Anionic Polymerization of Acrylic Monomers

Ballard et al. [64] found that bulky dialkyl aluminum phenolate additives would improve the anionic polymerization of acrylic monomers. They called their method Screened Anionic Polymerization (Scheme 28). 

The favorable effect of trialkylaluminium on the anionic polymerization of acrylic monomers induced by organolithium compounds prompted a DFT theoretical and 6Li,13C NMR study on the interaction between AlEt3 and ethyl lithioisobutyrate (EIBLi) in toluene at — 20 °C. Monomers, dimers and tetramers of 1 1 or 1 2 enolate and AlEt3 aggregates were identified (Scheme 67)286. 

Acrylate monomers do not generally polymerize by a cationic mechanism. However, the anionic polymerization of acrylic monomers to stereoregular or block copolymers is well known. These polymerizations are conducted in organic solvents, primarily using organometallic compounds as initiators. 

An important extension of anionic polymerization of acrylic monomers was the discovery of group-transfer polymerization (GTP), by Webster etal. [56], which allowed the synthesis of acrylic and methacrylic polymers in a Kving reaction at ambient temperature or above. A wide range of all-(meth) acrylic block copolymers as well hydrophilic-hydrophobic, as double-hydrophilic copolymers which are of special interest for micellization studies, could be prepared by Armes and co workers [57]. 

Hautekeer, J.R, Varshney, S.K., Fayt, R., Jacobs, G., JerOme, R. and Teyssi, Ph. (1990) Anionic polymerization of acrylic monomers. Synthesis, characterization and modification of polystyrene-poly(tert-butylacrylate) di- and triblock copolymers. Macromolecules, 23,... 

A brief review has appeared covering the use of metal-free initiators in living anionic polymerizations of acrylates and a comparison with Du Font s group-transfer polymerization method (149). Tetrabutylammonium thiolates mn room temperature polymerizations to quantitative conversions yielding polymers of narrow molecular weight distributions in dipolar aprotic solvents. Block copolymers are accessible through sequential monomer additions (149—151) and interfacial polymerizations (152,153). 

Lithium ester enolates are extremely important in polymer chemistry as initiators and active centers of the anionic polymerization of acrylic and methacrylic monomers in polar solvents. Thus, HF-SCF studies, comparable to those mentioned above, were undertaken on monomeric methyl isobutyrate (MIB) enolate210,211. The overall conclusions on the aggregation and solvation trends are exactly the same, the bent rj3-0,C mode being preferred over the rj1-O planar one by ca 3.3 kcalmol-1. While the dimeric MIB enolate solvated by four molecules of THF was found to be the enthalpically most stable aggregate, the prismatic S6 unsolvated MIB hexamer was computed as the preferred structure in non-polar solvents (Scheme 55)212. In the latter case, the supplementary oxygen of the ester acting as a side-chain ligand for the lithium seems to explain this remarkable stability. 

The surfactants used in the emulsion polymerization of acrylic monomers are classified as anionic, cationic, or nonionic. Anionic surfactants, such as salts of alkyl sulfates and alkylarene sulfates and phosphates, or nonionic surfactants, such as alkyl or aryl polyoxyethylenes, are most common (87,98—101). Mixed anionic—nonionic surfactant systems are also widely utilized (102—105). 

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. 

Lithium The anionic polymerization of some monomers, initiated by metallic lithium deposited on a cathode in organic solvents has been proposed by Albeck et al., who studied the electropolymerization of some acrylates in methanol120,121,122 This work could be correlated with that of Kikuchi and Mitoguchi, who reported the cathodic electropolymerization of acrylonitrile with LiC104 in DMF123. ... 

New developments in group transfer polymerization have made possible the living polymerization of acrylate and methacrylate monomers using silyl ketene acetal initiators with a nucleophilic or Lewis acid catalyst (73). By this method we may circumvent the side reactions which accompany conventional anionic polymerizations of acrylates and methacrylates and prepare almost mono-... 

A dramatic development in the anionic polymerization of acrylate and methacrylate monomers was the discovery that by addition of lithium chloride it was possible to effect the controlled polymerization of f-butyl acrylate [122]. Thus, using oligomeric (a-methylstyryl)lithium as initiator in THE at -78 °C, the molecular weight distribution of the... 

