Initiator hexafluoroantimonate

An analogous stepwise mechanism was also proposed by Wohrle [36] for the cation-radical-initiated cycloaddition of electron-rich allenes with pentamethyl-cyclopentadiene in the presence of tris (p-tolyl) aminium hexafluoroantimonate (TTA SbF6 ) (Equation 1.15). 

Materials. The 1-propanol, glycerol and anthracene were obtained from Aldrich Chemical Company and were used as received. As in a previous study,17 a commercially available bis-(4-dodecylphenyl) iodonium hexafluoroantimonate salt (UV9310C GE Silicones) was used as the initiator. In this initiator, various dodecyl isomers have been attached to the phenyl rings of the diphenyliodonium salt to impart solubility in the monomer and do not effect the reactivity of the initiator.1 All studies were performed with an order of magnitude excess of initiator relative to anthracene. 

The calorimetric method was applied by Bowyer et al. (1971) to the polymerisation of N-vinyl carbazole (NVC) by tropylium hexafluoroantimonate and perchlorate in CH2C12 at 0 °C and -25 °C. The reactions were very fast and the reaction curves had a monotonically decreasing rate from the start. The initial reaction rate, R0, was correlated with the initiator and monomer concentrations by the equation... 

At low temperatures in inert solvents (such as methylene dichloride) a controlled polymerization can be effected using various acids and alkylating agents. These initiators include boron trifluoride etherate, triethylaluminum, trityl hexachloroantimonate, triethylam-monium hexachloroantimonate, diethyloxonium hexafluoroantimonate, p-toluenesulfonic acid and diethylzinc or cadmium-1,2-dioI complexes. Crystalline, high molecular weight... 

Various other miscellaneous salts have been used from time to time to initiate cationic polymerisation. These include tris-p-bromophenyl aminium hexa-chloroantimonate (97), anilinium hexafluoroantimonate (98), nitronium tetra-fluoroborate (95), thianthrenium perchlorate (99), and hexachloroantimonate (100), and N-methyl acridinium and N-methylphenazonium salts (75). In general... 

The usefulness for this purpose of triarylsulphonium hexafluoroantimonate comes from its considerable thermal stability, stability in the presence of highly reactive monomer, and highly efficient photolysis to yield reactive cations capable of initiating cationic polymerization. These properties arise from the unique chemical composition of the photo-initiator, the effectiveness of which can be shown to be a result of the presence of a very weak cation and a similarly weak anion. 

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...

Even sodium methacrylate5S) can act as an unsaturated deactivator for living poly-THF. The reaction is slow but by a proper choice of the experimental conditions the macromonomers can be obtained in high yields55 without any change in molecular weight (with respect to the precursor polymer deactivated with sodium phenolate). The macromonomers thus obtained contain a methacrylic ester function at the chain end. They are identical with those prepared by cationic initiation with methacryloyl hexafluoroantimonate ... 

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 homopolymer of the pyridinium salt monomer readily initiates the cationic polymerization of BOE upon heating to around 120 °C, to yield grafted copolymers. The copolymer with styrene similarly catalyzed the polymerization to form the corresponding grafted copolymers. The initiation activity of the copolymer in the polymerization was higher than that of the homopolymer but lower than that of the low-molar mass analogue, N-benzyl-p-cyanopyridinium hexafluoroantimonate. 

The caged species may escape geminate recombination and produce various species that can initiate cationic polymerization. Solvent (RH) often participates in these reactions producing protonic acids. As shown in Eq. (44), protonic acids are also formed by reaction of radical cations with aryl radicals or by Friedel-Crafts arylation. Up to 70% of the protonic acid is formed upon photolysis of diaryliodonium salts [205]. In addition to initiation by protons, arenium cations and haloarene radical cations can react directly with monomer. The efficiency of these salts as cationic initiators depends strongly on the counterions. Those with complex anions such as hexafluoroantimonate, hexafluorophosphate, and triflate are the most efficient. 

To solubilize the initiator, an alkoxy substituent was attached to one or both rings of the A I+ X initiator. For example, (4-octyloxyphenyl)phe-nyl-iodonium hexafluoroantimonate, 70, was a good initiator for the reaction [23]. 

Halonium ions have received a great deal of attention in the last decade and a wdiole book has already been devoted to these species The stability of dialkyl-halonium ions decreases in the order R2l > R2Br > R2C (thefluoroniumhomolo-gues have never been observed), and iodonium and bromcmium hexafluoroantimonates have been prepared and characterised in Olah s laboratory Under specific conditions these salts are active initiators of the cationic polymerisation of cdefins, and this still little-explored field will be briefly analysed in Chap. IV. 

Rooney has recently revived work on this monomer in an investigation of its polymerisation by trityl hexafluoroantimonate - He used a spectroscopic stop-flow apparatus to follow initiation and an adiabatic calorimeter to measure rates of polymerisation. Propagation was shown to compete effectively with initiation to the point that some initiator was often present at the end of the polymerisations. These observations cast some doubts on the assumption made in the paper by the Liverpool school discussed above. A kinetic analysis of the initiation reaction showed it to be bimolecular, with a rate constant of about 130 sec at 20°C. The determination of the propagation rate constant was less strai tforward despite the fact that further monomer-addition experiments seemed to rule out any appreciable termination. The kp values fluctuated considerably as the initial catalyst concentration was varied, a fact which induced Rooney to propose that the empirical constant was a composite function of kp and kp. Experiments with a common-anion salt supported this proposal and their kinetic treatment led to the individual values of kp = 6 x 10 sec and kp = 5 x 10 sec. It is difficult to assess the reliability of these values in view of the following statement by the author the reaction at a 5 x 10 M concentration of initiator, thought to proceed exclusively through paired ions. .. . This statement is certainly incorrect as far as the initiator is concerned for which the proportion of ion pairs for a concentration 5 x 10 M at 20°C is only about 20% in methylene chloride However, the experiments... 

