Initiators Lewis acids

There is evidence that Lewis acids initiate a slow polymerization in some (but not most) systems by a self-ionization process in addition to the coinitiation process [Balogh et al., 1994 Grattan and Plesch, 1980 Masure et al., 1978, 1980], Two mechanisms are possible for self-initiation. One involves bimolecular ionization... 

A variety of protonic and Lewis acids initiate the cationic polymerization of lactams [Bertalan et al., 1988a,b Kubisa, 1996 Kubisa and Penczek, 1999 Puffr and Sebenda, 1986 Sebenda, 1988]. The reaction follows the mechanism of acid-catalyzed nucleophilic substitution reactions of amides. More specibcally, polymerization follows an activated monomer mechanism. Initiation occurs by nucleophilic attack of monomer on protonated (activated) monomer (XXIV) to form an ammonium salt (XXV) that subsequently undergoes proton exchange with monomer to yield XXVI and protonated monomer. The conversion of XXIV to XXV involves several steps—attachment of nitrogen to C+, proton transfer from... 

The reactivity of the five-membered heterocycles pyrrole, furan, thiophen and imidazole (Fig. 8-10) is characterised by interactions with electrophilic reagents. The precise nature of these reactions depends upon the particular ring system. Thiophens undergo facile electrophilic substitution, whereas the other compounds exhibit a range of polymerisation and other Lewis acid-initiated reactions upon treatment with electrophiles. We saw a number of examples of Lewis acid-promoted reactions of furans and pyrroles in Chapter 6. Although reactions of complexes of five-membered heterocyclic ligands have not been widely investigated, a few examples will illustrate the synthetic potential. 

Chiral 1,2-dihydropyridine complex 78 has been shown to undergo Lewis acid-initiated [5+2] cycloadditions with electron-deficient alkenes, such as methyl acrylate, to give, after demetalation using ceric ammonium nitrate, functionalized tropanes 80 (Scheme 22) <20000L3909, 1999JA5811>. Short reaction times and substoichiometric amounts of Lewis acid are sufficient to ensure regio- and enantioselectivity. It is thought that excess Lewis acid mediates a slow reversal... 

Heterobicyclic compounds often display specific behavior in their ring-opening polymerization. This is attributed to their rigid and bulky structures which contain, in most cases, two or more asymmetric carbon atoms. Sometimes, stereoelectronic effects involving heteroatoms also play an important role in regulating polymerization processes. For example, racemic bicyclic acetals such as 6,8-dioxabicy-clo[3.2.1]octane and its derivatives often undergo stereospecific polymerization even in the presence of conventional Lewis acid initiators. 

Lewis acids initiate cationic chain-growth polymerizations. There are several possible chain propagation reactions, and the mechanism of cationic chain growth is still open to... 

One very interesting new application of Lewis acids in curing epoxy adhesives has appeared within the last 25 years. The Lewis acid initiator for the cationic polymerization is formed by the heat or ultraviolet light-induced decomposition of Lewis acid Lewis base salts. Several patents by Crivello and coworkers43 47 describe compounds containing... 

Scheme 9 Mechanism of alkyl halide/Lewis acid-initiated polymerizations of alkenes. (From Ref. 57.)...

Scheme 11 Schematic energy profiles for the Lewis acid-initiated additions of various alkyl halides to a certain alkene. (From Ref, 60, reprinted with permission of Huthig Wepf Verlag, Zug, Switzerland.)...

The initiator stability must therefore be balanced with its required reactivity. For example, stable carbenium ions such as tropylium or trityl react very slowly with alkenes and are therefore not optimal initiators. Protonic acids may be sufficiently reactive but often have anions that are basic and cause transfer by j3-proton abstraction from the growing carbenium ion anions resulting from Lewis acid initiators are less basic. Many of the new efficient initiators are prepared by mixing two stable components, such as an alkyl halide and a Lewis acid, which provide reversibly active carbenium ions. 

Some Lewis acid-initiated polymerizations have been proposed to proceed by direct addition of the Lewis acid to the monomer s double bond. However, this is usually an exception, and has been clearly proven only for iodine [69,135] and boron halide [136,137] initiated systems. Iodi-nation and haloboration are reversible processes which produce deactivated alkyl halides due to the electron-withdrawing substituent at the neighboring carbon [Eq. (30)]. 

