Initiators oxocarbenium salts

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 ... 

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 ... 

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. 

With respect to the initiation of cationic chain polymerizations, the reaction of chlorine-terminated azo compounds with various silver salts has been thoroughly studied. ACPC, a compound often used in condensation type reactions discussed previously, was reacted with Ag X , X, being BF4 [10,61] or SbFa [11,62]. This reaction resulted in two oxocarbenium cations, being very suitable initiating sites for cationic polymerization. Thus, poly(tetrahydrofuran) with Mn between 3 x 10 and 4 x lO containing exactly one central azo group per molecule was synthesized [62a]. Furthermore, N-... 

For oxocarbenium and trialkyloxonium salts this is typically a true initiation. An attack of monomer on a-carbon follows, leading to forma-... 

Triphenylcarbenium salts (PhgC X") selectively oxidize secondary t-butyl or triph-enylmethyl (trityl) ethers derived from alcohols. The oxidation proceeds via initial hydride abstraction followed by loss of the group on oxygen. The secondary-overprimary selectivity results from preferential formation of an oxocarbenium ion intermediate at the secondary center (R2 C-OTr is formed faster than RH C-OTr). 

Some carbenium salts that form sterically hindered addition product, that is, triphenylmethylium (trityl) salts may initiate through hydride ion abstraction as shown in Scheme 9 (route Oxocarbenium ions thus formed serve as actual... 

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