Cationic Polymerization of Epoxides

Since the network density is changed by the reaction between epoxide and alcohol or water, the mechanical properties of the resulting polymer are also influenced. This can be used, for example, in the flexibiHzation of dental materials with poly(l,4-butanediol) [11] or coatings with polyester polyols [12]. It is not only alcohols that influence the polymerization kinetics and the properties of the polymer, but also carboxylic acids. By the addition of a polymer with carboxylic acid groups instead of the polyol, a polyester is formed as a reaction product and not a polyether. This was examined in detail by Wu and Soucek [13]. 

After this introductory review on the cationic polymerization of epoxides by latent initiators an overview of our work in recent years on the effect of moisture and different polyether diols on the photochemicaUy and thermally induced polymerization of different epoxides will be given. The polymerization behavior, as well as the final properties of the cured polymers, is covered. 

One of the focal points of this work was the influence of traces of moisture on the cationic polymerization of epoxides. Using the previously described iodonium salt as an example, it was shown that water not only played an important role as co-catalyst in the photochemical formation of the superacid, but it also caused the hydrolysis of the iodonium salt via a nucleophihc attack at the T center. This reaction was mainly investigated by X-ray photoelectron spectroscopy (XPS). Interestingly, this hydrolysis only occurs with the hexafluoroantimonate salt and not with the corresponding chloride. Here it should be mentioned that the chloride salt resembles an interhalogen compound and the hexafluoroantimonate salt forms a real ion pair. Hydrolysis of the hexafluoroantimonate anion of the iodonium salt was not observed, but slow hydrolysis was observed for sodium hexafluoroantimonate. The hydrolysis is so slow overall that the influence of this effect can be excluded from the discussions into the influences of water and alcohol on the cationic polymerization of epoxides. 

Due to the high effectiveness of the iodonium salt developed, it is used preferentially in the following investigations as a photoinitiator for the polymerization of epoxides. In some cases a commercial sulfonium salt is also used, in spite of the known disadvantages. In other cases the polymerization is initiated thermally. 

A requirement for a thermally induced polymerization with a latent initiator is that at room temperature the reaction mixture of monomer and initiator remains unchanged for as long as possible, but at moderate temperatures the polymerization should be fast. For the radical polymerization of, for example, aery- 

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The anionic polymerization of lactams proceeds by a mechanism analogous to the activated monomer mechanism for anionic polymerization of acrylamide (Sec. 5-7b) and some cationic polymerizations of epoxides (Sec. 7-2b-3-b). The propagating center is the cyclic amide linkage of the IV-acyllactam. Monomer does not add to the propagating chain it is the monomer anion (lactam anion), often referred to as activated monomer, which adds to the propagating chain [Szwarc, 1965, 1966]. The propagation rate depends on the concentrations of lactam anion and W-acy I lactam, both of which are determined by the concentrations of lactam and base. 

Cationic polymerization of epoxides by irradiation of charge-transfer complexes has been mentioned in the literature Fluorinated alkanesulfonic acid salts chromates and dichromates of alkali metals, alkaline earth metals and ammonium phototropic o-nitrobenzyl esters iodocyclohexene unsaturated nitrosamines and carbamates have been reported to act as cationic photoinitiators. 

TGA analyses were performed for polymer samples having different degrees of cross-linking. The decomposition of the linear oligomer starts at about 200 °C. Once cured and baked, the formed siloxane network is more thermally stable, and the decomposition begins at temperatures higher by 100-150 °C. The results are similar to those reported for analogous Tsi-modified siloxanes cross-linked by means of photo-initiated cationic polymerization of epoxides [8]. 

Compounds containing hydroxyl group accelerate photoinduced cationic polymerization of epoxides as explained previously vide ante) [109,110,122]. Accordingly, 1-pyrenemethanol was designed as an accelerator as well as photosensitizer [119]. 

