Anionic polymerization methacrylates/acrylates

Teyssie and Jerome [63] have solved the temperature problem to some extent by conducting anionic polymerization of acrylates and methacrylates in the presence of LiCl, LiOR, or Li0(CH2CH20)nMe. These agents complex with lithium enolate chain ends. 

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. 

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... 

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. 

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]. 

Despite numerous efforts, there is no generally accepted theory explaining the causes of stereoregulation in acryflc and methacryflc anionic polymerizations. Complex formation with the cation of the initiator (146) and enoflzation of the active chain end are among the more popular hypotheses (147). Unlike free-radical polymerizations, copolymerizations between acrylates and methacrylates are not observed in anionic polymerizations however, good copolymerizations within each class are reported (148). 

Unlike ftee-tadical polymerizations, copolymerizations between acrylates and methacrylates ate not observed in anionic polymerizations however, good copolymerizations within each class ate reported (99). 

Group-Transfer Polymerization. Du Pont has patented (29) a technique known as group-transfer polymerization and appHed it primarily to the polymerization of acrylates and methacrylates. It is mechanistically similar to anionic polymerization, giving living chains, except that chain transfer can occur (30). 

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. 

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]. 

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. 

Penultimate effects have been observed for many comonomer pairs. Among these are the radical copolymerizations of styrene-fumaronitrile, styrene-diethyl fumarate, ethyl methacrylate-styrene, methyl methacrylate l-vinylpyridine, methyl acrylate-1,3-butadiene, methyl methacrylate-methyl acrylate, styrene-dimethyl itaconate, hexafluoroisobutylene-vinyl acetate, 2,4-dicyano-l-butene-isoprene, and other comonomer pairs [Barb, 1953 Brown and Fujimori, 1987 Buback et al., 2001 Burke et al., 1994a,b, 1995 Cowie et al., 1990 Davis et al., 1990 Fordyce and Ham, 1951 Fukuda et al., 2002 Guyot and Guillot, 1967 Hecht and Ojha, 1969 Hill et al., 1982, 1985 Ma et al., 2001 Motoc et al., 1978 Natansohn et al., 1978 Prementine and Tirrell, 1987 Rounsefell and Pittman, 1979 Van Der Meer et al., 1979 Wu et al., 1990 Yee et al., 2001 Zetterlund et al., 2002]. Although ionic copolymerizations have not been as extensively studied, penultimate effects have been found in some cases. Thus in the anionic polymerization of styrene t-vinylpyri-dine, 4-vinylpyridine adds faster to chains ending in 4-vinylpyridine if the penultimate unit is styrene [Lee et al., 1963]. 

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... 

As explained in Sections 16.3.4, 6.4.1, and 16.4.2, SEC is a nonabsolute method, which needs calibration. The most popular calibration materials are narrow molar mass distribution polystyrenes (PS). Their molar mass averages are determined by the classical absolute methods—or by SEC applying either the absolute detection or the previously calibrated equipment. The latter approach may bring about the transfer and even the augmentation of errors. Therefore, it is recommended to apply exclusively the certified well-characterized materials for calibrations. These are often called PS calibration standards and are readily available from numerous companies in the molar mass range from about 600 to over 30,000,000g moL. Their prices are reasonable and on average (much) lower than the cost of other narrow MMD polymers. Other available homopolymer calibration materials include various poly(acrylate)s and poly(methacrylate)s. They are, similar to PS, synthesized by anionic polymerization. Some calibration materials are prepared by the methods of preparative fractionation, for example, poly(isobutylene)s and poly(vinylchloride)s. 

The most widely investigated optically active poly-acrylic-esters are the polymers of bomyl and menthyl acrylate and methacrylate in this case the monomers have been polymerized by radical polymerization using benzoyl-peroxide (135), A. I. B. N. (134, 135), y-rays (131, 135), U. V. rays (4) in the presence of benzoin (134), and by anionic polymerization using LiC4H9 (4, 135) or C6H5MgBr (134, 135) as catalyst. 

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). 

Acrylic Macromers. Thus far we have shown applications of SFC to the characterizations of monomers and crosslinkers. The next couple applications will focus upon the analysis of oligomeric methacrylates, specifically methacrylate macromers. Methacrylate macromers are frequently used as building blocks for larger architecturally designed polymers. Unfortunately, macromers far exceed the capability of GC and do not possess a chromophore for HPLC analysis. Hatada et. al. has used packed column SFC to analyzed the stereoisomers of oligomeric methylmethacrylate (MMA) prepared by anionic polymerization (13). 

Acrylates polymerize two orders of magnitude faster than methacrylates by anion catalyzed GTP however, the polymerization dies at about 10,000 MW. During the anion catalyzed polymerization of acrylates the silyl ketene acetal end groups migrate to internal positions. These ketene acetals are too hindered to act as initiators for branch formation [9]. 

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. 

Pentadienyl-terminated poly(methyl methacrylate) (PMMA) as well as PSt, 12, have been prepared by radical polymerization via addition-fragmentation chain transfer mechanism, and radically copolymerized with St and MMA, respectively, to give PSt-g-PMMA and PMMA-g-PSt [17, 18]. Metal-free anionic polymerization of tert-butyl acrylate (TBA) initiated with a carbanion from diethyl 2-vinyloxyethylmalonate produced vinyl ether-functionalized PTBA macromonomer, 13 [19]. 

Narrow distribution in the backbone length as well as in the chemical composition or the branch frequency may be expected from a living-type copolymerization between a macromonomer and a comonomer provided the reactivity ratios are close to unity. This appears to have been accomplished to some extent with anionic copolymerizations with MMA of methacrylate-ended PMMA, 29, and poly(dimethylsiloxane) macromonomers, 30, which were prepared by living GTP and anionic polymerization, respectively [50,51]. Recent application [8] of nitroxide (TEMPO)-mediated living free radical process to copolymerizations of styrene with some macromonomers such as PE-acrylate, la, PEO-methacr-ylate, 27b, polylactide-methacrylate, 28, and poly(e-caprolactone)-methacrylate, 31, may be a promising approach to this end. 

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