Lewis acids cationic initiators

Rashkov, I. B., and Gitsov, I., Cationic polymerization initiated by intercalation compounds of Lewis acids. II. Initiating ability and mechanism of action of the initiators, J. Polym. 

Cationic Poiymerization with Lewis Acids as initiators... 

Cationic Polymerization. Cationic polymerization is initiated by the transfer of a cation from the catalyst to the monomer. It allows a wider choice of monomers with double bonds, including carbonyls, cyclic ethers, and lactones. The ion may be within a carbonium or an oxonium ion. Friedel-Crafts halides, like AlCls or A CoHsJCL, are strong Lewis acids and initiate the polymerization directly. Weak Lewis acids need a... 

In cationic polymerizations, initiation occurs by attachment of a proton or some other Lewis-acidic cation X" to the H2C=CR2 double bond of a vinyl monomer to form a new carbon-centred cation of the type XH2C-CR2, which then grows into a polymer chain by subsequent H2C=CR2 additions (Figure 2, bottom). This type of polymerization works well - and is used in practice - only for olefins such as isobutene, where 1,1-disubstitution stabilizes the formation of a cationic centre. Since side reactions, such as release of a proton from the cationic chain end, occur rather easily, cationic polymerization usually gives shorter chains than anionic polymerization. 

Since a direct and crucial proof for the occurrence of an initiation reaction consisting of the addition of a Lewis acid cation to the monomer double bond is not yet available, we will expound some relevant observations which seem to us highly suggestive indications of such an interaction. 

Simple Lewis acid cationic catalysts, e.g. BF3, SnCl4, AIR3, PFj, SbCls etc., have been extensively used in the study of the polymerization of THF [118]. With these initiators the problem of estimating the concentration of prop ating species becomes acute, and is not made any easier by the use of so-called promoters such as propene oxide,... 

Much work on cationic polymerization has been done with Lewis acids as initiators. We must realize that Lewis acids by themselves are not the actual initiators but a reaction product of unknown structure is formed either with the monomer or with impurities in the system. Some of the Lewis acids are extremely active initiators and are active at very low... 

Because strong Bronsted (proton) acids and Lewis acids can initiate styrene polymerization, other cationically polymerizable monomers can be added to the styrene-based copolymer list. Due to the facile occurrence of chain transfer processes of polymer chains with impurities, cationically prepared polystyrene-based polymers are low molecular weight materials. Nevertheless, low molecular weight polystyrenes still find important applications as additives, as tackifiers for pressure sensitive adhesives, and in hot melt adhesives. However, the market for low molecular weight polystyrene is small. 

Cationic Polymerization with Lewis Acids as Initiators... 

Another type of chain reaction is possible by ionic initiation. Both, positive and negative ions may start polymerization reactions (cationic and anionic polymerization). The existence of ions in solution is illustrated in Fig. 3.24 (B = base, M = metal). The interaction with the solvent plays an important role in the activity of the catalyst. In the covalent combination, no reactivity is expected. As the ions separate, they become increasingly available for reaction. While one of the ions supports the reaction, the other, the counterion, is in the vicinity and may also affect the rate of reaction. Since cationic initiators accept a pair of electrons, one finds these among the Lewis acids. Anionic initiators donate an electron pair, so they are Lewis bases. 

Similarly, cationic initiators such as protic or Lewis acids jne also not very effective as polymerization catalysts for these systems. The basicity of the skeletal nitrogen atoms of the cyclophosphazene rings overwhelms the reactivity of the cationic initiators towards olefin polymerization. Thus, the cyclophosphazene rings are readily protonated or metalated and thus prevent both protic and Lewis acids from initiation of polymerization. 

Due to the highly electron-rich character of their double bonds, vinyl ethers are susceptible to cationic polymerization using a variety of Bronsted and Lewis acids as initiators. Bronsted acids as weak as H2SO3 (SO2-I-H2O) and H3PO4 effect the cationic polymerization of... 

This is also probably the case with the other classes of photoinitiators, with the exception of the diazonium and ferrocenium salts. The photolysis of diazonium salts is well known to generate Lewis acids which initiate polymerization either by themselves or in combination with a protogen. Similarly, the photolysis of ferrocenium salts proportedly proceeds by the mechanism shown in equation (24), which involves the formation of the Lewis acid shown. Apparently, this iron-containing Lewis acid is strong enough to initiate many types of cationic polymerization, including those of vinyl ethers. 