As mentioned before, addition of plasticizing agents, which decrease the viscosity, into polymerization medium increase the rate of polymerization. This effect was observed in photoinitiated cationic polymerization of epoxides in the presence of epoxidized soybean oil [133],... 

Crivello s group followed either or both of two strategies that described for the additives in acceleration of photoinitiated cationic polymerization of epoxide monomers. These are stabilization of free radicals and cations by resonance and inductive effect, and the activated monomer mechanism. Comparative studies of novel monomers with conventional monomers show that newly designed monomers given in... 

Similar to the strategy employed in vinyl ether polymerization, addition of sulfides was considered as a way to achieve living cationic polymerization of epoxides. The... 

As discussed in a previous section, a number of studies have been conducted to increase the rate of cationic polymerization of epoxides. In curing applications, polymerization should be rapid enough for high output of production. In a recent work, the effect of addition of tetraethylene glycol (TEG) or polyEPB on the rate of photoinitiated cationic polymerization of CY179, limonene dioxide (LDO), and 1,2,7,8-diepoxyoctane (DEO) has been investigated [150]. These hydroxyl containing additives were shown to obviously accelerate the polymerization, increase the total epoxide conversion and decrease the induction period. 

T. Takata, K. Takuma, and T. Endo, Photoinitiated cationic polymerization of epoxide with phosphonium salts as novel photolatent initiators. Makromol. Chem. Rapid Commun. 1993, 14(3), 203-206. 

K. Takuma, T. Takata, and T. Endo, Cationic polymerization of epoxide with benzyl phosphonium salts as the latent thermal initiator. Macromolecules 1993,26(4), 862-863. 

J.V. Crivello and R.A. Ortiz, Benzyl alcohols as accelerators in the photoinitiated cationic polymerization of epoxide monomers. J Polym. Sci. A Polym. Chem. 2002, 40(14), 2298-2309. 

Unlike free-radical propagation, photoinitiated cationic polymerizations of epoxides are unaffected by oxygen and thus require no blanketing by an inert atmosphere. However, water and basic materials present in UV-curable epoxy formulations can inhibit cationic cures and should be excluded. 

Fig.2 Influence of the epoxy content on the photoinitiated cationic polymerization of epoxidized liquid natural rubber (ELNR). ( ) Photolysis of the photoinitiator ( A) [TAS] = 3 wt %.

The metal arene bond in (arene)FeCp complexes is photolabile [93], and this forms the basis of the use of these complexes as photoinitiators for cationic polymerizations of epoxides [94]. Arene exchange via this route has not been used extensively. On photolysis of (p-xylene)FeCp in CH2CI2 or acetone by a medium pressure Hg-arc or by bright sunlight , the complex undergoes arene exchange with hexamethylbenzene, paracyclophane and thiophene under mild conditions... 

Influence of Proton Donors on the Cationic Polymerization of Epoxides... 

Analogously to water, alcohols (used in the form of polyols) also cause a chain-transfer reaction during the cationic polymerization of epoxides. Polyols (polyvalent alcohols with an average molecular weight from several hundred to several thousand grams/mole) are often utihzed in technical formulations. These serve to reduce the network density of the polymerized epoxide and therefore make the material less brittle. At the same time the reactivity also is influenced and the susceptibiHty of the polymerization rate and polymer properties to the influence of air humidity is reduced. This is important, as the influence of air humidity is difficult to reproduce in technical appHcations. 

The preparation and properties of (9-oxo-9//-fluoren-2-yl)phenyliodonium hexafluoroantimonate (14) as a new photoinitiator for the cationic polymerization of epoxides have been reported [37]. Compound 14 was prepared by the reaction of (diacetoxyiodo)benzene with fluorenone followed by treatment with sodium hexafluoroantimonate (Scheme 7.3). Photoinitiator 14 has the advantage of intramolecular photosensitization and it is a more effective initiator than the conventional iodonium salts [37]. 

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