Steric Control in Anionic Polymerization

Use of hydrocarbon solvents has an advantage in polymerizations of conjugated dienes because they yield some steric control over monomer placement. This is true of both tacticity and geometric isomerism. As stated earlier, the insertions can be 1,2 or 3,4 or 1,4. Furthermore, the 1,4-placements can be cis or trans. Lithium and organolithium initiators in hydrocarbon solvents can yield polyisoprene, for instance, which is 90% cis-1,4 in structure . The same reaction in polar solvents, however, yields polymers that are mostly 1,2 and 3,4, or trans-1,4 in structure. There is still no mechanism that fully explains steric control in polymerization of dienes. High cis-1,4 polyisoprene forms with lithium or alkyl lithium initiators in non-polar solvents because, propagation takes place through essentially covalent or intimate ion-pair lithium to carbon bonds . An intermediate pseudo-six-membered ring is believed to form in the process of addition of the diene :

Formation of such intermediates is favorable for lithium because it has a small ionic radius and is high in the proportion of p-character. Organometallic compounds of the other alkali metals (sodium, potassium, rubidium, and cesium) are more polar and more dissociated. They react essentially as solvated ions even in a hydrocarbon medium, yielding high 3,4 placement. O’Driscoll et al. [198] proposed a mechanism for steric control. In isoprene polymerization the terminal charges are complexed with the metal cations. These cations are close to the active centers through the occupied p-orbitals of the chain ends and the unoccupied p-orbitals of the lithium ions. In the transition state the monomers are complexed with the cations in the same way . The lithium cations are assumed to be in hybridized tetrahedral sp³ configurations with four vacant orbitals. The chain ends are presumed to be allylic and the diene monomers are bidentate . During the propagation steps both the monomers and the chain ends complex with the same counterions:

In hydrocarbon solvents, the complexes are tight and the rotations of the C2–C3 bonds are sterically hindered by the methyl groups. This constrains the 1,4-additions to cis-configurations. In polar solvents, however, like tetrahydrofuran, the complexes are loose and thermodynamically favored trans additions take place . It was observed, however, that the polymerizations of 2,3-dimethylbutadiene with organolithium initiators in non-polar solvents result in high trans-1,4 structures . This appears to contradict the above-proposed mechanism. Proton NMR spectra show that solvation shifts the structures of the carbanionic chain ends from localized 1,4-species to delocalized “p-allylic” type structures :

The s-bonded lithium chain can be expected to predominate. In highly solvating solvents, such as ethers, the p-allyl structure is dominant leading to high 1,2 placements. Because the 2,3-bond is maintained, the above shown equilibrium should not be expected to lead to cis–trans isomerization [200]. In fact, such isomerizations do not take place for butadiene or for isoprene when they are polymerized in hydrocarbon solvents. They do occur, however, in polar solvents at high temperatures. This suggests that additional equilibrium exist between the p-allylic structures and the covalent 1,2 chain ends. Table 4.3 shows the manner in which different polymerization initiators and solvents affect the microstructures of polyisoprene.

Polymerizations of polar monomers, like acrylic and methacrylic esters with alkyl lithium initiators yield the greatest amount of steric control. An almost all isotactic poly(methyl methacrylate) forms at low temperatures. Addition of Lewis bases such as ethers or amines reduces the degree of isotactic placement. Depending upon the temperature, atactic or syndiotactic polymers form . Also, butyllithium in heptane yields an isotactic poly(N,N'-dibutyla- crylamide) at room temperature .

The propagation rates for methyl methacrylate polymerization in polar solvents like tetrahydrofuran- ran or dimethylformamide are lower than the rates of initiation . There is no evidence, however, that more than one kind of ion pairs exist . The ion pairs that form are apparently contact- ion pairs . Furthermore, based on the evidence, the counterions are more coordinated with the enolate oxygen atoms of the carbonyl groups than with the a-carbons. As a result, they exert less influence on the reactivity of the carbanions . The amount of solvation by the solvents affects the reaction rates. In addition, “intramolecular solvation" from neighboring ester groups on the polymer chains also affects the rates. In solvents like dimethylformamide, tetrahydrofuran, or similar ones , the propagating chain ends-ion pairs are pictured as hybrid intermediates between two extreme structures. This depends upon the counterion, the solvent, and the temperature :

where S means a solvent molecule; Me represents a metal.