Overview Cationic polymerizations proceed via a chain growth process which involves a carbenium ion (R3C+) at the chain end [19, 28]. Most examples of living cationic polymerizations utilize low temperatures (-90 °C to -30 °C) and electrophilic solvents, such as dichloromethane. In contrast to anionic polymerizations that require Lewis bases as initiators, cationic polymerizations employ Brpnsted acids or Lewis acids as initiators or coinitiators. For example, common Lewis acids include the halides of aluminum, boron, tin, and titanium. 

An active catalyst site requires a metal-carbon bond that may have existed in the pre-catalyst, may have been formed upon initial activation by cocatalyst (via ligand exchange), or may exist because of a previous migratory insertion event. In most cases, the starting precursor of the catalyst is a metallocene dichloride (dichlorides are usrraUy the most artive precursors for coordination polymerization) complex, which obtains a vacant site as a consequence of reaction with cocatalyst (see Section 3.21.3.1 below). In the case of metallocene activation by MAO, the produced active center is a strongly Lewis acidic cationic metal complex stabilized by a bulky MAO anion the transition metal bears a vacant coordination site ready for complexation of the olefinic monomer (Figure 4(a)). 

For cracking reactions, combinations of zeolites, alumina, clay, and silica are used as the catalyst. These acidic materials, which contain both Br0nstead and Lewis acidic sites, initiate a complex set of carbonium- and carbenium ion-based reactions. Note that carbonium ions are protonated alkyl groups (e.g., C Hg ), while carbenium ions refer to alkyl cations (e.g., To enhance the acidic properties, rare... 

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. 

A second type of uv curing chemistry is used, employing cationic curing as opposed to free-radical polymerization. This technology uses vinyl ethers and epoxy resins for the oligomers, reactive resins, and monomers. The initiators form Lewis acids upon absorption of the uv energy and the acid causes cationic polymerization. Although this chemistry has improved adhesion and flexibility and offers lower viscosity compared to the typical acrylate system, the cationic chemistry is very sensitive to humidity conditions and amine contamination. Both chemistries are used commercially. 

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. 

Cationic polymerization with Lewis acids yields resinous homopolymers containing cycHc stmctures and reduced unsaturation (58—60). Polymerization with triethyl aluminum and titanium tetrachloride gave a product thought to have a cycHc ladder stmcture (61). Anionic polymeriza tion with lithium metal initiators gave a low yield of a mbbery product. The material had good freeze resistance compared with conventional polychloroprene (62). 

Catalytic curing agents initiate resin homopolymerization, either cationic or anionic, as a consequence of using a Lewis acid or base in the curing process. The Lewis acid catalysts frequently employed are complexes of boron trifluoride with amines or ethers. 

The most important reaction with Lewis acids such as boron trifluoride etherate is polymerization (Scheme 30) (72MI50601). Other Lewis acids have been used SnCL, Bu 2A1C1, Bu sAl, Et2Zn, SO3, PFs, TiCU, AICI3, Pd(II) and Pt(II) salts. Trialkylaluminum, dialkylzinc and other alkyl metal initiators may partially hydrolyze to catalyze the polymerization by an anionic mechanism rather than the cationic one illustrated in Scheme 30. Cyclic dimers and trimers are often products of cationic polymerization reactions, and desulfurization of the monomer may occur. Polymerization of optically active thiiranes yields optically active polymers (75MI50600). 

The initial step is the coordination of the alkyl halide 2 to the Lewis acid to give a complex 4. The polar complex 4 can react as electrophilic agent. In cases where the group R can form a stable carbenium ion, e.g. a tert-buiyX cation, this may then act as the electrophile instead. The extent of polarization or even cleavage of the R-X bond depends on the structure of R as well as the Lewis acid used. The addition of carbenium ion species to the aromatic reactant, e.g. benzene 1, leads to formation of a cr-complex, e.g. the cyclohexadienyl cation 6, from which the aromatic system is reconstituted by loss of a proton ... 

Closely related to the polyepoxide cascade procedure for the synthesis of polycyclic systems is Corey s biomimetic-type, nonenzymatic, oxirane-initiated (Lewis acid-promoted) cation-olefin polyannulation. By this strategy, compound 96, containing the tetracyclic core of scalarenedial, was constructed by exposure of the acyclic epoxy triene precursor 95 to MeAlCl2-promoted cyclization reaction conditions (Scheme 8.25) [45]. 

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