Metal enolates polymerization

Block copolymers have been successfully synthesized because many metallocene polymerizations of MMA proceed as living polymerizations, and it is possible to have a single one-way crossover from carbanion (alkene) polymerization to MMA (enolate) polymerization with metallocene and related initiators, especially when group 3 transition metal initiators are used [Boffa and Novak, 2000 Desurmont et al., 2000a,b Jin and Chen, 2002 Yasuda et al., 1992],... 

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). 

The sol-gel process is a versatile solution process for making ceramic and glass materials involving the transition of a system from a colloidal suspension (sol) into a solid phase (gel) ". The resulting porous gel can be chemically purified and consolidated at high temperatures. In the classical sol-gel process, the precursor (e.g. a metal enolate or a metal aUcoxide) is exposed to a series of hydrolysis and polymerization reactions to form... 

The two parts of the present volume contain seventeen chapters written by experts from eleven countries. They cover computational chemistry, structural chemistry by spectroscopic methods, luminescence, thermochemistry, synthesis, various aspect of chemical behavior such as application as synthons, acid-base properties, coordination chemistry, redox behavior, electrochemistry, analytical chemistry and biological aspects of the metal enolates. Chapters are devoted to special families of compounds, such as the metal ynolates and 1,2-thiolenes and, besides their use as synthons in organic and inorganic chemistry, chapters appear on applications of metal enolates in structural analysis as NMR shift reagents, catalysis, polymerization, electronic devices and deposition of metals and their oxides. 

Precatalyst 4(Sm) was utilized as a standard system [60]. The mechanism follows a coordination anionic polymerization via an eight-membered transition state (Scheme 3, see p. 985). Formation of a metal enolate turned out to be essential for the initiation of the MMA polymerization and was confirmed by the initiation activity of the enolate complex [(C5H4SiMe3)2Y(OCH=CH2)]2- The rate of polymerization is directed by steric factors depending on the metal (Sm > Y > Yb > Lu) and the auxiliary ligand (Cp > Cp ). Ethyl, isopropyl and f-butyl methacrylates are also stereospecifically polymerized, but the rate of poly-... 

Thus, one should expect similar behavior for transition metal enolates where there is significant covalent character to the M-O (or M-G) bond. This section will focus on polymerization of (meth)acrylate esters by group 4 metallocene (or the related group 3 and lanthanocene ") initiators where the mechanism of this process is analogous to the classical GTP process. Of course, the polymerization of (meth)acrylates by other transition metal complexes has been reported frequently in the literature however, in many cases the mechanisms of these processes are less well understood or involve free radical or other forms of initiation. Recent examples of other transition metal-mediated methyl methacrylate (MMA) polymerization processes that may proceed via a GTP or anionic mechanism are given. " "- " ... 

At the first step, the insertion of MMA to the lanthanide-alkyl bond gave the enolate complex. The Michael addition of MMA to the enolate complex via the 8-membered transition state results in stereoselective C-C bond formation, giving a new chelating enolate complex with two MMA units one of them is enolate and the other is coordinated to Sm via its carbonyl group. The successive insertion of MMA afforded a syndiotactic polymer. The activity of the polymerization increased with an increase in the ionic radius of the metal (Sm > Y > Yb > Lu). Furthermore, these complexes become precursors for the block co-polymerization of ethylene with polar monomers such as MMA and lactones [215, 217]. 

As will be discussed more thoroughly in Section 3.2.5, transition metal carbene complexes can mediate olefin metathesis. Because heteroatom-substituted carbene complexes are usually less reactive towards olefins than the corresponding nonheteroatom-substituted complexes, it is, e.g., possible to use enol ethers to terminate living polymerization or other types of metathesis reaction catalyzed by a non-heteroatom-substituted carbene complex. Olefin metathesis can also be used to prepare new heteroatom-substituted carbene complexes (Figure 2.15, Table 2.11). 

Grubbs and co-workers have developed a new class of neutral Ni complexes incorporating bulky phenoxy-imine ligands F13-16 that show considerable tolerance toward functional groups and even remain active in the presence of water. These complexes can co-polymerize ethylene with functionalized NBs such as 5-norbornen-2-ol to provide cyclic olefin co-polymers with hydroxy or ester functionality. It has been reported that these Ni complexes are ineffective for the polymerization of MA due to /3-H transfer from the enolate-Ni complex to the Ni metal. 

MMA can function as a chain-transfer agent for late transition metal-catalyzed ethylene polymerization. Gibson and Tomov revealed that Ni complexes with bulky phosphino-enolate ligands, F13-17, were capable of polymerizing ethylene in the presence of MMA to afford MMA-end-capped PEs through immediate /3-H transfer after MMA 337... 

Examination of the pH dependence of the polymerization process established that the enolate anion of the activator was the species responsible for initiation (46). Metal chelates of the dienones were also found to be effective. Chaberek believed that the semireduced dye radicals were active initiators in his system (47), in contradistinction to the work of Chen (17). Chaberek proposed that an excited dye-enolate complex was produced during the photochemistry and that this adduct reacted with monomer to yield a radical, capable of initiation, and a semireduced dye radical. Figure 4 shows the proposed mechanism. Several aspects of this mechanism should be modified in view of present knowledge. The semireduced radical (D ) has been conclusively shown to be a terminator (17) and not an initiator. 

Nevertheless, polymerizations other than the anionic ones have also been carried out using enolates as ligands of transition metal cations. Metal acetalyacetonates are indeed organometallic compounds that directly contributed to the control of radical polymerization, ring-opening polymerization and coordination polymerization of specific monomers, as discussed hereafter. 

Since most of the synthetically useful enolate anions described in the previous section are prepared by the reactions of enolizable substrates with alkali metal amide bases, it is appropriate to note a few structures of these amide bases. The common bases in synthetic organic chemistry include LDA and LHMDS. The structures of both of these bases are known as the THF solvates.Both of these compounds form bis-solvated dimers corresponding to structure (201). The diethyl ether solvate of LHMDS also forms a bis-solvated dimer (202).Sodium hexamethyldisilazide crystallizes as an unaggregated monomer from benzene solution.Two different cryst line forms of KHMDS are known as the polymeric dioxane solvate (203), ° and the unsolvated dimer (204). ... 

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