No comparative data are available for these temperatures in Lewis acid-initiated polymerizations of styrene. However, DP increases linearly with polymer yield in TiCl4-initiated polymerizations in CH2CI2 at temperatures from -90° C to -25° C (DP 60 at -25° C, DP 200 at -60° C, [M]o 0.25 mol/L), indicating that transfer to monomer is small (CtrM - 4-10-4 and <5 I0-5) [308]. 

Both protonic and Lewis acids initiate ring-opening polymerization of siloxanes. In spite of the practical importance [240] and the very extensive studies of this system, the mechanism of the polymerization is not completely understood. Even the nature of ionic propagating species is still a matter of controversy. The silicenium [241] or oxonium [242,243] ion structure has been postulated. 

Figure 5.13. Reactions involved in cationic addition polymerization. Shown are (a) generation of a carbo-cation intermediate from a Lewis acid initiator, (b) propagation of the polymer chain through the combination of the carbocationic polymer chain and additional monomers, and (c) termination of the polymer growth through either proton abstraction (i) or anionic attachment (ii) routes.

To summarise our views on this topic we can say that the Kennedy s mechanism is probably operative, but the overwhelming process is the one originating from the formation of highly chlorinated aluminium catalysts formed in halogen-alkyl exchange reactions and possessing a much more pronounced acidity than the Lewis acids initially added. 

Presumably this reaction proceeds via an allylic carbocation intermediate 301 formed by Lewis acid-initiated C-S cleavage, then ring closure to a new carbocation 302 stabilized by the silyl group. As illustrated in Scheme 55, the subsequent steps, leading to final product 300, involve disulfide bond formation, double-bond shift, and hydride migration. 

Extension of these studies to strong Lewis acid initiators such as SbCls gave similar results except that a high concentration of THF. Lewis acid complex was formed very early and remained high throughout the polymerization. Lyudvig et al. [89] reached a similar conclusion in an... 

Treatment with a Lewis acid initiates a rearrangement very like those occurring when 3- The new seven-membered... 

FeCl3 is an efficient Lewis acid initiator for the cationic polymerization of monomers like NVC [60]. It is also an efficient dopant for PNVC enhancing its bulk conductivity [61]. Accordingly, in the study of the NVC-MMT polymeriza-tion/nanocomposite formation system, the addition of FeCl3 was considered to be interesting. Results of a recent study [33] (Table 2) indicated that in NVC-... 

Cationic polymerisation employs a strong protic or Lewis acid initiator. A proton, for example, adds to an electron-rich alkene to form the most stable carbocation. The carbocation then adds to another electron-rich alkene to build the polymer chain. The polymerisation is terminated by, for example, deprotonation. 

M. Kamigaito, M. Sawamoto, and T. Higashimura, Living cationic polymerization of vinyl ethers by electophile lewis acid initiating systems. 6. Living cationic polymerization of isobutyl vinyl ether by RCOOH/lewis acid initiating systems effects of carboxylate ions and lewis acid activators. Macromolecules 1991, 24(14), 3988-3992. 

Lewis acids initiate polymerization through the formation of carbonium ions, and Plesch (17) has proposed that a suitable coinitiator is necessary to produce these ions. The mechanism for the initiation of the homopolymerization of epoxy resins by BF3 complexes has been proposed by Arnold (11) to proceed as follows ... 

Exo-exo-, and endo-exo-2,3-dimethyl derivatives weue polymerized at 0 °C with 60% yield to polymer having Mn = 1000, whereas under the same conditions exo-exo- and endo-endo-2,5-dimethyl monomers gave only 15% polymer, and for exo-endo-derivatives the yield was 25 %55). Polymerization of these monomers with Lewis acid initiators is acompanied by termination. Thus, it is not clear whether the reported limited conversions reflect genuine monomer-polymer equilibria or are due to termination (cf. p. 12). 

Polymerization of (8-8) with Lewis acid initiators involves opening of the oxirane ring and leads to low-molecular-weight polymers in addition crosslinked gel is... 

This is in contrast to the already discussed polymerization of cyclic ethers and acetals, where two growing species are formed, yielding dicationically growing macromolecules. The amide group, which could be a potential site for the second growing species, is unreactive. As shown in Table 10.1 the majority of polymers were prepared with Lewis acid initiators, most frequently with the BF3 OEt2 complex. 

Metallocene methylene polymers have also been prepared via the preparation of an a-metallocenylcarbocation with an acidic or Lewis acid initiator, followed by polycondensation involving electrophilic substitution of the metallocene nucleus... 

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