Lewis acids as initiators

Cationic Poiymerization with Lewis Acids as initiators... 

The newest condensation reaction to poly(dichlorophosphazene), developed by Allcock, Manners, and their coworkers,44 47 uses an N-silylphosphoraniminc, Me3SiN=Cl3 (3.43), as a monomer for bulk or solution phase polymerizations at room temperature in the presence of small amounts of Lewis acids as initiators. The process yields... 

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... 

It was demonstrated that DBO polymerizes with Lewis acids as initiators. BF3 OEt SbCl5 and PFS were used and the best results were obtained with PFS however, no systematic studies of the kinetics of polymerization or molecular weights have been published yet. The stereochemistry of DBO polymerization was investigated in detail, and it was shown that DBO polymerizes exclusively through opening of the 5-membered ring ... 

Cationic Polymerization with Lewis Acids as Initiators... 

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... 

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. 

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. 

Titanium alkoxides are also effective and sought-after initiators for the ROP of lactides due to a low toxicity, which minimizes the problems linked to the presence of catalyst residues in commercial PLA products [18, 19]. Despite impressive advancements in the use of Lewis acidic metal initiators in the preparation of PLAs, surprisingly little attention has been paid to the group 4 metal (Ti, Zr, Hf) initiators, probably due to the highly oxophilic nature of M(1V) which has a natural tendency to form aUcoxy-bridged multinuclear complexes. Verkade and coworkers previously demonstrated a series of titanium aUcoxide complexes 118-122 (Fig. 17) that function as moderately efficient initiators in bulk homopolymeization of L-lactide and rac-lactide, some of these initiators displaying a well-controlled polymerization behavior [119]. 

Use of Lewis acid anions, which disproportionate on heating to yield neutral Lewis acid polymerization initiators, improves the efficiency of this safety strategy. The most common Lewis acid anions and their associated elevated temperature disproportionation products are shown in the table. Since these anions exhibit significant dissociation equilibria at temperatures less than 100°C, they are not as useful for applications demanding long-term elevated temperature operation. 

The activators (coinitiators) are not only molecular iodine but also other mild Lewis acids (MtX ) that are mostly metal halides such as zinc halides. The suitable metal halides and other Lewis acids should be able to effectively activate the carbon-B linkage of the adducts 5 but they should not be too strongly Lewis acidic to initiate uncontrolled polymerization in the presence of adventitious protogens like water. 

A comparison of Eq. (22) with Eq. (23) reveals that the apparent rate constant, k, equals the product kp K. Thus, the rate of polymerization is affected by the concentrations of monomer, initiator, and Lewis acid, as well as by the rate and equilibrium constants. Although the ionic propagation rate constant is not very sensitive to the nature of the counterion, solvent and temperature, the equilibrium constant K usually depends strongly on the temperature, solvent, Lewis acid, and the leaving group X. 

Initiation efficiency can also be improved by using an initiator with a leaving group better than the one in the growing species. This is the case for alkyl acetates used as initiators with boron halides as Lewis acids as activators [35,117]. The initiating acetates are ionized stronger and faster than macromolecular alkyl halides (fc,° > k"), resulting in an enhancement of the relative initiation rate ... 

Salt/molecule reactions with fluoride acceptors or Lewis acids appear, in all cases studied to date, to form ion pair complexes as the initial reaction product for such Lewis acids as HF, SiFi, BF3 and COF2. There has been no evidence of abstraction as the initial step, although some systems may exist where this step will be preferred. Some spectroscopic effects of ion pairing are observed in the spectra of the product anions, but these still allow for the study of imusual anions in a relatively unperturbed environment. 

A different notation with the protogen as "initiator" and the Lewis acid as "co-initiator" has been suggested [84], but has so far not been generally accepted. 

The isolation of the initial aldol products from the condensation of the enolates of carbene complexes and carbonyl compounds is possible if the carbonyl compound is pretreated with a Lewis acid. As indicated in equation (9), the scope of the aldol reaction can also be extended to ketones and enolizable aldehydes by this procedure. The condensations with ketones were most successful when boron trifluoride etherate was employed, and for aldehydes, the Lewis acid of choice is titanium tetrachloride. The carbonyl compound is pretreated with a stoichiometric amount of the Lewis acid and to this is added a solution of the anion generated from the caibene complex. An excess of the carbonyl-Lewis acid complex (2-10 equiv.) is employed however, above 2 equiv. only small improvements in the overall yield are realized. 

Fhal and Renaud have examined the alkylation of a radical generated from the a-iodoimide 333 with a variety of Lewis acids, as shown in Sch. 42 [70]. The stereogenic step in this process would be hydrogen atom transfer from tin to a Lewis acid-com-plexed radical generated from 333. Initial screening was performed for the reaction of allyltributylstannane and imide 333, which was conducted by precomplexation of the imide with the Lewis acid and then addition of the stannane in the presence of AIBN under irradiation at 10 °C. The Lewis acid prepared from BINOL was ineffective whereas that prepared from the bis-sulfonamide 337 was slightly superior to that from the TADDOL ligand 339. 

In practice such salts do function as photo-initiators but via a mechanism which involves formation of protonic acid, not Lewis acid, as shown in Scheme 10. Furthermore, though the salts themselves require irradiation with u.v. light, photolysis is readily sensitized by dyes and allows the use of visible wavelengths. ... 

One of the first examples of Lewis-acid free initiators based on tungsten(VI), W(O-2,6-i-Pr2-C6H3)2Cl2(CH-t-Bu)(OR)2, (R=Et, i-Pr), which allowed the polymerization of substituted NBEs, was reported by J.M. Basset et al. [57]. Additionally, Lewis-acid free W(=C(CH2)4)(OCH2-t-Bu)2Cl2 was reported to effectively polymerize substituted NBEs such as exo-norborn-5-ene-2,3-dicarboxylic anhydride [58, 59[. 

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