Acrylate anionic polymerization polar solvents

The stereoselective polymerization of various acrylates and methacrylates has been studied using initiators such as atkyllithium [Bywater, 1989 Pasquon et al., 1989 Quirk, 1995, 2002]. Table 8-12 illustrates the effects of counterion, solvent, and temperature on the stereochemistry of the anionic polymerization of methyl methacrylate (MMA). In polar solvents (pyridine and THF versus toluene), the counterion is removed from the vicinity of the propagating center and does not exert an influence on entry of the next monomer unit. The tendency is toward syndiotactic placement via chain end control. The extent of syndiotacticity... 

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. 

Currently this technology is of minor commercial significance, but stereoregular forms of numerous polyacrylates have been prepared and characterized These include poly(/-butyl acrylate) (138—141), poly(isopropyl acrylate) (142), and poly(isobutyl acrylate) (143,144). Carefully controlled reaction conditions are usually required to obtain polymers with some measurable degree of crystallinity. In nonpolar solvents the anionic polymerization of acrylates generally yields isotactic polymer, whereas in polar solvents syndiotactie polymerization is favored. The physical and chemical properties of the various forms are often quite different. A general review covers these and other aspects of the anionic polymerization of acrylates (145). 

Ester enolates can be used as molecular models of the active centers in the anionic polymerization of acrylates and methacrylates. Thus, knowledge of the structure of these models in polar and nonpolar solvents is important for the understanding of the polymerization processes. Earlier C and Li NMR smdies by Wang and coworkers of methyl... 

Solvents influence the rate of free-radical homopolymerization of acrylic acid and its copolymerization with other monomers. Hydrogen-bonding solvents slow down the reaction rates. Due to the electron-withdrawing nature of the ester groups, acrylic and methacrylic ester polymerize by anionic but not by cationic mechanisms. Lithium alkyls are very effective initiators of a-methyl methacrylate polymerization yielding stereospecific polymers.Isotactic poly(methyl methacrylate) forms in hydrocarbon solvents. Block copolymers of isotactic and syndiotactic poly(methyl methacrylate) form in solvents of medium polarity. Syndiotactic polymers form in polar solvents, like ethylene glycol dimethyl ether, or pyridine. This solvent influence is related to Lewis basicity in the following order ... 

Coordinated anionic polymerizations of methyl methacrylate with diethyliron-bipyridyl complex in nonpolar solvents like benzene or toluene yield stereoblock polymers. In polar solvents, however, like dimethylformamide or acetonitrile, the products are rich in isotactic placement. There are many reports in the literature on polymerizations of acrylic and methacrylic esters with Ziegler-Natta catalysts. " The molecular weights of the products, the microstructures, and the rates of the polymerizations depend upon the metal alkyl and the transition metal salt used. The ratios of the catalyst components to each other are also important. ... 

Additives that contain alkoxy and alkoxide functionalities for efficient coordination with propagating enolate ion pairs have been developed by Wang et as a new class of ligands for the controlled polymerization of alkyl (meth) acrylates in both polar and nonpolar solvents. Alkoxyalkoxides combine advantages of alkoxides (jr-type ligands) and Lewis bases (o-type ligands) in one molecule. These additives allow a fast and controlled anionic polymerization of methacrylates and even primary acrylates. Since these ligands were mostly used in nonpolar solvents, they are reported in Section 3.19.2.4.2 further below. 

Not only acrylic esters that have intermediate solubility in water due to additional hydroxy or amino groups can be polymerized in water, but also conventional acrylic monomers with a relatively low water solubility (MMA 15g/L at room temperature) [36] can be polymerized in water. Acrylate monomers of intermediate solubility in water, such as hydroxyalkyl acrylates and methacrylates or aminoalkyl acrylates or methacrylates, undergo free-radical polymerization with a variety of initiator systems. Both monomer classes have been reviewed in the literature [37]. Highly soluble monomers such as 2-sulfoethyl methacrylates or the corresponding alkali salts are easily polymerized to high molar mass by hydrogen peroxide in aqueous solution [38]. Anionic initiation has been accomplished in a variety of solvents, both polar and nonpolar. 

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