Triethyloxonium tetrafluoroborate initiation

In 1968, Yamashita et al. [127] reported a study of the kinetics of 1,3-dioxolane polymerization initiated by Et30 BF4 and carried out in CH2 CI2 solution. They concluded that the mechanism, deduced from the structure of the initiation product and the molecular weight of the polymer, is simply 

The mechanism of initiation is based on an examination of reaction products in an early stage of the polymerization. After short-stopping the polymerization by addition of sodium methoxide/methanol solution, the main product subsequently identified by gas chromatography was C2H5OCH2CH2OCH2OCH3. The rate of polymerization is presumed to follow the standard eqn. (6) for an equilibrium polymerization without termination and with rapid initiation. It is acknowledged that there is an induction period (presumably due to reaction of catalyst with adventitious water, since rigorous drying reduced the induction time to only 

There was an induction period before this rate was achieved. 

A somewhat later study of the polymerization of 1,3-dioxolane by Et3 0 BF4 was reported by Jones and Plesch [129] in 1969. Their studies were carried out with CHj Clj solvent, but they used a reactor fitted with Pt electrodes and followed conductivity and polymerization simultaneously. They eed that the formation of active centres is slow and found that the rate obtained from the linear portion of the conversion-time curve varied with initiator concentration. Their measurements showed a sharp rise in conductivity initially, then a steady fall to a very low constant value. They conclude from further studies that the concentration of propagating ions is much lower than that of the ions present initially. They suggest that the following reactions occur when Et3 0 BF4 is added to dioxolane, viz. 

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Since the tertiary oxonium ion is the propagating species, preformed oxonium ions such as triethyloxonium tetrafluoroborate can be used for initiation (Eq. 7-28) [Meerwein et al., I960] ... 

Kern (7) and Entelis and co-workers (15-17) found tetramer and small amounts of larger rings when the monomer was treated with borontrifluoride or triethyloxonium tetrafluoroborate. With the first initiator the ratio of tetramer to polymer is greater in the early stages of the reaction than after longer reaction periods (15). From this observation it was concluded that tetramer and polymer are formed by two independent reaction paths. The ratio of oligomers to polymer increased with temperature (16)-... 

Bucquoye (22) made a kinetic study of the formation of cyclic tetramer during the polymerization of 3,3-dimethyloxetane initiated by triethyloxonium tetrafluoroborate. With this monomer also, the oligomer formed only during and not after the polymerization (Fig. 2). 

The cationic polymerization of styrene sulfide has been reinvestigated by Van Craeynest (15). With triethyloxonium tetrafluoroborate as initiator, a rapid and quantitative polymerization was observed, followed by a slow degradation of die polymer to a mixture of cis and tram 2,5-diphenyl-l, 4-dithiane and as and tram 2,6-diphenyl-1,4-dithiane. Since the BF4 counter ion is not capable of forming a covalent bond, a back-biting reaction via sulfonium ions seems the plausible mechanism for the dimer formation. The polymerization initiated with dimethyl sulfate showed the same characteristics a fast polymerization is followed by degradation to the same mixture of isomeric diphenyl- 1,4-dithianes. However, the mwts-2,5-diphenyl derivative was the only isomer that crystallized from the solution. It is therefore reasonable to accept that with dimethyl sulfate also, the cyclic dimers of styrene sulfide are formed by a back-biting type of degradation of the polymer and not by the mechanism shown above. 

Fig. 4. Gel permeation chromatograms showing the polymerization and degradation of N-(2-phenylethyl)aziridine in methylene chloride with triethyloxonium tetrafluoroborate (40). Initial monomer cone. 1.0 mol 1—1, initiator cone. 0.012S mol l-1. A 12 hrs after initiation (at 0 °C) B 120 hrs after initiation (at 20 °C)...

The use of an unsaturated nucleophile to introduce into the chain end of the macromolecule a double bond has also proved successful for the synthesis of poly-THF macromonomers. The oxolane polymerization is started with any efficient initiator. When the growing chains have reached the desired length, the unsaturated deactivator is added. The reaction between the oxonium sites and the nucleophile should be fast and free of side reactions. Various unsaturated nucleophiles have been employed, e.g. p-vinylphenoxide used by Asami50). The THF polymerization was initiated with triethyloxonium tetrafluoroborate and carried out atO °C. Addition of the nucleophile (obtained by reaction of the phenol with NaH) yields the corresponding macromonomer the structure of which was characterized by various techniques ... 

