Triflic acid cationic polymerization

A variety of initiator systems of the types used in the cationic polymerization of alkenes (Chapter 8) can be used to generate the tertiary oxonium ion prpoagating species. Strong protonic acids such as sulfuric, trifiuoroacetic, fluorosulfonic, and trifluoromethanesulfonic (triflic) acids initiate polymerization via the initial formation of a secondary oxonium ion ... 

Triflic acid has become a widely applied catalyst in various polymerization processes, and a few selected characteristic examples are discussed here. Additional basic information, examples of practical significance, and recent trends of cationic polymerization can be found in books and monographs.942 945... 

A rare example of cationic polymerization of emulsified epoxy resins has been reported by Walker et al.973 Polymerization of water emulsion of epoxy resins with a variety of superacids (triflic acid, HCIO4, HBF4, HPF6) results in polyols with two glycidyl units (294) in contrast to commercial epoxy resins with one unit separating the aromatic moieties. The level of residual glycidyl ether and Bisphenol-A units is also much lower than in conventional epoxy resins. 

Perchloric and triflic acids polymerize nearly all cationically polymerizable monomers, except a-olefins and isobutene. Unsaturated dimers... 

Cationic polymerization, initiated with strong protonic acids (e.g., triflic acid), occurs purely by opening of the Si—O bond giving high molecular weight regular polysilaethers. For the former monomer, yields up to 80% and M up to 40,000 were reported, whereas the latter polymerized with yields up to 90% of polymer and with M 34,000. 

The role of VO(acac) is presumably identical with that of the oxidative polymerization of diphenyl disulfide. The V(IV) catalysts disproportionate to give a V(ll) and V(III) species. The V(V) species oxidizes monomer 311 producing cation-radical 328 and V(1V). V(III) is readily oxidized in triflic acid by oxygen to regenerate the V(IV) catalyst [211]. 

Anhydrides of strong i x)tonic acids provide a group of initiators able to give dicationically terminated macromolecules °° The anhydride of trifluoromethane-sulfonic acid (triflic anhydride) initiates the polymerization of THF in this way both reactions, with rate constants ki and kai, are faster than the formation of the alkyltetrahydrofuranium cation with the corresponding triflic acid ester ... 

This explanation is in a grod eement with differences observed when cationic polymerization of vinyl and heterocyclic monomers is initiated with triflic acid. In the former case some acid is bound to anions (e.g. 3 molecules of acid per anion ) wdieteas in the latter (e.g. 1,3-dioxolane) every molecule of acid used gives one macTomolecule. 

It must be considered that silyl triflates are not stable in THF for an extended time, because they can initiate the cationic ring-opening polymerization (ROP) of cyclic ethers. Therefore, the silyl triflate (in diethyl ether) is added in drops to the (aminosilyl)lithium compound dissolved in THF. The reaction proceeds very quickly, and ROP of THF is not observed under these conditions. The trisilanes la - Ic can be converted by reaction with triflic acid into the silyl bis (triflates) 2a - 2c. These compounds are precursors for further chain elongation. The reaction with (diethylamino)-diphenylsilyllithium leads to the pentasilanes 3a - 3c. The reaction of bis(diethylamino)phenylsilyl-lithium [9] with silyl triflates proceeds analogously. The formation of the disilane 4 is shown in Scheme 2 as an example. Conversion with triflic acid and chain elongation with LiSiPh2(NEt2) leads to the branched tetrasilane 6. 

B-90 and B-91, respectively.390 Another route coupled with cationic ring-opening polymerizations is accomplished for polymer B-92 with the use of a hydroxyl-functionalized initiator with a C—Br terminal, where the OH group initiates the cationic polymerizations of 1,3-dioxepane in the presence of triflic acid.329 Polyethylene oxide)-based block copolymers B-93 are obtained by living anionic polymerization of ethylene oxide and the subsequent transformation of the hydroxyl terminal into a reactive C—Br terminal with 2-bromopropionyl bromide, followed by the copper-catalyzed radical polymerization of styrene.391... 

Cationic polymerization of cyclic siloxanes could not be adequately treated in Vol. I of this monograph (Adv. Polymer Sci. 37 (1980), because the nature of the elementary reactions was obscure and rate constants were unknown. Recently more detailed information has become available, mostly provided by the work of two groups, one in Lodz (Chojnowski a. o.)6) and the other in Paris (Sigwalt and Sauvet)7,8). In our further analysis we will mostly rely on the results of these groups. Their success stems from the use of triflic acid as initiator, following other works in cationic polymerization of heterocyclics. 

Initially, Gandini and Plesch proposed that the perchloric acid-initiated low temperature polymerization of styrene is based on monomer insertion on the nonionic perchlorate chain ends, which was based on the observation that the polymerization mixture was not conductive [68, 69]. These nonionic polymerizations were referred to as pseudo-cationic polymerizations. However, more detailed investigations by stopped-flow UV-vis spectroscopy revealed the presence of short-lived carbocations indicating that these are the propagating species in the cationic polymerization of styrene [70, 71]. This was also confirmed for the polymerization of styrene with trifiic acid for which Matyjaszewski and Sigwalt showed that the covalent triflic ester adduct was unstable even at -78 °C leading to carbocationic propagating species [72]. 

In contrast to these initial reports on the living CROP of tetrahydrofuran which were performed without additional solvents, Penczek and coworkers demonstrated that the solvent plays an important role in the cationic ROP of tetrahydrofuran since it controls the proximity and stability of the ion pair at the living chain end [100, 101]. The polymerization rate increases in more polar solvents because of stabilization of the ion pair, whereby it was demonstrated that the methyl-triflate-initiated CROP of tetrahydrofuran involves an equilibrium between the cationic propagating oxonium species and the covalent triflic acid adduct, which can be shifted by the solvent choice as depicted in Scheme 8.19. Nonetheless, as a result of the much higher reactivity of the cationic propagating species, the polymerization rate is almost exclusively determined by the concentration of oxonium ions. 

Figure 4. Degree of polymerization ( ) and polydispersity (O) resulting from cationic polymerizations of (a) 5-[(4 -(4"-cyanophenyl)phenoxy)pentyl]vinyl ether [125, 126] and (b) 8-((4 -(2/f,3S)-2-fluoro-3-methyl-pentyloxycarbonyl)-3 -fluorophenyl-4"-phenoxy)octyl] vinyl ether [ 139] initiated by triflic acid in CH2CI2 at 0 °C in the presence of dimethyl sulfide.

A combination of RAFT and cationic ROP was used to synthesize a series of (poly(methyl methacrylate)](poly (l,3-dioxepane)](polystyrene) 3-miktoarm star terpoly-mers. The synthetic approach involved the sjmthesis of PS functionalized with a dithiobenzoate group, using RAFT polymerization, and subsequent reaction of this group with hydroxyethylene dnnamate in THF (Scheme 73). The hydroxyl group served as the initiating site for the cationic ROP of 1,3-dioxepane in the presence of triflic acid. Finally, the diblock copolymer with the dithiobenzoate group situated between the two blocks was used for the RAFT polymerization of MMA. The... 

Bourissou et al. reported recently controlled cationic polymerization of lactones using a combination of triflic acid with a protonic reagent as the initiators [90]. The reaction was carried out in CH2CI2. Results indicated that the process is controlled is a linear relationship between the molecular... 

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