FACTORS INFLUENCING STEREOREGULATION

The low-pressure polymerization of ethylene, reported by Ziegler in 1955, signaled the emergence of a new phase in polymer science. The catalyst, prepared from TiCl₄ and (C₂H₅)₃Al, was heterogeneous, and it was subsequently demonstrated by Natta (1961) that coordination catalysts of this type could be used to exercise control over the stereoregular structure of the polymer. Initially, it was thought that only hetero-geneous catalysis would lead to stereoregular polymers, but we now know that this is untrue and that stereoregulation can be effected under specific rigorously defined conditions, regardless of the solubility of the catalyst system.

If stereoregulation is simply the control of the mode of entry of a monomer unit to a growing chain, examination of the factors influencing this addition should provide an understanding of how to exert such control.

FIGURE 6.4 A diagrammatic representation of various other ordered helical structures adopted by isotactic polymers. (From Natta,G. and Corradini, P., Rubber Chem. Technol., 33, 703, 1960. With permission.)

X CH

H₂C CH₂ C H X

X CH H₂C CH₂

C X C H

X H CH₂

X CH H₂C CH₂

C X C

XH CH₂

2

2 1

2 1 1 H₂C CHX

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166 Polymers: Chemistry and Physics of Modern Materials

Free-radical initiation can usually be thought of as generating a chain by a Bernoulli-trial propagation, in which the orientation of the incoming monomer is unaffected by the stereostructure of the polymer. It can then add on in one of the two ways in which the active end is assumed to be a planar sp² hybrid, and the configuration of the adding monomer is finally determined only when another mono-mer adds on to it in the next step. In other words, this addition leads to an isotactic or syndiotactic placement of the pseudo-asymmetric center 1 with respect to 2. When the chain carrier is a free species, i.e., a radical, the stereoregularity of the polymer is a function of the relative rates of the two methods of addition, and this is governed by the temperature. Consideration of the relative magnitudes of the enthalpy and entropy of activation for isotactic and syndiotactic placements shows that whereas the differences are small, the syndiotactic structure is favored. This is, of course, aided by the greater steric hindrance and repulsions experienced by the substituents in the isotactic configuration and will vary in extent with the nature of the group X.

Thus, for a free-radical polymerization at 373 K, the fraction of syndiotactic place-ments is 0.73 for methyl methacrylate monomer but only 0.51 when vinyl chloride is used. A decrease in the polymerization temperature increases the tendency toward syndiotactic placements, but as radical reactions are normally high-temperature processes, atactic structures predominate. Low-temperature free-radical propagation has been found to produce syndiotactic polymers from the polar monomers, isopropyl and cyclohexyl acrylate, and methyl methacrylate.

The same general principles apply for freely propagating ionic chain carriers, but if coordination between the monomer and the active end takes place, the stereo-regulation is altered. The configuration of the monomer is then influenced by the stereochemistry of the growing end, and the possible number of ways the monomer can join the chain is in excess of two. These coordination catalysts include the Ziegler–Natta type as the largest group, and others such as butyl lithium, phenyl magnesium bromide, and boron trifluoride etherate. The resulting polymer is nor-mally isotactic, although some cases exist in which highly syndiotactic polymers are obtained (see Table 6.1).

The orienting stage in coordination polymerization can be pictured as being mul-ticentered, with the monomer position governed by coordination with the gegen-ion

TABLE 6.1

Polymerizations Using Coordination Catalysts Where Quoted Tacticities Are

>90%

Monomer Catalyst Structure

Isobutyl vinyl ether BF3(C2H5)2O in propane at 213 K Isotactic Methyl acrylate C6H5MgBr or n-C4H9Li in toluene at 253 K Isotactic Propylene TiCl4 + (C2H5)3Al in heptane at 323 K Isotactic Propylene VCl4 + Al(i-C4H9)2Cl in anisole or toluene at 195 K Syndiotactic 9813_C006.fm Page 166 Tuesday, June 12, 2007 12:35 PM

Polymer Stereochemistry 167

and the propagating chain end. As the gegen-ion will tend to repel the substituent X on the incoming monomer, it is forced to approach in a way that leads to predominantly isotactic placement. If coordination plays a major role in determining the configuration of the incoming monomer, then the greater the coordinating power the more regular the resulting polymer should be, but the nature of the monomer is also important. Polar monomers (the acrylates and vinyl ethers) are capable of taking an active part in the coordination process and will only require catalysts with moderate powers of orientation, but nonpolar monomers such as the α-olefins will require stronger coordinating catalysts to maintain the required degree of stereoreg-ulation in the addition process. In extreme cases, the heterogeneous Ziegler–Natta catalysts are required, in which severe restrictions are imposed on the method of monomer approach to the growing chain end, and these must be used for the nonpolar monomers, which yield only atactic polymers with homogeneous catalyst systems.

6.7 HOMOGENEOUS STEREOSPECIFIC CATIONIC POLYMERIZATIONS

An example of this type of reaction is provided by the alkyl vinyl ethers (CH₂=CHOR). Isobutyl vinyl ether was the first monomer studied, which produced a stereoregular polymer using a BF₃ + (C₂H₅)₂O catalyst and will be used as the illustrative monomer. A homogeneous-stereospecific polymerization can be carried out in toluene at 195 K using such soluble complexes as (C2H5)2TiCl2AlCl2 or (C₂H₅)₂TiCl₂Al(C₂H₅)Cl, or, if a suitable choice of mixed solvents is made, a homo-geneous system with BF3 + (C2H5)2O can be obtained, which is capable of producing the isotactic polymer.

