 2. Relative rates of initiation in cationic polymerizations initiated by covalent initiators in the presence of a Lewis acid catalyst [113].

Explain what is meant by the expression that initiation in cationic polymerization results from transposition of either one or two electrons. What type of initiating species are involved ... 

J. P. Kennedy, Cationic polymerization, in Macromolecular Science (Vol. 8 of Physical Chemistry Series 1), C. E. H. Bawn (ed.), MTP International Review of Science, 1972, p. 49. J. P. Kennedy, Self-initiation in cationic polymerization, J. Macromol Sci. (Chem.) A6, 329... 

Rissoan, G., Randriamahefa, S., Cheradame, H. Heterogeneous Cationic Polymerization Initiators, In Cationic Polymerization Faust R-> Shaffer, T. D., Eds., American Chemical Society Washington, D.C., 1997 pp. 135-150. 

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