Free-radical-promoted cationic polymerization

Free radical promoted, cationic polymerization also occurs upon irradiation of pyridinium salts in the presence of acylphosphine oxides. But phosphonyl radicals formed are not oxidized even by much stronger oxidants such as iodonium ions as was demonstrated by laser flash photolysis studies [51, 52]. The electron donor radical generating process involves either hydrogen abstraction or the addition of phosphorus centered or benzoyl radicals to vinyl ether monomers [53]. Typical reactions for the photoinitiated cationic polymerization of butyl vinyl ether by using acylphosphine oxide-pyridinium salt combination are shown in Scheme 10. 

Free radical promoted cationic polymerization was successfully employed [77] for the preparation of new classes of liquid crystalline (LC) block copolymers comprising a semicrystalline block, poly(cyclohexene oxide), and LC block of different structures ... 

This process is usually termed as the free radical promoted cationic polymerization. This so-called free radical promoted cationic polymerization is an excellent and fairly flexible type of indirect initiation of cationic polymerization. 

Y. Yagci and W. Schnabel, New aspects on the photoinitiated free-radical promoted cationic polymerization. Makromol. Chem. Macromol. Symp. 1992, 60, 133-143. 

Scheme 4. Synthesis of block copolymer by combination of ATRP and Free Radical Promoted Cationic Polymerization.

Carbocations generated in this way can add directly to appropriate monomers (e.g., tetrahydrofuran, cyclohexene oxide, n-butyl vinyl ether) or can form Bronsted acids by abstracting hydrogen from surrounding molecules. This method, which is commonly referred to as free-radical-promoted cationic polymerization, is quite versatile, because the user may rely on a large variety of radical sources. Some of them are compiled in Table 10.9. 

Table 10.9 Free radicals that may be employed in free-radical-promoted cationic polymerizations.

DUR 03] Dursun C., Degrimenci M., Yagci Y. et al, Free radical promoted cationic polymerization by using bisacylphosphine oxide photoinitiators ... 

DUR 08] Durmaz Y.Y., Moszner N., Yagci Y., Visible light initiated free radical promoted cationic polymerization using acylgermane based photoinitiator in the presence of onium salts , Macromolecules, vol. 41, no. 18, pp. 6714-6718, 2008. 

On the contrary, in CP, the photocuring of cationic coatings in industrial lines under visible light appears to be rather complex (the usual Pis mainly absorb in the UV the photosensitization of the onium salt decomposition is relatively difficult to achieve or scarcely efficient when along-wavelength excitation is desired). Fortunately, free radical promoted cationic polymerization (FRPCP) has become really interesting and is ruling out this limitation (see below). 

Bi, Y. and Neckers, D.C., A visible light initiating system for free radical promoted cation polymerization, Macromolecules, 27, 3683,1994. 

Photoinitiators absorb light in the UV-visible spectral range, 250-450 nm. The photoinitiator, PI, is raised to an electronically excited state, PI by promotion of an electron to a higher-energy orbital, and then it converts this light energy into chemical energy in the form of reactive intermediates, such as free radicals or cations, which subsequently initiate polymerization of monomers. 

Cationic cure mechanisms are an alternative approach to uv curing. This involves the photogeneration of ions, which initiate ionic polymerization. This process is not subject to oxygen inhibition, as are some of the free radical mechanisms. Cationic cure mechanisms generally also provide less shrinkage and improved adhesion. The disadvantages are that the photoinitiators are sensitive to moisture and other basic materials. The acidic species can also promote corrosion. As a result, the vast majority of uv formulations are acrylate-based and cure by a free radical mechanism. 

Recently, Ledwith described combined systemscomposed of a radical initiator and a cationic photoinitiator, which are very suitable for hybrid systems. It is particularly convenient to employ common photochemical sources of free radicals for this purpose. Since many of them possess aromatic carbonyl groups (e.g. benzoin and acetophenone derivatives), these groups provide an extension of the absorption to longer wavelengths and promote the initiation of cationic polymerization. Figure 19 shows the proposed formation mechanism of free-radical and cationic active species by electron transfer ... 

Free Radical Promoted. Many photolytically formed radicals can be oxidized by onium salts. The cations thus generated are used as initiating species for cationic polymerizations [82-84]. 

Monothiocarbonates such as 5,5-dimethyl-l,3-dioxane-2-thione reportedly polymerize by using cationic photoinitiators [2]. It was shown that onium salt based direct, and free radical promoted and photosensitization via exciplexes are practicable photoinitiation systems for such cyclic monomers. 

M.U. Kahveci, M.A. Tasdelen, and Y. Yagci, Photochemically initiated free radical promoted living cationic polymerization of isobutyl vinyl ether. Polymer 2007, 48(8), 2199-2202. 

VEs such as MVE polymerize slowly in the presence of free-radical initiators to form low mol wt products of no commercial importance (9). Examples of anionic polymerization are unknown, whereas cationic initiation promotes rapid polymerization to high mol wt polymers in excellent yield and has been extensively studied (10). 

The use of iV-alkoxy pyridinium salts is not limited to cationic polymerization. Since, in addition to cationic species, ethoxy radicals are also formed upon direct and sensitized irradiation of pyridinium salts (see above), pyridinium salt based photoinitiating systems may be used to initiate the polymerization of vinyl monomers that are prone to free radical polymerization. Kayaman et al. [71] recently polymerized mono- and bi-functional acrylate monomers by photosensitization of pyridinium salts. It therefore appears that pyridinium salts can promote both cationic and free radical polymerization and are, thus, eminently suitable for use in hybrid systems. 

Contrary to the views of Kiss and Lederer, cations other than ferrous will act as reducing agents for free-radical production, in a similar fashion. Thus, chromium (II), copper (I), titanium (II), and manganese (II) cations, and also mercury (0) are known to be effective polymerization catalysts. Indeed, manganese (II) compounds will, like ferrous salts, cause degradation of starch, glycogen, and inulin. Ferric salts also promote free-radical formation in hydrogen peroxide, but at a much lower rate. ... 

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