Enolate anions, living polymerization

When compared with the multifunctional initiators, the corresponding terminators are less available in cationic polymerization [202]. The situation is in sharp contrast to anionic living polymerization, where a variety of multifunctional terminators are developed (e.g., Cl2MeSiCH2CH2Si-MeCl2) [203,204]. However, a series of multifunctional silyl enol ethers were recently found to be effective in multiple termination of living cationic polymers of vinyl ethers [142,147,205,206] and a-methylstyrene [159,207] (Scheme 10). 

In contrast, the order of monomer addition is critical among monomers with different reactivities. As described in Section 5.1, a more-reactive chain-end anion is produced by a less-reactive monomer, and vice versa. Accordingly, less-reactive monomers should first be polymerized, followed by the polymerization of more-reactive monomers. In the block copolymer of styrene and MMA, for instance, it is necessary first to polymerize styrene, after which MM A is polymerized to prepare the second block, as the chain-end enolate anion produced by MMA cannot initiate the polymerization of styrene. Similarly, and for the same reason, the synthesis of P(2)-b-PMMA is possible only by the addition of 2-vinylpyridine first, and then MMA. For the successful design and synthesis of block copolymers, the pJ values of the conjugated acids of chain-end anions, as well as the e- and a-values of monomers (as mentioned above) are valuable guides. The details of almost all block copolymers synthesized to date, using living anionic polymerization, have been summarized by Quirk and Hsieh [190]. With the monomer addition order in mind, ABC triblock terpolymers composed of PS (A), PB (B), and PMMA (C), as well as PS (A), poly(2-vinylpyridine) (P(2VP)) (B), and P BMA (C), could be successfully... 

Zagala et al. investigated the polymerization of methacrylates in the presence of tetraphenylphosphonium (TPP) ion at ambient temperature. The polymerization appears to have living character [228]. In case of MMA number average molecular masses increase linearly with conversion and molecular mass distributions are narrow (< 1.30). Results of H, and P NMR studies indicated the presenee of phosphorylides formed by the addition of the PMMA enolate anion to one of the phenyls of the TPP cation. Muller et al. managed to synthesize another metal-free initiator, namely the salt of the tetrakis[tris(dimethylamino)-phosphoranylideneamino]phosphonium (P5) cation with the... 

Most of the published material has dealt with kinetics, mechanisms and stereochemical aspects of the polymerization. Thus early work by the Rohm and Haas group showed that under proper conditions the anionic polymerization of methacrylates is truly living. That is to say, if monomer, solvent and reagents meet stringent criteria of purity, and the temperature is maintained at -75°C or thereabouts, a carbanion or anion-radical source such as fluorenyllithium (FlLi) or naphthalenesodium will add to methyl methacrylate (MMA) to form an enolate anion( ) which then propagates to form a polymer chain with an anionic terminus (2). 

Since living chain-end enolate anions generated from methacrylate monomers are less reactive than those (carba-nions) generated from styrene monomers, living anionic polymerization of the following functional methacrylate derivatives can be achieved without protection in THF at -78 °C with the use of the above initiator systems glyddyl methacrylate, 3 -ethyl-3- (methacryloyloxymethyl) oxe-... 

Initiation of MMA polymerization by complexes such as (192) was shown to proceed via a bimetallic bis(enolate) intermediate, arising from the dimerization of a radical anion.478" 80 Such a mechanism481,482 explains why efficiencies with such initiators (calculated from polymer molecular weights) are always <50%. Using a similar methodology, the bimetallic bisallyl complex (198) was shown to polymerize MMA in a living fashion (Mw/Mn 1.1) and triblock copolymers with methacrylate and acrylate segments have been prepared. 

By using the aluminum porphyrin-Lewis acid system, we attempted the synthesis of a narrow MWD block copolymer from oxetane and methyl methacrylate (MMA). Methacrylic monomers can be polymerized radically and anioni-cally but not cationically, so a block copolymer of oxetane and methyl methacrylate has never been synthesized. As already reported, methacrylic monomers undergo accelerated living anionic polymerization with the (TPP)AlMe (1, X= Me)-3e system via a (porphinato)aluminum enolate as the growing species. 

Living anionic polymerization can also be used to produce well-controlled block copolymers. For PMMA, the best procedures need temperatures below O C and are therefore unlikely to be commercially attractive. Hiey are, furthermore, largely unsuccessful for the controlled polymerization of acrylates, which are far too reactive. The use of tetraalkyl ammonium ate complexes, in conjunction with an appropriate aluminum catalyst, solved fhis problem [225]. The function of the ammonium counterion is to promote dissociation of the complex ion to form the reactive ate complex of the aluminum enolate of the ester (Scheme 6.176). Thus, polymerization was initiated by the lithium enolate of isobutylate in the presence of the ate complex of Me3Al-R3NCl. A controlled block copolymer (PMMA-block-... 

This new technology offers considerable promise for commercial preparations of living polymers of methyl methacrylate without resorting to low-temperature anionic polymerizations. Although the mechanism or polymerization is not completely explained, the propagation is generally believed to be covalent in character. A silyl ketene acetal is the initiator. It forms from an ester enolate ... 

The dienyl methacrylates in Table II copolymerize readily by our standard anionic techniques. The polymers listed in Table III were prepared with diphenyl hexylithiurn initiator in THF at -78 C. NMR and GPC analyses provided the characterization included in Table III. We have not seen any evidence that the diene groups either copolymerize with the methacrylates or interfere with the polymerizations by providing acidic hydrogens. These observations are to be expected since the acidity difference between the ester enolate living end and any potential side products based on hydrocarbon anions is very large. 

The second method involves end-quenching of living polymers with appropriate nucleophiles. Although this approach appears to be more attractive than the first one, in situ end funaionali-zation of the living ends is limited to nucleophiles that do not react with the Lewis add coinitiator. Because the ionization equilibrium is shifted to the covalent spedes, the concentration of the ionic active species is very low. Quantitative functionalization can only be accomplished when ionization takes place continuously in the presence of nudeophile. Quenching the vinyl ether polymerization with the malonate anion,certain silyl enol ethers " and silyl ketene acetals have been successfully used to synthesize end-functionalized poly(vinyl ethers). Alkyl amines, " ring-substituted anilines, " " alcohols, " and water " have also been used to quench the vinyl... 

When living poly(methyl methacrylate) (PMMA) prepared by group transfer polymerization (GTP) is used as a macroinitiator for the ROP of cyclic carbonates, a site transformation from the silyl ketene acetal (GTP-mechanism) to an alcoholate (anionic ROP-mechanism) with a metal-free counterion occurs (Scheme 12.5). The GTP of PMMA was initiated with l-methoxy-l-trimethylsilyloxy-2-methyl-l-propene (MTS) in combination with catalytic amounts of tetrabutyl ammonium cyanide in THF as solvent. Towards the end of the reaction, DTC is dissolved in the reaction mixture and lequiv. of fluoride anions (e.g. tris(dimethylamino) sulfonium difluorotrimethylsilicate TASF), with respect to the active species, is added. In this way, good yields of the respective block copolymers were obtained. A model experiment for this site transformation is the polymerization of DTC with MTS as the initiator and TASF as the desilylating agent. The fluoride anion promotes desilylation of the silyl ketene acetal with formation of an enolate, which reacts as a carbon-centered nucleophile with the carbonyl carbon of DTC, thereby... 

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