Initiators for cationic polymerization

Yagci, Y. and Endo, X N-Benzyl and N-Alkoxy Py ridium Salts as Thermal and Photochemical Initiators for Cationic Polymerization. Vol. 127, pp. 59-86. 

Developments in the Theory of Cationic Polymerization VII. Theoretical Attempts at Improving Initiators for Cationic Polymerization of Alkenes, S.D. Pask and P.H. Plesch. European Polymer Journal, 1982, 18, 839-846. 

Some common initiators for cationic polymerization reactions are protonic acids, Friedel-Crafts catalysts (Lewis acids), compounds capable of generating cations, or ionizing radiation. 

Photo-initiators for cationic polymerization may be classified in four groups the diazonium salts, the diaryliodonium salts, the triarylsulphonium salts, and the mixed ligand arene cyclopentadienyl Fe11 salts. 

Use of triphenylmethyl and cycloheptatrienyl cations as initiators for cationic polymerization provides a convenient method for estimating the absolute reactivity of free ions and ion pairs as propagating intermediates. Mechanisms for the polymerization of vinyl alkyl ethers, N-vinylcarbazole, and tetrahydrofuran, initiated by these reagents, are discussed in detail. Free ions are shown to be much more reactive than ion pairs in most cases, but for hydride abstraction from THF, triphenylmethyl cation is less reactive than its ion pair with hexachlorantimonate ion. Propagation rate coefficients (kP/) for free ion polymerization of isobutyl vinyl ether and N-vinylcarbazole have been determined in CH2Cl2, and for the latter monomer the value of kp is 10s times greater than that for the corresponding free radical polymerization. 

Touring the past five years we have shown that many stable organic cations are useful initiators for polymerizing reactive olefins and cyclic ethers (I, 2, 3, 4, 5, 27). Compared with more common initiators for cationic polymerization (31), the stable salts allow for complete characterization of the catalyst system and give rapid, highly reproducible polymerizations. In addition many of the salts used are stable indefinitely in the crystalline state, which makes the experimental techniques easy and convenient. 

V-Benzyl and iV-Alkoxy Pyridinium Salts as Thermal and Photochemical Initiators for Cationic Polymerization... 

Table 1. The externally stimulated initiators for cationic polymerization...

N-Benzyl and iV-alkoxy pyridinium salts are suitable thermal and photochemical initiators for cationic polymerization, respectively. Attractive features of these salts are the concept of latency, easy synthetic procedures, their chemical stability and ease of handling owing to their low hygroscopicity. Besides their use as initiators, the applications of these salts in polymer synthesis are of interest. As shown in this article, a wide range of block and graft copolymer built from monomers with different chemical natures are accessible through their latency. 

Diazonium Salts. The first use of a diazonium salt as an initiator for cationic polymerization of cyclic ethers was reported in 1965 by Dreyfuss and Dreyfuss (1 ), who announced the polymerization of tetrahydrofuran initiated by thermal decomposition of a "new catalyst" which they identified as benzene-diazonium hexafluorophosphate. In a later publication (13) this material was correctly identified as p-chlorobenzenediazomum hexaf1uorophosphate). 

The Schieman reaction (J ) was cited by Schlesinger (j ) and by Licari and Crepeau (14) as the probable mechanism by which initiators for cationic polymerization are released by diazonium salts. Rutherford et al ( ) showed side reactions to be minimized when the diazonium salt is a hexafluorophosphate. 

In recent years, research at this laboratory has centered about the development of new Initiators for cationic polymerization. Among the most Interesting and useful of these Initiators are dlaryllodonlum salts whose structure Is shown below. 

Mengoli and Vidotto in 197158 proposed the use of the electrogenerated radical cation of 9,10-diphenylanthracene as initiator for cationic polymerization of styrene and n-butyl vinyl ether (NBVE). 

Y. Yagci and I. Reetz, Addition fragmentation type initiators for cationic polymerization. Macromol. Symp. 1998, 732, 153-164. 

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