The mechanism proposed by Bawn and Ledwith (1962) postulates the existence of an sp³ configuration for the terminal carbon in the growing chain due to the attendant gegen ion, and especially in low-dielectric solvents. They also point out that the structure of the alkyl vinyl ethers, with the exception of the ethyl and isopropyl members, will be subject to steric shielding of one side of the double bond, i.e.,

This blocks one mode of double-bond opening and assists stereoregulation. This conclusion is supported by the lack of any crystalline polymer in the product when the ethyl and isopropyl groups are used where no blocking is possible.

The formation of a loose six-membered ring is thought to stabilize the growing carbonium ion in the reaction so that the only route for monomer approach is past the counterion.

C H H

C O H CH₂

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168 Polymers: Chemistry and Physics of Modern Materials

A four-centered cyclic-transition state is involved in the propagation stage lead-ing to the insertion of a monomer unit between the catalyst and the chain end, with subsequent regeneration of the cyclic structure. An alternative transition state, pro-posed by Cram and Kopecky, has a similar but more rigid structure.

Both mechanisms ignore the nature of the catalyst forming the gegen-ion, but obviously, as this will act as a template for the attacking monomer, it will exert an influence on the rate of reaction and the type of stereoregularity imposed. The most probable configuration is isotactic because of the tendency for the gegen-ion to repel the substituent group of the incoming monomer.

6.8 HOMOGENEOUS STEREOSELECTIVE ANIONIC POLYMERIZATIONS

The various factors influencing the stereoregularity, when the propagating chain end is a carbanion, are conveniently highlighted in a study of the polymerization of methyl methacrylate by organolithium catalysts.

The propagating chain end in an anionic reaction initiated by a reagent such as n-butyl lithium can be thought of as existing in one of the following states, analogous to carbonium ion formation.

The extent of the separation will depend on the polarity of the reaction medium and in nonpolar-hydrocarbon solvents, such as toluene, covalent molecules, or contact

CH₂

ion pair free ions 9813_C006.fm Page 168 Tuesday, June 12, 2007 12:35 PM

Polymer Stereochemistry 169

ion pairs, are most likely to exist. With increasing solvent polarity, there is a greater tendency to solvate the ions, eventually producing free ion for strictly anionic polymerizations. These lead to conditions similar to a free-radical polymerization in which the stereoregulation is reduced and syndiotactic placements are favored at low-temperatures.

The effects of solvent and temperature are manifest in the polymerization of methyl methacrylate with n-butyl lithium at 243 K in a series of mixed solvents prepared from toluene and dimethoxyethane (DME). The NMR spectra of the prod-ucts indicate the compositions in Table 6.2 and reveal that a predominantly isotactic material is produced in a low-polarity medium, but that this becomes highly syn-diotactic as the solvating power of the medium increases.

An additional point emerges from this; higher syndiotactic contents are obtained when the Lewis-base strength of the solvent increases, and this factor probably accounts for the efficiency of the ether in this system. When the catalyst is 9-fluorenyl lithium, the reaction of methyl methacrylate at 195 K in toluene leads to isotactic polymer, whereas a change of solvent to tetrahydrofuran results in a syndiotactic product.

Stereoregulation is also altered by the nature of the solvent when Grignard reagents and alkali metal alkyls are used as initiators. In toluene, for example, the isotactic placements in the chain decrease as reagents change from Li to Na to K.

If general conclusions can be drawn from the behavior of methyl methacrylate, it appears that stereoregulation in anionic polymerizations, involving either polar monomers or monomers with bulky substituents, will lead to predominantly syndio-tactic polymers when a free, dissociated ion occurs at the propagating end. This is because it is the most stable form arising from a minimization of steric and repulsive forces. If, however, some strongly regulating kinetic mechanism is available, for instance monomer + gegen-ion coordination, then the less favorable isotactic place-ment occurs.

In an attempt to explain the mechanism of stereoregulation in systems catalyzed by lithium alkyls, Bawn and Ledwith (1962) proposed that the penultimate residue plays an important role in the addition. A loose cyclic intermediate is formed when the Li+

TABLE 6.2

The Effect of Mixed Solvent Composition on the Tacticity of Poly(methyl methacrylate) Initiated by n-Butyl Lithium at 243 K

Toluene/DME Isotactic Heterotactic Syndiotactic

100/0 0.59 0.23 0.18

64/36 0.38 0.27 0.35

38/62 0.24 0.32 0.44

2/98 0.16 0.29 0.55

0/100^a 0.07 0.24 0.69

Note: The mole fractions of the various configurations are given.

a Measured at 203 K.

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170 Polymers: Chemistry and Physics of Modern Materials

counterion coordinates with the carbonyl of the penultimate unit and with the ter-minal unit in a resonance enolic structure V.

This can be represented alternatively as a transition state VI similar to that in an SN2 reaction. With one side of the Li⁺ shielded, monomer approach is restricted, and the path of least resistance places the α-methyl group on the incoming monomer in a trans position, relative to the α-methyl group on the carbanion, during the π-complex formation. Addition then proceeds through a series of bond exchanges as the carbanion joins the monomer methylene group. The carbonyl group of the monomer coordinates with the ion by replacing the interaction with the previous penultimate group, and the cyclic intermediate is regenerated.

The steric restriction imposed by the α-methyl group aids the formation of an isotactic polymer, and in its absence (i.e., methyl acrylate), there is a reduced probability of isotactic placements. A compensating feature in the higher acrylates arises from the shielding of one side of the monomer by the bulkiness of the ester group. In the branched homologues, isopropyl and t-butyl acrylate, π-bonding with the Li⁺ ion is forced to take place on one side of the monomer only, thereby enhancing the formation of isotactic polymer quite markedly.

As this and other mechanisms all postulate the existence of structures stabilized by intramolecular solvation, the addition of Lewis-bases or polar solvents should disrupt the required template and encourage conventional anionic propagation by free ions. This automatically reduces the probability of an isotactic placement occurring.