Lewis acids polymerization reaction

An interesting bifunctional system with a combination of In(OTf)3 and benzoyl-quinine 65 was developed in p-lactam formation reaction from ketenes and an imino ester by Lectka [Eq. (13.40)]. High diastrereo- and enantioselectivity as well as high chemical yield were produced with the bifunctional catalysis. In the absence of the Lewis acid, polymerization of the acid chloride and imino ester occurred, and product yield was moderate. It was proposed that quinine activates ketenes (generated from acyl chloride in the presence of proton sponge) as a nucleophile to generate an enolate, while indium activates the imino ester, which favors the desired addition reaction (66) ... 

The majority of the condensation polymerizations can be considered extensions of typical Lewis acid-base reactions. 

Nafion-H is a convenient acid catalyst for pinacolone rearrangements. Hydration of acetylenes can be conducted with Nafion-H impregnated with mercury(II) ions. Diels-Alder catalyst. This protic resin catalyzes Diels-Alder reactions, but longer reaction times are needed than in reactions catalyzed by Lewis acids. The reactions are generally conducted in refluxing benzene or chloroform. In the case of dienes that polymerize readily, the reaction is conducted at room temperature for 1-2 days.°... 

Alkyl halides are widely used as cocatalysts in combination with aluminum alkyl halides or aluminum halide Lewis acids. Tlie reaction scheme in Fig. 9-2 illustrates the complicated equilibria which may affect the initiation process. Each carbenium ion can initiate polymerization or remove an ethyl group from the counterion to produce a saturated hydrocarbon, REt, and a new more acidic Lewis acid. The propagating macrocarbenium ions can also terminate by the same process to produce ethyl-capped polymers and new Lewis acids. Thus, even though the initiator is ostensibly dielhylaluminum chloride there may be major contributions to the polymerization from ethyl aluminum dichloride or aluminum chloride. 

Although copper reagents, hahdes and triflates, are widely used in atom-transfer polymerization reactions (ATRP) [63], these processes do not fall under the category of Lewis acid-mediated reactions. Sherrington and co-workers have shown that a vinyl monomer coordinated to a chiral copper Lewis acid (122) undergoes stereoselective polymerization (Sch. 29) [64]. A chiral block-copolymer 124 was prepared under radical conditions. 

Ring-opening polymerization of cyclic ethers and lactones has been a major area of research in Lewis acid-promoted reactions. In particular, aluminum compounds have been investigated in depth not only because of their high oxophilicity and ability to initiate polymerization but also because of their commercial availability and low cost. 

After the photolysis of the dlazonlum derivative and the generation of the Lewis acid, the light may be removed and cationic polymerization can proceed further In the dark as any other cationic polymerization Initiated by Lewis acids. The reaction Is terminated by the disappearance of available epoxide or by interaction of the growing chain with moisture. 

Acid catalysis by Br nsted acids provided the first examples of cationic polymerization. The later discovered (but still before the salts) polymerization initiation and start by Lewis acids was not initially well understood. In many cases of polymerization started by Lewis acids, polymerization rates and degrees of polymerization were not very reproducible. Today, it is accepted that many pure Lewis acids cannot induce any polymerization at all in monomers of high degrees of purity, and that the initiator requires at least a trace of cocatalyst. Water, HCl, and CCI3COOH are examples of this kind of cocatalyst, but in certain cases alkyl halides and ether, the solvent used, or even the monomer itself are effective. Since it is difficult to free the initiator, monomer, solvent, and the reaction vessel from traces of these cocatalysts, the poor reproducibility in qualitative, and above all in quantitative, measurements is quite understandable. 

The intramolecular Diels-Alder reactions of the Fischer carbene complexes, (L)(0C)4W=C(0R)CH=CH(CH2) CH=CHCH=CH2, (L = CO, PPh3 R = Me or Pri n = 3 or 4) are compared with known reactions of the analogous methyl esters. The stereoselectivites are comparable to those of the Lewis acid catalysed reactions but are more tolerant of functionalised diene units." The reaction between Mo(CO)6 and All(H)N(CH2)2NHAll/CH(NMe2)(OMe)2 (All = allyl) in methylcyclohexane at 1(X) C affords the chelated cyclic carbene complex 122, while polymeric carbene materials 123 are available from electron rich alkene precursors containing pendant phosphine donor groups such as 124. ... 

The hydrocarbon resins can be produced by a simple thermal polymerization process (48-50) or by Lewis acid catalyzed reaction (51). The thermal process is carried out at a high temperature in the range of 200-280°C and a reactor pressure above 300 psig. At temperatures below 200°C, the Diels-Alder polymers are formed. They are not desirable in most resins because they are insoluble in aromatic solvents. If reaction temperature exceeds 280°C, decomposition of the resins would occur. 

Under acidic conditions, furfuryl alcohol polymerizes to black polymers, which eventually become crosslinked and insoluble in the reaction medium. The reaction can be very violent and extreme care must be taken when furfuryl alcohol is mixed with any strong Lewis acid or Brn nstad acid. Copolymer resins are formed with phenoHc compounds, formaldehyde and/or other aldehydes. In dilute aqueous acid, the predominant reaction is a ring opening hydrolysis to form levulinic acid [123-76-2] (52). In acidic alcohoHc media, levulinic esters are formed. The mechanism for this unusual reaction in which the hydroxymethyl group of furfuryl alcohol is converted to the terminal methyl group of levulinic acid has recendy been elucidated (53). 

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. 

Uses ndReactions. Some of the principal uses for P-pinene are for manufacturing terpene resins and for thermal isomerization (pyrolysis) to myrcene. The resins are made by Lewis acid (usuaUy AlCl ) polymerization of P-pinene, either as a homopolymer or as a copolymer with other terpenes such as limonene. P-Pinene polymerizes much easier than a-pinene and the resins are usehil in pressure-sensitive adhesives, hot-melt adhesives and coatings, and elastomeric sealants. One of the first syntheses of a new fragrance chemical from turpentine sources used formaldehyde with P-pinene in a Prins reaction to produce the alcohol, Nopol (26) (59). 

An extremely wide variety of catalysts, Lewis acids, Brmnsted acids, metal oxides, molecular sieves, dispersed sodium and potassium, and light, are effective (Table 5). Generally, acidic catalysts are required for skeletal isomerization and reaction is accompanied by polymerization, cracking, and hydrogen transfer, typical of carbenium ion iatermediates. Double-bond shift is accompHshed with high selectivity by the basic and metallic catalysts. 

Polymerization of ethylene oxide can occur duriag storage, especially at elevated temperatures. Contamination with water, alkahes, acids, amines, metal oxides, or Lewis acids (such as ferric chloride and aluminum chloride) can lead to mnaway polymerization reactions with a potential for failure of the storage vessel. Therefore, prolonged storage at high temperatures or contact with these chemicals must be avoided (9). 

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). 

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