Epoxides cationic polymerization

Cationic polymerization in hot melts has been applied to epoxidized polymers [38,39]. No hot melts based on vinyl ether or other cation-sensitive functionalized polymers have been described in the literature. With cationic systems, it is important that the other ingredients in the adhesive be of low basicity to avoid scavenging the initiating acid generated by the photoinitiator. 

The reactivity of I in photoinitiated cationic polymerization is due to several factors associated with the structure of this monomer. Most importantly, the presence of the ester groups in I which can interact with oxiranium ions generated at either of the two epoxide groups both intra- and intermolecularly produces dioxacarbenium ions of reduced activity in the propagation reaction. Taking this into account, a series of diepoxides were prepared which did not possess ester groups. Some of these monomers show enhanced reactivity as measured by RTIR in photoinitiated cationic polymerization compared to I. 

It has been shown for many metal halides and monomers that binary mixtures of these can be prepared (usually in a solvent) without any polymerization taking place. Such a quiescent mixture can be made to react by the addition of a suitable third compound, which is called the co-catalyst. This term is preferable to the word promoter , because in certain contexts a substance is called promoter which enhances the rate or yield of a reaction that will also go in the absence of the promoter herein lies the true distinction between promoter and co-catalyst [28]. (For example, small quantities of epoxides or epichlorohydrin act as promoters in the cationic polymerization of tetrahydrofuran.) I will take it that in the above quotation the word promoter was inadvertently used in place of co-catalyst , for only thus does it become really meaningful. 

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. 

Further papers in this section include the hydrosilylation of a-pinene and of j8-pinene, ° thiocyanation and selenocyanation of a-pinene, ° cationic polymerization of a- and /3-pinene epoxides,bromination of ( )-c/s-pinonic acid with dioxan dibromide, and straightforward myrtenol (224 R=CH20H) ° and tetrahydroperillyl alcohol syntheses from -pinene epoxide. 

It has long been accepted that cyclic ethers with more than four-membered rings are, in contrast to epoxides, only cationically polymerizable due to the high basicities of their ether oxygen atoms. The cationic polymerization involves 0-... 

Homopolymerization, epoxides aluminate-Lewis acid catalyst system, 11, 602 via aluminum-porphyrin-Lewis acid catalysts, 11, 599 aluminum-tetradentate ligand catalyst system, 11, 601 anionic polymerization, 11, 598 cationic aluminum catalyst system, 11, 603 cationic polymerization, 11, 598 characteristics, 11, 597 zinc-based catalyst system, 11, 605 Homopolymers, cyclic olefins, 11, 716... 

Once the initiating cation has been formed the polymerization is controlled by temperature (technical difficulties may arise if the control is not adequate). A reaction scheme for cationic polymerization of an epoxide using triarylsulphonium hexafluoroantimonate as photo-initiator is shown in Figure 105. Termination of the polymerization often is adventitious, particularly with anions and other bases (theoretically the reaction can continue until the supply of monomer is exhausted). 

Figure 105 A reaction scheme for cationic polymerization of an epoxide using triarylsulphonium hexafluoroantimonate as photo-initiator...

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. 

As we will see, cationic polymerization is the only polymerization mechanism open to the higher cyclic ethers. It is also available to epoxides, but the reactions are complex. The mechanism has not been applied commercially since the polymers which result are only of rather low molecular weight. 

As in the case of the zinc catalysts, active catalysts are formed by reaction of alkyl aluminium compounds with water. It is generally felt that since aluminium compounds are usually fairly strong Lewis acids, the catalysts also are somewhat more acidic in nature. Thus a coordinate cationic mechanism is generally favoured for these polymerizations. In contrast, a more anionic coordinate mechanism is usually suggested for the zinc catalysts. In fact, as will be seen in the discussion of the higher cyclic ethers, some of these catalysts are distinctly able to initiate true cationic polymerizations. However, the catalysts under discussion here as applied to epoxides are clearly considered to be coordinate. 

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]. 

In this paper, we would like to report some recent work which has led to the development of triarylsulfonium salts (III) as a third class of useful photoinitiators for cationic polymerization and in particular, describe their application to the polymerization of epoxides. 

The release of a Lewis acid such as BF3 or PFs in the presence of a material capable of undergoing cationic polymerization would be expected to yield polymeric products, and the photo-induced polymerization of epoxides can be explained according to the mechanism for cationic polymerization proposed by Rose (25) and others (, 28). 

This is followed by cationic polymerization of the epoxide initiated by the proton from HPFe-... 

Sulfonium Salts. Other onium salts beside the diazonium and halonium salts will, on exposure to light, initiate polymerization of epoxides and other monomers capable of undergoing cationic polymerization. The use of onium salts of the elements of Groups Va and Via was first described in 1975 (41, ) and 1976 ( ). 

Typical of cationic polymerization, the rate of cure is influenced by temperature and the presence of moisture. By a judicious choice of epoxides, it is possible to formulate coatings which cure rapidly under ambient conditions. 

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