The second part of Johnson and Pearce s investigation dealt with trityl hexafluoroantimonate. This initiator was found to be extremely active for the polymerisation of styrene, as already reported by Pepper and coworkers ). Monomer conversions reached 100% before complete bleaching of the trityl salt, and as polymerisations proceeded the... 

We can summarise the situation with styrene by a few observations. Obviously this monomer is not easily attacked by the trityl ion and the most favourable case encountered is that in which trityl hexafluoroantimonate it used. Termination reactions seem to plague most of these polymerisations and we believe that halogen-ion transfer from the anion to the growing species soon after initiation is responsible for the loss of activity of these systems. Again, the SbF counterion is an exception in that it does not seem to undergo this termination reaction. Except for the last study discussed, little reliable evidence is available to date to elucidate these difficult problems. Much fundamental information could be obtained by continuing on the line followed by Johnson and Pearce. 

WhUe this nomomer cannot be polymerised either by trityl or by tropylium salts, Plesdi and Pask were able to promote its polymerisation by using diphenylethylium hexafluoroantimonate. This recent discovery opens up new interesting potentials in the field of carbenium-salt initiated polymerisatkm of aliphatic olefins. 

Acylium hexafluoroantimonates have been successfully used as initiators in the polymerisation of tetrahydrofuran but little work has been undertaken to our knowledge to test their effectiveness for the polymerisation of olefins. Hesch and Padc discovered recently that some of these salts can induce the polymerisation of isobutene. This area of research diould provide much useful and fundamental information. 

As was noted in Scheme 12, distonic radical cations obtained from cyclopropane bond cleavages add oxygen rapidly, producing products with two CO bonds. So do some alkene radical cations. Addition of O2 to an alkene radical cation is formally a nucleophilic attack by the single alkene n electron on O2, and oxidizes both carbon atoms (an alkene radical cations has formally two -f carbons, and the adduct a 1+ and an oxygen-bound carbon). The oxygenation of the radical cation of bia-damantylidine (96) leads to dioxetanes such as 98 in chain reactions (see Scheme 21) [110]. The reactions may be initiated electrochemically or photochemically, but tris(o,p-dibromophenyl)amine hexafluoroantimonate, 97, is a superior catalyst for the dark reaction of certain tetraalkylalkenes, with turnovers up to ca. 800 at... 

The importance of both the cationic and anionic portions of the initiator was revealed by a study of various initiators for THF polymerization having different cations and different counter-ions. In this study, Yamashita et al. [84,85] concluded that the kp value with CH2CI2 solvent at 0°C is almost independent of counter-ion when triethyloxonium ions are used as initiators. They found much slower apparent rates when the different cation initiators, acetyl hexafluoroantimonate and 2-methyl-1,3-dioxolenium perchlorate, were used. They explained the slower rates by decreased rates of initiation. The apparent fep s can be increased to the... 

Evans and coworkers have reported that cationic copper(II)-bis(oxazoline) complexes derived from ferf-leucine are effective Lewis acids for a wide range of enantioselective Diels-Alder reactions. While initial investigations employed cy-clopentadiene as the diene and triflate catalyst 31a (Scheme 24) as the Lewis acid [82], subsequent studies revealed that the reaction rate is strongly dependent on the counterion X [83]. The hexafluoroantimonate catalyst 31b is approximately 20 times more reactive than 31a and is typically more stereoselective. The heightened reactivity and selectivity conferred by catalyst 31b allows access to more substituted adducts in uniformly high enantioselectivity. The active catalyst is easily prepared and robust exposure to air is not deleterious and the reactions may be conducted in the presence of free hydroxy groups. However, reduction of the metal center can be problematic with electron-rich dienes this side reaction may be controlled by a judicious choice of temperature. 

Saturated aliphatic hydrocarbons are readily carbonylated by CO in liquid HF in the presence of SbF5. The final product is a carboxylic acid, or ester, or a simple ketone, depending on whether the reaction mixture is solvolyzed with water, ROH, or a hydrocarbon. A carbonium hexafluoroantimonate is probably the initial product. 

A. Kikkawa, T. Takata, and T. Endo, Cationic polymerization of vinyl monomers with latent initiators. 3. l-(Para-methoxybenzyl)tetrahydrothiophenium hexafluoroantimonate as a new potent cationic initiator. Makromol. Chem. Macromol. Chem. Phys. 1991,192(3), 655-662. 

In the cationic polymerization of p-propiolactone ( 3PL), initiated with dimethyl-iodonium hexafluoroantimonate (a source of CH cations), the initiator induces rapid and quantitative formation of the active centers (by transfer of the methyl cation). The structure of the end-groups has been determined, the terminal one by cation trapping with triphenyl phosphine. These end-groups indicate O-alkyl ringopening 16) (route a in Equ. 9.1.) ... 

---