Enolate chain ends

The fact that trimethylsilyl methacrylate is a sluggish monomer under GTP conditions [45, 46] also bodes well for a dissociative mechanism. The excess silyl carboxy groups are silylating enolate chain ends Thus lowering the rate of polymerization and changing the nature of the carboxylate catalyst (Scheme 23c). 

The fact that known anionic initiators for MMA can act as catalysts for GTP and the need for low amounts of catalysts in itself nearly puts to rest the associative mechanism. Seven of the other factors support the dissociative process. Except for the low temperature exchange studies, none supports the associative mechanism. Based on the lack of exchange of added silyl fluoride with silyl ketene acetal ends it looks like fluoride and bifluoride catalysts operate by irreversible generation of ester enolate chain ends [1] (Scheme 19b). On the other hand carboxylate catalysts appear to operate by reversible generation of ester enolate ends as evidenced by rapid exchange of silyl acetate with silyl ketene acetal ends [36] (Scheme 19c). 

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. 

The complexity of the polymerization arises with the nucleophilic attack of either anionic initiators or the propagating enolate chain-ends at the carbonyl groups of monomer or the polymer chain. Moreover, the reactive proton in the monomer is susceptible to transfer during the polymerization that limits the formation of high-molecular-weight polyacrolein. 

The same authors chose another very reactive nucleophilic function, the silyl enol ether group, which upon reaction with living cationic chain ends of poly(vinyl ether)s, also leads to a carbon-carbon bond with formation of a ketone (Scheme 4). Model reactions of living poly(IBVE) with various monofunctional silyl enol ethers [47] showed that the a-substituent R should have electron-donating properties in order to increase the electron density on the double bond. 

Silyl group transfer can also be used to functionalize chain ends. For example, allyl silanes, silyl ketene acetals, and silyl enol ethers [301-304] generate polymers with terminal allyl and methacrylate groups [Eq. (103)]. This type of transfer becomes degradative (termination) if reinitiation with silyl halides is not possible. 

Despite numerous efforts, there is no generally accepted theory explaining the causes of stereoregulation in acrylic and metliacrylic anionic polymerizations. Complex formation with the cation of the initiator (146) and enolization 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). 

Enolates, which are the actual propagating species, exist in the E (11) and Z (12) configurations. The E/Z ratio of the living chain-ends can be indirectly determined by reacting the propagating enolates 11 and 12 with chlorotrimethylsilane and converting them into the corresponding ketene silylacetals 13 and 14, which are characterized by NMR spectroscopy (equations 23 and 24). ... 

In order to shed light on the polymerization mechanism, the attention of several research groups was focused on the structure of the living chain-ends. A major problem is that enolates are known for condensation at a rate that depends on solvent, temperature and structure of the ester group (f-Bu esters being less reactive than Me esters). This undesired reaction makes the structural analysis of the chain-ends more complex . In order to get rid of any contribution of the chain in the structural analysis, unimeric, dimeric and oligomeric models of the chain-ends were considered. 

The silylation route shown in equations 23 and 24 was used to investigate the configuration of the ester enolate at the chain-end. Although ligation by LiCl changed the ElZ molar ratio (11.5/88.5 instead of 0/100), no influence on the tacticity was observed . 

Zune and coworkers studied the structure of the species propagating the anionic polymerization of tBMA by NMR spectroscopy . The spectrum of the lithium ester enolate was perturbed by LiCl, as result of an equilibrium established between free lithium chloride and complexed active end-groups. The structure of the chain-end was not modified by a large excess of LiCl. 

Formation of a dihydropyridine -adduct at the chain-end was shown by NMR and UV" spectral analysis. Anderson and collaborators proposed that the actual initiator would be adduct 34, formed by reaction of s-BnLi with pyridine (equation 40), which implies that the a-end-group of PMMA is a dihydropyridine group. This hypothesis was not experimentally confirmed. However, in the specific case of the e-caprolactone (3, n = 4) polymerization initiated by the BnLi/pyridine addnct, no characteristic NMR signal of dihydropyridine could be detected. The polymerization mechanism was therefore revised, based on the alkyllithium as the actual initiator and the establishment of an equilibrium between an active uncomplexed enolate (35) and a dormant cr-complex (36) as the basis for polymerization control (equation 41)" . 

The incoming monomer unit would then be forced, either because of steric interactions, or by the interaction of its carboxyl group with lithium at the chain-end, to add in a specific manner to re-form the same loose ring structure present initially. One variant of this mechanism [192] involves a covalently bonded six membered ring formed by enolization of the active chain end followed by alkoxide ion attack on the penultimate carboxyl group. In polar solvents, or in the presence of moderate amounts of them, competition for solvation of the counter-ion would be produced and the intramolecular solvation producing the stereospecificity would be reduced in effectiveness as the ether concentration is increased. Replacing the lithium counter-ion with sodium or other alkali metal would be... 

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