Polymerization, anionic methacrylates, alkyl

Reactions catalyzed by phosphazenes have also been described. The catalytic enantioselective alkylation of the amino acid intermediate Ph2C = NCH2C02-t-Bu by alkyl halides was reported to occur efficiently in the presence of BEMP or BTTP [52], and such bases catalyze Michael additions in non-aqueous [53] and aqueous media [54]. Methyl [55] and butyl [56] methacrylates are anionically polymerized using a phosphazene base as an initiator in the presence of an ester that is apparently deprotonated in the process. Functioning as promoters in the... 

Another quite recently developed method for the controlled polymerization of methacrylates via anionic polymerization is the screened anionic polymerization (SAP), investigated by Haddleton et al. The systems are based on lithium aluminum alkyl/phenoxide initiators, which are synthesized in situ following the equation shown... 

The difficulties encountered in the early studies of anionic polymerization of methyl methacrylate arose from the unfortunate choice of experimental conditions the use of hydrocarbon solvents and of lithium alkyl initiators. The latter are strong bases. Even at —60 °C they not only initiate the conventional vinyl poly-addition, but attack also the ester group of the monomer yielding a vinyl ketone1, a very reactive monomer, and alkoxide 23). Such a process is described by the scheme. 

Alkyl methacrylates, hydrolysis of polymeric ester functionality, 259 Aluminum-hydrogen bond, nucleophilic substitution, 264 Amines alkylation, 28 benzyl-group cleavage, 25 Aminomethylation chloromethylated polymers, 19 Deltfpine reaction, 19 Anionic polymerization advantages, 85... 

Alkyl derivatives of the alkaline-earth metals have also been used to initiate anionic polymerization. Organomagnesium compounds are considerably less active than organolithiums, as a result of the much less polarized metal-carbon bond. They can only initiate polymerization of monomers more reactive than styrene and 1,3-dienes, such as 2- and 4-vinylpyridines, and acrylic and methacrylic esters. Organostrontium and organobarium compounds, possessing more polar metal-carbon bonds, are able to polymerize styrene and 1,3-dienes as well as the more reactive monomers. 

Diphenylmethylcarbanions. The carbanions based on diphenyknethane (pKa = 32) (6) are useful initiators for vinyl and heterocyclic monomers, especially alkyl methacrylates at low temperatures (94,95). Addition of lithium chloride or lithium /W -butoxide has been shown to narrow the molecular weight distribution and improve the stability of active centers for anionic polymerization of both alkyl methacrylates and tert-huXyi acrylate (96,97). Surprisingly, these more stable carbanions can also efficiendy initiate the polymerization of styrene and diene monomers (98). 

Enolate Initiators. In principle, ester enolate anions should represent the ideal initiators for anionic polymerization of alkyl methacrylates. Although general procedures have been developed for the preparation of a variety of alkali metal enolate salts, many of these compounds are unstable except at low temperatures (67,102,103). Useful initiating systems for acrylate polymerization have been prepared from complexes of ester enolates with alkali metal alkoxides (104,105). 

Classic anionic polymerization must be conducted at too low temperature for commercial feasibility. Anionic polymerization of hindered acrylates and methacrylates with stable large counterions works at temperatures close to those required for condenser cooling. The anionic polymerization of lower alkyl acrylates and methacrylates is still a problem. 

Lithium and magnesium alkyl catalysts yield metal-polymer bonds with appreciable covalent character and their cations coordinate strongly with nucleophiles. Therefore, these catalysts will initiate simple anionic polymerization only under the most favorable conditions, e. g., in basic solvents and with monomers which produce resonance stabilized polymer anions. As examples of stereoregular anionic polymerization, a-methyl-methacrylate yields syndiotactic polymer with an alkyl lithium catalyst in 1,2-dimethoxyethane at — 60° C. (211, 212) or with a Grignard catalyst at -40° C. (213). 

The initiation of methyl methacrylate polymerization by alkyl- and aryl-lithium compounds is a very complex process which has not been completely elucidated so far. Even less information is available on the efects of or-ganometals not containing lithium. Recently, the interest of some authors has centred on the application of Grignard reagents for the initiation of anionic polymerizations, especially of MMA [171, 172],... 

If the monomers contain reactive groups that could be attacked by carbanions they will not be suitable for anionic polymerizations. Halogen-containing vinyl monomers are difficult to polymerize in these systems because of the elimination of alkyl halides. Interactions between many initiators and the carbonyl groups of methacrylates or acrylates necessitate the use of special reaction conditions, like very low temperatures, for the anionic polymerization of these monomers. 

