Search

As an illustration of the frantic development of (meth)acrylic esters anionic polymerization during the last decade, three significant breakthroughs will be presented and analyzed: - extension of the LAP (ligated anionic polymerization) concept to the practical synthesis of both high iso- or syndio high MW PMMA; - the production of perfect hydrocarbon gels from tailored PMMA-polybutadiene-PMMA triblock copolymers; - the use of reactivity boosters allowing to climb back up the nucleophilic reactivity scale, i.e. initiating styrene, dienes and (meth)acrylates polymerization with alcoholates and silanolates.

It has been well-known for a very long time that, for anionic homo-polymerization or sequential copolymerization to occur, it is mandatory that the relative nucleophilic reactivity of the initiating species be equal to or larger than the one of the (co)monomer. However, the results described in this paper demonstrate that such a classic rule can be circumvented in some cases. Indeed, it is reported for the first time that potassium trimethylsilanolate, and living potassium poly(ethylene oxide), both of which are unable to initiate styrene or methyl methacrylate (MMA) polymerization, can be converted from oxyanionic active ends to silyl anionic ones by reacting with cyclic disila derivatives, so allowing the homopolymerization and sequential (co)polymerization of styrene or MMA to proceed in a controlled manner.

A simple alkyl lithium initiator (sBuLi) complexed with LiCl (10 molar equiv.) has been used to initiate the anionic polymerization of various methacrylic monomers, i.e. methyl methacrylate (MMA), tert-butyl methacrylate (tBuMA), glycidyl methacrylate (GMA), and dimethyl amino ethyl methacrylate (DMAEMA) in tetrahydrofuran at −78°C. Only the homopolymerization of tBuMA proceeds in a living manner, as evidenced by both the linear plot of experimental molecular weight (Mnexp) vs theoretical ones (Mncal) (slope ≅ 0.93) and monomer resumption experiments. However, three types of block copolymers, i.e. PtBuMA-b-PMMA, PtBuMA-b-PGMA, PtBuMA-b-PDMAEMA, have been successfully synthesized with a predictable molecular weight, and narrow molecular weight distribution.

Tailored polymerization of acrylic and methacrylic esters, particularly methyl methacrylate (MMA), has been the topic of a frantic research effort during the last decade. It is the purpose of this paper to present one of the main lines of that quest, and to evaluate its potential versus other new approaches.

Well defined polymers can be produced by radical polymerization using three different approaches: degenerative, transfer, reversible homolytic cleavage of covalent species, and reversible homolytic cleavage of persistent radicals. The first method requires high degenerative transfer coefficients but allows for slow initiation; the other two methods require that the equilibrium be strongly shifted towards dormant species, and that initiation is fast. All three approaches may provide controlled polymerizations, but not living polymerizations, because chain breaking reactions can not be eliminated although their contribution can be strongly reduced.

The recently developed ‘ligated’ technique has proved to be very successful in promoting living anionic polymerization of (meth) acrylic ester monomers. The efficient ligands discovered to date include μ, σ and μ–σ dual types. The roles of these ligands in the control of livingness and stereoregularity of anionic polymerization of (meth) acrylates are discussed in detail. There is convincing evidences that both ligation thermodynamics (electron density redistribution and steric hindrance in the formed active complex) and kinetics (how fast and in which way ligation occurs) are very critical in promoting the livingness. Moreover, steric hindrance appears to be a determining factor maintaining the process ‘living’ under more demanding conditions.

