Protic initiation, mechanism

Friedel-Crafts (Lewis) acids have been shown to be much more effective in the initiation of cationic polymerization when in the presence of a cocatalyst such as water, alkyl haUdes, and protic acids. Virtually all feedstocks used in the synthesis of hydrocarbon resins contain at least traces of water, which serves as a cocatalyst. The accepted mechanism for the activation of boron trifluoride in the presence of water is shown in equation 1 (10). Other Lewis acids are activated by similar mechanisms. In a more general sense, water may be replaced by any appropriate electron-donating species (eg, ether, alcohol, alkyl haUde) to generate a cationic intermediate and a Lewis acid complex counterion. 

Lactams can also be polymerized under anhydrous conditions by a cationic mechanism initiated by strong protic acids, their salts, and Lewis acids, as weU as amines and ammonia (51—53). The complete reaction mechanism is complex and this approach has not as yet been used successfully in a commercial process. 

Two competing chain-transfer mechanisms in copolymerization of CO and ethene catalyzed by Pd11 acetate/dppp complexes were found. One involves termination via an isomerization into the enolate followed by protonation with methanol the rate of this reaction should be independent of the concentration of the protic species. The second chain-transfer mechanism comprises termination via methanolysis of the acylpalladium species, and subsequent initiation by insertion of ethene into the palladium hydride bond.501... 

The mechanism of the tin(II) bis-(2-ethylhexanoate)-mediated ROP of lactones remained a matter of controversy for many years, and many different mechanisms were proposed. Indeed, tin(II) bis-(2-ethylhexanoate) is not made up of alkoxides but of carboxylates, known as poor initiators for the ROP of lactones. In 1998, Penczek and coworkers made a major contribution in this field. They reported that, if the polymerization is carried out in THF at 80 °C, then tin(II) bis-(2-ethylhexanoate) is converted in situ into a new tin alkoxide by the reaction with either an alcohol, purposely added in the reaction medium, or with any other protic impurity present in the polymerization medium (Fig. 14) [37]. Tin alkoxides formed in situ are the real initiators of the polymerization, which takes place according the usual... 

Cationic polymerizations can be initiated with protic acids (e.g., sulfuric, perchloric, trifluoroacetic acid), with Lewis acids (see Sect. 3.2.1.1), and with compounds that form suitable cations (e.g., iodine, acetyl perchlorate). Some monomers are also polymerized by high-energy radiation according to a cationic mechanism. 

The mechanism of the Baylis-Hillman reaction has been re-evaluated in terms of implications in asymmetric catalysis.39 These studies have shown that in the absence of added protic species, the initial stage of the Baylis-Hillman involves rate-limiting proton transfer. 

Recommendations on the synthesis of metal phthalocyanines. It is still difficult to evaluate real reaction mechanisms in each synthetic procedure applied. It is clear that the use of such polar protic solvents as alcohols contributes to higher yields of Pc from PN in the electrosynthesis conditions due to the ease of nucleophilic attack of the generated additional RO-. In the further steps of Pc formation from PN or 1,3-D, a solvent s nature has no significant importance. These data about the importance of, first of all, the initial stage correspond to those reported on UV irradiation [40] of PN solutions, where such a treatment is effective only at the beginning of the process. However, in the case of the use of urea and PA, a solvent must be completely inert (or be close to urea s nature) to carry out the one-step synthesis of metal phthalocyanines, in order to exclude any negative influence on the reaction course. The fact that the yields are almost always higher in the case of direct electrosynthesis could serve as an additional confirmation about the usefulness and necessity of this technique. 

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. 

Hydrostannation of carbonyl compounds with tributyltin hydride is promoted by radical initiation and Lewis or protic acid catalysis.The activation of the carbonyl group by the acidic species allows the weakly nucleophilic tin hydride to react via a polar mechanism. Silica gel was a suitable catalyst allowing chemoselective reduction of carbonyl groups under conditions that left many functional groups unchanged. Tributyltin triflate generated in situ from the tin hydride and triflic acid was a particularly efficient catalyst for the reduction of aldehydes and ketones with tributyltin hydride in benzene or 1,2-di-chloromethane at room temperature. Esters and ketals were not affected under these conditions and certain aldehydes were reduced selectively in preference to ketones. 

The presence of a substituent in the 3-position of the NCA precludes equilibrium (6) and hence prevents the occurrence of reaction (18). It follows that, on the basis of the mechanisms described above, such NCA s should not polymerize unless a protic base (for example a primary or secondary amine) or other source of protons (for example, 3-methyl hydantoin) is present. If it could be established that polymerization does proceed with an aprotic base in aprotic media then some other mechanism of polymerization must be operative. This matter has been of central importance in discussions of various mechanisms of polymerization which have been advanced (Section 3). Experimentelly, it is not easy to obtain definitive evidence because of the high sensitivity of NCA s to protonic impurities (such as water and alcohols) in the presence of bases. It has been shown [18, 19, 38a] that proline NCA (X) and sarcosine NCA (I Ri = R2 = H, R3 = CH3) do not polymerize in the presence of tertiary bases under strictly aprotic conditions. With alkoxides, realization of such conditions is difficult, but it would appear that, at least with proline NCA, such strong bases can bring about ionization of the methine hydrogen and hence initiate polymerization as shown in (26). Evidence for this mechanism is provided by the observation that while sodium methoxide enriched... 

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