Several mechanisms were offered to explain steric control in polymerizations of polar monomers. Furukawa and coworkers [157] based their mechanism on infra-red spectroscopy data of interactions between the cations and the growing polymeric chains in polymerizations of methyl methacrylate and methacrylonitrile. They observed a correlation between the tacticities of the growing molecules and the carbonyl stretching frequencies. The higher the frequency, the higher is the amount of isotactic placement in the resultant chains. The adducts, just as in the initiation reactions, are resonance hybrids of two structures, A and B:

where Me represents a metal. Furukawa and coworkers concluded that due to the extremely low tendency of the adducts to dissociate , the carbonyl absorption can only be ascribed to undissociated ion pairs. The magnitude of the carbonyl absorption and the shifts to higher frequencies show the degree of contribution of structure B, shown above. The absorption and the shifts were also explained by the configurations of the electrons in the anti-bonding orbitals of the carbonyl groups. The higher the stretching frequencies, the nearer are the positions of the counterions to the carbonyl groups of the terminal units . This is accompanied by higher tacticity. The carbanions on the terminal units in the transition states are located near the b-carbons of the incoming monomers. At the same time, rotations around the axis through these two carbons may be quite restricted when the cations are in the vicinity of the carbonyl groups of the terminal unit and near the incoming monomers. In this manner, isotactic placement is enhanced :

The same mechanism was proposed for the polymerization of methacrylonitrile [157]. Cram and Kopecky [219] offered a different mechanism of steric control. According to their mechanism, during a methyl methacrylate polymerization the growing ether enolate possesses a complete alkoxide character:

Attacks by the alkoxide ion on the carbonyl groups of the penultimate units lead to formations of six-membered rings:

The six-membered ring is destroyed in the process of propagation:

The transition state formed by a 1,4-dipolar addition to a polarized double bond. Coordination of the lithium atom to two oxygen atoms determines stereo regulation. Each new incoming monomer must approach from below the plane because the other side is blocked by an axial methyl group. This favors isotactic placement. There is doubt, however, whether it is correct to assume a rigid six membered cyclic alkoxide structure for a propagating lithium enolate [209]. Aslightly similar model was suggested by Bawn and Ledwith [209]. It is based on the probability that a growing polymeric alkyl lithium should have some enolic character, with the lithium coordinating to the carbonyl oxygen of the penultimate unit:

The cyclic intermediate forms due to intramolecular solvation of the lithium and due to intramolecular shielding of one side of the lithium ion. The nucleophilic attack by the monomer, therefore, has to occur from the opposite side. The transition state is similar to an SN2 reaction. When the bond between the lithium and the incoming monomer forms, the oxygen lithium bond ruptures. Simultaneously, the charge migrates to the methylene group of the newly added monomer. The resultant new molecule is stabilized immediately by intramolecular solvation as before. In this manner, the retention of configuration is assured, if the incoming monomer always assumes the same configuration toward the lithium ion. NMR spectra of poly (N,N-dimethylacrylamide) formed with sec-butyllithium in both polar and non-polar solvents show that the penultimate unit does affect monomer placement . Also, a coordination was observed with both heteroatoms , the one on the ultimate and the one on the penultimate unit. Many refinements were introduced into the various proposed explanations of steric control in anionic polymerizations  over the last 30 or more years. Two important features of these mechanisms are: (1) coordinations of the chain ends with the counterions and (2) counterion solvation. Use of complex lanthanide catalysts allows a very high cis-1,4 placement of isoprene monomer and preparation of polymers that are very close to natural rubber . Thus, complex neodymium catalysts can yield polymers that are greater than 98% cis-1,4 polyisoprenes. The preparation of such catalyst, however is difficult. Evans et al. reported, however, that simple TmI₂,NdI₂,andDyI₂ will initiate polymerization of isoprene without any additives and can also yield high cis-1,4 placement :

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مواضيع ذات صلة

Polymerization of Aldehydes

Isomerization Polymerizations with Coordination Catalysts

Reduced Transition Metal Catalysts on Support

Post Ziegler and Natta Coordination Polymerization of Olefins

Steric Control in Polymerization of Conjugated Dienes

Steric Control with Homogeneous Catalysts

Homogeneous Ziegler–Natta Catalysts

Steric Control with Heterogeneous Catalysts

Heterogeneous Ziegler–Natta Catalysts

Coordination Polymerization of Olefins

Termination in Anionic Polymerization

Hydrogen Transfer Polymerization