A dramatic development in the anionic polymerization of acrylate and methacrylate monomers was the discovery that by addition of lithium chloride it was possible to effect the controlled polymerization of f-butyl acrylate (86). Thus, using oligomeric (o -methylstyryl)lithium as initiator in THF at —78°C, the molecular weight distribution (M /Mn) of the polymer was 3.61 in the absence of lithium chloride but 1.2 in the presence of lithium chloride ([LiCl]/[RLi] = 5). In the presence of 10 equiv of LiCl, f-butyl acrylate was polymerized with 100% conversion and 95% initiator efficiency to provide a polymer with a quite narrow molecular weight distribution (My,/Mn = 1.05). More controlled anionic polymerizations of alkyl methacrylates are also obtained in the presence of lithium chloride. Other additives, which promote controlled pol5unerization of acylates and methacrylates, include lithium f-butoxide, lithium (2-methoxy)ethoxide, and crown ethers (47,48). The addition of lithium chloride also promotes the controlled anionic polymerization of 2-vinylpyridine. 

There are, however, several highly reactive vinyl monomers such as 2-(trifluoromethyl) acrylates and 2-cyanoacrylates that undergo anionic polymerizations in the presence of even weak bases. The photoinitiated anionic polymerizations of these monomers have been achieved using a number of photosensitive metal complexes. For example, the irradiation of alkali salts containing the trans-[Cr(NH3)2(NCS)4] anion at wavelengths in the range of 350-532 nm releases the thiocyanate anion (SCN"). As depicted in Scheme 34, the thiocyanate anion is capable of initiating the anionic chain polymerization of ethyl 2-cyanoacrylate. ... 

Anionic polymerization of polar monomers is complicated by side reactions that involve the polar groups. For example in the polymerization of acrylates and methacrylates, the initiator species and propagating active centres can react with the C=0 groups in the monomer (or polymer)... 

Group transfer polymerization (GTP) is a technique for the polymerization of acrylic monomers discovered by the scientists at Dupont in 1983 [7-10]. The technique gives living polymers, remarkably free of termination or transfer reactions, at room temperatures or above. This is in distinct contrast to anionic polymerization of methacrylic monomers, which can be performed in a truly living manner only at low temperatures (much below 0°C). GTP... 

To be eligible to living anionic polymerization a vinylic monomer should carry an electron attracting substituent to induce polarization of the unsaturation. But it should contain neither acidic hydrogen, nor strongly electrophilic function which could induce deactivation or side reactions. Typical examples of such monomers are p-aminostyrene, acrylic esters, chloroprene, hydroxyethyl methacrylate (HEMA), phenylacetylene, and many others. 

These TMS-carbamate-mediated NCA polymerizations resemble to some extent the group-transfer polymerization (GTP) of acrylic monomers initiated by organo-silicon compounds [40]. Unlike GTPs that typically require Lewis acid activators or nucelophilic catalysts to facilitate the polymerization [41], TMS-carbamate-mediated NCA polymerizations do not appear to require any additional catalysts or activators. However, it is still unclear whether the TMS transfer proceeds through an anionic process as in GTP [41] or through a concerted process as illustrated in Scheme 14. 

The activity of transition metal allyl compounds for the polymerization of vinyl monomers has been studied by Ballard, Janes, and Medinger (10) and their results are summarized in Table II. Monomers that polymerize readily with anionic initiators, such as sodium or lithium alkyls, polymerize vigorously with allyl compounds typical of these are acrylonitrile, methyl methacrylate, and the diene isoprene. Vinyl acetate, vinyl chloride, ethyl acrylate, and allylic monomers do not respond to these initiators under the conditions given in Table II. 

The controlled polymerization of (meth)acrylates was achieved by anionic polymerization. However, special bulky initiators and very low temperatures (- 78 °C) must be employed in order to avoid side reactions. An alternative procedure for achieving the same results by conducting the polymerization at room temperature was proposed by Webster and Sogah [84], The technique, called group transfer polymerization, involves a catalyzed silicon-mediated sequential Michael addition of a, /f-unsaluralcd esters using silyl ketene acetals as initiators. Nucleophilic (anionic) or Lewis acid catalysts are necessary for the polymerization. Nucleophilic catalysts activate the initiator and are usually employed for the polymerization of methacrylates, whereas Lewis acids activate the monomer and are more suitable for the polymerization of acrylates [85,86]. 

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