Although the anionic polymerization of tert-butyl methacrylate (tBMA) induces a narrow and unimodal molecular weight distribution (MWD) when performed in either pure THF or pure toluene at -78 o C in the presence of a lithium counterion, that in THF/toluene mixtures, keeping other conditions unchanged, surprisingly gives rise to broad bi- and even multimodal MWD's. Moreover, with increasing polymerization temperature from -78 to 0 o C, that multimodal MWD gets narrower, atypical of termination reactions, and indeed at 0 o C, a very narrow unimodal MWD is again obtained. These results have been satisfactorily interpreted in terms of a multiple solvation equilibria mechanism: there coexist several types of THF-solvated and nonsolvated active species and the exchange among them is slow at -78 o C but fast at 0 o C compared to the monomer propagation rate, thus resulting in a multimodal MWD in the former case and a narrow one in the latter. Furthermore, this solvation mechanism also dominates the stereoregulation of the tBuMA polymerization in mixed solvent. It has indeed been demonstrated that addition of various types of ligands, i.e., lithium chloride (LiCl) and lithium tert-butoxide (LiOtBu) (both μ-ligands), 12-crown-4 (12CE4) and cryptand 211 (K211) (both α-chelating ligands), and lithium 2-(2-methoxyethoxy)ethoxide (LiOE 2 M) (a μ/ dual type of ligand), affects the tBMA anionic polymerization in a way which can be accounted for by these so-called solvation/ligation multiple equilibria among THF-nonsolvated, THF-solvated, and ligated ion pairs. Once again, the fact that, in comparison with 12CE4 and LiOtBu, such ligands as LiOE 2 M, K211, and LiCl can effectively promote a much narrower MWD is nicely related to their high propensity to coordinate with a lithium ester enolate and at the same time to either promote a fast-exchanging ligation equilibrium or form a single type of active complex, in good agreement with the NMR investigation of corresponding model systems as previously reported by us

Different initiators combined with a new family of μ/σ dual ligands, i.e., polydentate alkoxides, were used to initiate the anionic polymerization of 2-ethylhexyl acrylate (2EtHA) at low temperatures. It has been found that lithium 2-(2-methoxyethoxy) ethoxide (LiOEEM) is the most efficient μ/σ ligand in promoting a living polymerization of that monomer. Although LiOEEM gives rise to a molecularly well-controlled P2EtHA at -78°C in a 9/1 toluene/THF mixture, i.e., quantitative field, high initiator efficiency (> 90%), and narrow molecular weight distribution (M w /M n down to 1.05), two-step monomer resumption experiments always lead to a bimodal distribution, suggesting that the LiOEEM-complexed P2EtHA anions are not stable

A mixed complex of a polydentate lithium alkoxide, i.e., lithium 2-(2-methoxyethoxy) ethoxide (LiOEEM), and of a mono- or bisfunctional organolithium initiator, i.e., (diphenylmethyl)lithium (DPMLi) or lithium naphthalene/diphenylethylene, has been used to synthesize well-controlled AB (BA), ABA, and BAB block copolymers of methylmethacrylate (MMA) (A) and 2-ethylhexyl acrylate (2EtHA) (B), irrespective of the monomer addition order. Although the 2EtHA block copolymerization initiated with monofunctional living PMMA macroanions at -78°C in a 75/25 toluene/THF mixture gives rise to a precisely-tailored pMMA-b-P2EtHA diblock polymer,the reverse sequence, i.e., from P2EtHA anions to the MMA type monomer, always results in contamination by homo-P2EtHA

A unique ligand-promoted-loose mixed complex MIB-(Li 3 (OEEM) 2 ) + has been characterized for the first time by means of NMR spectroscopy in solution. This new complex, which is a dual μ/σ one, is obviously different from either a simple tight μ-mixed complex, such as nMIBLi-mX (X=Cl or OtBu), or a σ-cation-binding one with more charge separation, such as K211/MIBLi. It originates from a simultaneous dual complexation (μ/σ) of MIBLi by both the lithium alkoxide (μ-mixed complexation) and the chelating polyether components (a-cation-binding complexation) of LiOEEM

The effect of several types of ligands, i.e., 12-CE-4 (1), K211 (2), LiOtBu (3), and LiOEEM (4), on the stereochemistry of MMA anionic polymerization at -78°C in various solvents, i.e., toluene, THF, and a 9/1 toluene/THF mixture, has been studied. Similar to the LiCl-complexed system, the stereoregularity of MMA polymerization in the presence of these additives is seemingly controlled by association/complexation equilibria among associated, nonassociated, and ligand-complexed active species. Accordingly, complexation of active species by σ-chelating ligands such as 12-CE-4 and K211 simply shifts the association equilibrium toward the formation of ligand-complexed (nonassociated) species, giving preferably syndiotactic placements (rr) in a 9/1 toluene/THF mixture as well as in toluene

