Oxocarbenium salt

The cationic ring opening polymerization of oxolane (THF) or of N-substituted aziridines can be initiated by oxocarbenium salts [42]. The methacrylic ester unsaturation is insensitive to cationic sites, and polyoxolanes (poly-THF) macromonomers are obtained in good yields. 

This polymeric oxocarbenium salt readily initiates the cationic ring opening polymerization of oxolane to produce a polystyrene-polyTHF block copolymer. Molecular weight control is provided, polydispersity is narrow and compositional heterogeneity is small59). 

To initiate efficiently, rapidly and quantitatively the polymerization of these heterocycles it was searched for cationic species which add to the monomer and are associated to stable counterions. Suitable initiators are oxocarbenium salts (I), stabilized car-benium salts (II) and trifluorosulfonic derivatives (III) (triflic esters or anhydrides)42"44 ... 

As already mentioned, oxocarbenium salts are efficient initiators42,45 for the polymerization of oxolane. They can be prepared in situ by a metathetic reaction between the corresponding acyl halide and silver hexafluoroantimonate ... 

Tropylium (C H7 +, A-) and related Oxocarbenium salts Methoxymethylium (CH3OCH2+, A )... 

Oxocarbenium and carboxonium ions have an oxygen atom in the exposition to the carbon atom formally bearing the positive charge. Oxocarbenium salts are usually generated in situ [19], although some dioxolenium... 

The copolymerization between trioxane and suitable comonomers (ethylene oxide, 1,3-dioxolane, diethylene glycol formal, 1,4-butane diol formal in amounts of 2-5% by weight) is performed using cationic initiators. The cationic initiators could be Lewis acids, such as BF3 or its etherate BF3Bu20 which was used, for example by Celanese (the mechanism of this reaction was studied in detail [163,164]) or protic acids such as perchloric acid, perfluoroalkane sulfonic acids and their esters and anhydrides. Heteropoly acids were used and also a series of carbenium, oxocarbenium salts, onium compounds, and metal chelates. To regulate the molecular weight chain-transfer agents, such as methylal and butylal, are added. 

Plesch showed by electrochemical methods that at the sufficiently low concentrations, the components exist in the 2-to-2 equilibrium with unpaired ions These salts were used to initiate polymerization of cyclic ethers and acetals but the structure of the tirst addition products has not yet been determined. Nevertheiess, several authors observed the corresponding end groups in oligomers (at the early stages of polymerization) or in high polymers . These observations indicate that the oxocarbenium salts initiate by simple addition, for instance ... 

Several multifunctional oxocarbenium salts were also prepared by Franta and used in the polymerization of... 

The mechanistic aspects of Friedel-Crafts acylation have been widely developed, in particular concerning identification of the intermediate complexes between the acylating reagent and the catalyst (refs. 1, 5, 6, 43). The general opinion is that these species exist in solution as an equilibrium mixture of ionic (oxocarbenium salts) and molecular (donor-acceptor complexes) forms whose relative concentrations depend on solvent and temperature. 

The cationic catalysts used for THF polymerisation, with the general formula R+X" are trialkyloxonium salts of superacids, esters of superacids, oxocarbenium salts, Lewis acid - oxirane complexes, superacid anhydrides, (e.g., triflic anhydride), and others [1-38] (Figure 7.2) ... 

Another industrial route to obtain a,oo hydroxy-telechelic PTHF is based on the transfer reaction with anhydrides (7.11). The polymerisation reaction is based on a mixture of anhydrides (usually acetic anhydride) and a superacid (HSbF6, HC104, CF3S03H or even in the presence of a polymeric superacid with -CF2-CF2-S03H groups (Nafion resins, solid analogue of triflic acid) [26, 27] or Lewis acids (BF3, SbF5) [20, 24], or solid acidic clays [29]. The real catalyst is the oxocarbenium salt formed by the reaction of acetic anhydride and the superacid ... 

Oxonium, carbenium, or oxocarbenium salts generate tertiary oxonium ions in single step, as shown in Scheme 8 for initiation with triethyloxonium salt. 

The backbone chain is made by free radical copolymerization of styrene with some acryloyl chloride. It is reacted in THF solution with silver hexafluoroantimonate. The oxocarbenium salt formed immediately initiates the polymerization of THF (Scheme 23). However, the growth of the grafts has to be stopped at an early stage (by induced deactivation) to prevent formation of bridges between individual chains. The graft copolymers obtained have been characterized accurately. 

It has also been shown that living polydioxolane can be reacted directly onto polystyrene, whereby grafting takes place (Scheme 26). This is a kind of Friedel-Crafts reaction, resulting from the attack of the active site onto the benzene nucleus of a monomer unit. In this case, however, polydioxolane (PDXL) may remain ungrafted. The absence of any proton-donating impurity in the medium is required. The dioxolane polymerization has to be initiated by oxocarbenium salts and not by systems containing protons. 

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