All these cationic deactivation processes were performed with oxolane as the monomer and with various initiators such as triethyloxonium tetrafluoroborate and benzoyl, acetyl or propionyl hexafluoroantimonate. Efficient difunctional cationic initiators such as adipoyl- or terephthaloyl hexafluoroantimonate) can also be used 42 to synthesize bifunctional macromonomers containing at both chain ends a polymerizable double bond. 

The most convincing evidence is Vofsi and Tobolsky s (24) work with 14C-labelled triethyloxonium tetrafluoroborate at 0°C. They found good agreement among the molecular weights calculated from the amounts of catalyst charged, determined by 14C end-group analysis and determined by vapor pressure osmometry. Dreyfuss et al. have also obtained NMR data on this initiation reaction (9). The spectra are entirely consistent with Reaction 4. No side reactions are indicated. 

Kinetics with specific initiators (a) Triethyloxonium tetrafluoroborate... 

Problem 10.4 (a) The kinetics of polymerization of tetrahydrofuran was studied [6] with the use of triethyloxonium tetrafluoroborate, (C2Hs)30+BF4, as im tiator and dichloromethane as solvent. Conversion versus time was measured at 0°C with initial catalyst concentration [I]o = 0.61 x 10 mol/L and monomer concentration [M] varying from 3 to 9 mol/L. The initial rates, Rp, determined from these data are given in Table A. 

THF polymerization was initiated with triethyloxonium tetrafluoroborate. BF anions are known to decompose during THF polymerization, producing dead polyTHF carrying an alkyl fluoride end-group. Another reason for the formation of polyTHF may be chain transfer to polymer during THF polymerization, yielding dead blocks and dicationically living chains ... 

Conversion of carboxylic acids to aldehydes. Raber and Guida have reported a general method for partial reduction of carboxylic acids to aldehydes. The acid is converted into the 2-methoxyethyl ester (1), which is then alkylated with triethyloxonium tetrafluoroborate in CH2CI2. Crystalline 1,3-dioxolanium tetrafluoroborates (2) are obtained by initial alkylation of the methoxy oxygen... 

Reactions.—Oishi and his co-workers have reported that thiono-benzoates may be converted into thiolobenzoates by means of triethyl-oxonium tetrafluoroborate in a process that is most probably initiated by the formation of the S-alkylated thiono-ester intermediate (268). However, the latter is also a powerful alkylating agent towards thiono-esters, yielding the new cationic alkylating agent (269) by the liberation of ethyl thiolo-benzoate. In the authors opinion, (268) and (269) are the important reagents in the above-mentioned reaction, whereas triethyloxonium tetrafluoroborate is effective merely as an initiator of it. The statement was... 

The polymerization of l-oxa-3-thiacyclopentane by two different initiators gives widely contrasting results. With boron fluoride etherate the system is regarded as quite uncomplicated," whereas triethyloxonium tetrafluoroborate was claimed to induce simple and fast initiation, but slow propagation with extensive chain-transfer." ... 

With triethyloxonium tetrafluoroborate, the initiation reaction is fast compared with propagation, so that f a- 1. The type of termination reaction defines the functional form of mg, m and Cq, i.e. eqn. [4] or [s]. Consequently the (grafical) solution cf these equations permits to distinguish between first order and second order termination reaction and to determine the values of p t ... 

Figure 1. Determination of f k,A<, according to Equation 4, for the polymerization of 1-benzyl aziridine 1 2-cyano-ethyl)aziridine (O), and l- 2-phenyletlwl)aziridine ( ) in CHy Clt at 0°C uHth triethyloxonium tetrafluoroborate as initiator, mo = 1.0 mol r ...

It was not possible to polymerise this monomer at tenq>eratures between 0° and 120 C, in bulk or in solution. BDMA does react with triethyloxonium tetrafluoroborate to form the expected aziridinium salt but this salt does not give a ring-opening reaction with BDMA monomer. It is however an excellent initiator for the polymerization of BA ... 

The reactions of cyclopropenone with a wide variety of reagents have been reported by Breslow, Oda, and Pecoraro. " Bromine gives an acyclic product after initial electrophilic attack at the carbonyl oxygen rather than at the double bond. As expected, triethyloxonium tetrafluoroborate gives ethoxycyclo-propenium tetrafluoroborate, and trifluoroacetic acid opens the C-1—C-2 bond, but this could conceivably occur after initial addition to the C-2—C-3 double bond. A cyclopropenyl cation is also a proposed intermediate in the... 

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