Although μ-coordinating ligands such as LiCl are effective in promoting the well-controlled living anionic block copolymerization of methyl methacrylate (MMA) and tert-butylacrylate (tBuA), whichever monomer is first polymerized, it has been previously shown that a copolymerization of mixtures of these two monomers does not proceed as expected, giving poor results in terms of both conversion and molecular weight distribution: that was accounted for by the easy back-biting nucleophilic attack of a highly sensitive methyl ester group by a moderately hindered PtBuA anion. It is now reported that such a situation can be completely modified, when a cation-binding σ-ligand, i.e., dibenzo-18-CE-6, is used in chelating a sodium counterion and surrounding it with a steric barrier, blocking a large enough space area around the ion pair

The aggregation equilibrium and electronic structure of methyl α-lithioisobutyrate (MIBLi), a monomeric model structurally corresponding to a living poly(methyl methacrylate) chain end associated with a lithium countercation, has been examined by 7 Li and/or 13 C NMR spectroscopy in THF in the presence of various lithium cation-binding ligands. The addition of dimethoxyethane (DME), triglyme (glyme-3), 12-crown-4 (12-CE-4), and hexamethylphosphoric triamide (HMPA) to coexisting tetrameric and dimeric MIBLi in THF induces an increase in dimeric population in the order DME< glyme-3< 12-CE-4< HMPA

A new family of ligands (dual σ-π ligand), i.e. chelating lithium alkoxides, Li-O-(CH2-CH2O)nCH3, is shown to be very effective in promoting the living anionic polymerization of methacrylic and acrylic esters, including a number of primary alkyl acrylates such as n-butyl acrylate and 2-ethylhexylacrylate.

The mixed complexation of methyl α-lithioisobutyrate (MIBLi) (a monomeric model structurally corresponding to the living end of PMMA-Li + ) with lithium tert-butoxide (LiOtBu) in THF has been studied for the first time by using 13 C NMR spectroscopy. It is shown that LiOtBu, like LiCl, is very effective in coordinating with MIBLi tetramer in THF, incrementally replacing MIBLi with the formation of other tetrameric complexes, MIB x Li 4 (OtBu) 4-x (x=1-3). Complextion of MIBLi by LiOtBu induces a shielding 13 C shift of the MIBLi Ca resonance, which might suggest that lithium cations associated with the electron-rich ligands, i.e., LiOtBu, can also form waeker bonds with the carbonyl oxygen of MIBLi (i.e., μ-type complexes), athereby decreasing the localization of the negative charge on that latter atom

Solutions of methyl α-lithioisobutyrate (MIBLi) in THF added with various amounts of LiCl have been analyzed by 7 Li NMR spectroscopy in a wide range of temperatures. It is concluded that a multinuclei NMR investigation of MIBLi chosen as a model for living PMMA - Li + is a very useful tool for the mechanistic study of the anionic polymerization of (meth)acrylates. It particularly provides extensive valuable insights into the structure and fate of the species active in the polymerization which allows us to account for the polymerization kinetics and the polymer characteristics

LiCl has been found to have an important effect on the E/Z ratio of the living chain ends of poly(methyl methacrylate) (PMMA) in THF, in contrast to the main-chain tacticity. In contrast, LiCl has a significant influence on the stereoregularity of the anionic polymerization of MMA in a 9/1 toluene/THF mixture. When the LiC/initiator molar ratio is increased, the syndiotactic placements (rr) are favored at the expense of the isotactic ones (mm)

A monomeric model structurally comparable to the active end of a living poly(methyl methacrylate) (PMMA), i.e., a methyl α-lithioisobutyrate (MIBLi)/THF solution, was studied by both 7 Li and 13 C NMR. It has been shown that two different coordination aggregates exist in a MIBLi/THF solution. That situation corresponds to a slow exchange process between tetramer and dimer, i.e. (MIBLi) 4 + 2nS ⇄ 2(MIBLiS n ) 2 (S=THF) the later being favored by a decrease in temperature or concentration. Such an equilibrium is characterized with ΔH= -2.1 kcal/mol and ΔS= -13.7 cal/mol K

Although the anionic polymerization of styrene cannot be initiated by living poly(tert-butyl methacrylate) (PTBMA - K + ), the para substitution of the aromatic ring by an electron-withdrawing ester group such as tert-butyloxycarbonyl is very effective in increasing the styrene reactivity to the point where it is polymerized in a living manner by PTBMA - K + in THF at -78°C