Chlorine atoms are prompt to enter into side reactions with metal alkyls. On the other hand, they do not appreciably facilitate anionic polymerization. Polymerization rates are relatively slow, even at room temperature. It is then difficult to suppress the side reactions by lowering the polymerization temperature, as is commonly done for methacrylates and other systems [8]. 

MMA toward an alkyl radical [302]. Because of its low electron density (e=2.7) [302], this fluoromonomer readily undergoes anionic polymerization with amines and organic and inorganic salts in the presence of 18-crown-6 [301, 303]. However, anionic initiators commonly used for polymerization of MMA such as butyllithium and Grignard reagents fail to polymerize this monomer due to Sn2 addition-elimination [303,304]. All-acrylate polymers for 157 nm imaging have been prepared by radical terpolymerization of 2-trifluoromethyl-acrylate and methacrylate or acrylate [288] (Fig. 90). 

Yamamoto and collaborators have pioneered efforts to prepare polymers of methacrylates containing very bulky ester substructures. For the most part, the approaches reported involve anionic polymerization utilizing chiral amines and alkyl lithium initiators. Polymers prepared in this way are highly helical and have many useful properties, including their use in chiral chromatography. 

Polar Vinyl Monomers The anionic polymerization of polar vinyl monomers is often complicated by side reactions of the monomer with both anionic initiators and growing carbanionic chain ends, as well as chain termination and chain transfer reactions. However, synthesis of polymers with well-defined structures can be effected under carefully controlled conditions. The anionic polymerizations of alkyl methacrylates and 2-vinylpyridine exhibit the characteristics of living polymerizations under carefully controlled reaction conditions and low polymerization temperatures to minimize or eliminate chain termination and transfer reactions [118, 119]. Proper choice of initiator for anionic polymerization of polar vinyl monomers is of critical importance to obtain polymers with predictable, well-defined structures. As an example of an initiator that is too reactive, the reaction of methyl methacrylate (MMA)... 

Like the anionic polymerization of dienes, the anionic polymerization of alkyl methacrylates, especially MMA, is dependent on the counterion, solvent, and, to a certain extent, temperature [156, 173, 121]. In general, the stereochemistry of the anionic polymerization of alkyl methacrylates in toluene solution with lithium as the counterion is highly isotactic (68-99% mm) and the isotacticity increases with the steric requirements of the alkyl ester. For example 90% mm triads are obtained for t-butyl methacrylate at —70 °C in toluene [174], while 68% mm triads are observed for MMA [120]. Isospecificity for polymerizations in toluene... 

A variety of stereoregulating mechanisms have been invoked to explain the stereochemistry of anionic polymerization of alkyl methacrylates. As discussed by Pino and Suter [189], although syndiotactic diads are thermodynamically slightly more favored over isotactic diads, the free energy differences are so small that the formation of stereoregular... 

Microreactors have also been used for ionic polymerization or polycondensation processes. Nagaki et al. [136] have synthesized polystyrene-poly(alkyl methacrylate) block copolymers by butyllithium initiated anionic polymerization in an integrated flow microreactor system. A high level of control of molecular weight was achieved at temperatures between -28 and +24 °C due to fast mixing, fast heat transfer, and residence time control. Santos and Metzger... 

Table 1. Anionic polymerizations of n-[4 -(4"-methoxyphenyl)phenoxy]alkyl methacrylates (n=2-6).

Themodynamically induced shear degradation also has been investigated using solutions of anionically polymerized poly(n-alkyl methacrylate)s in poor solvents Three different lengths of the alkyl substituents (methyl PMMA n-butyl PBMA -decyl PDMA) were used in the course of the study. As an example for the thermodynamic characterization of these systems, Fig. 6 shows the cloud point curves of PDMA dissolved in -butanol... 

Schlaad, H., Muller, A. H. E., Kolshom, H., and Kruger, R.-P., Mechanism of Anionic Polymerization of (Meth)acrylates in the Presence of Aluminium Alkyls. 3. MALDI-TOF-MS Study on the Vinyl Ketone Formation in the Initiation Step of Methyl Methacrylate with ferf-Butyl Lithium, Polymer Bull., 35, 177, 1995. 

Sugimoto, H., Aida, T., and Inoue, S,. 1994, Lewis Acid-Promoted Living Anionic Polymerization of Alkyl Methacrylates Initiated with Aluminum Porphyrins. Importance of Steric Balance between a Nucleophile and a Lewis Acid, Macromolecules, 27 3672... 

Controlled/Living Anionic Polymerization of Alkyl Methacrylates Using Flow Microreactor Systems... 

Fig. 16 Flow microreactor system for anionic polymerization of alkyl methacrylates initiated by 1,1-diphenylhexyllithium. Ml, M2 T-shaped micromixers Rl, R2 microtube reactors...

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