6 Polymer Stereochemistry
6.9 HOMOGENEOUS DIENE POLYMERIZATION
The principles applied in the previous section to essentially polar monomers can be extended to the stereoregular polymerization of dienes by alkali metals and metal alkyls. We have already seen that the cis-trans isomerism presents a variety of possible structures for the polydiene to adopt and complicates the preparation of a sample containing only one form rather than a mixture. Thus polyisoprene may contain units in the 1,2 or 3,4, or cis-1,4 or trans-1,4 configuration without even considering the tacticity of the 1,2 or 3,4 monomer sequences in the chain.
Most work has centered on the preparation of a particular form of geometric isomer because the type and distribution of each isomeric form in the chain has a profound influence on the mechanical and physical properties of the sample.
The original discovery that metallic lithium in a hydrocarbon solvent catalyzed
CH3 9813_C006.fm Page 170 Tuesday, June 12, 2007 12:35 PM
Polymer Stereochemistry 171
the production of an all cis-1,4 polyisoprene stimulated interest in this area and quickly raised two points that must be satisfied if a suitable mechanism is to be postulated.
1. Lithium and lithium–alkyl catalysts produce highly specific stereostruc-tures, but when replaced by Na or K, this effect diminishes.
2. Stereospecific polymerization takes place in the bulk state or in hydrocar-bon solvents, but the addition of a polar solvent leads to drastic changes.
To explain these features, the following mechanism has been put forward. Ini-tiation produces a “Schlenk” adduct VII.
The lithium ion then forms a chelate complex with the isoprene monomer locking it into a cis-configuration, which is maintained during the addition reaction. This type of complex is suitable when a small ion like Li+ is used but will be disrupted by the larger gegen-ions Na+ and K+, thereby allowing freer approach of the reactants.
The presence of ethers also alters the stereospecificity by competing for the Li+ and altering the spatial arrangement of the chelating pattern.
The monomer can then enter in a random fashion. The absence of significant 1,2- or 3,4-addition is thought to be caused by the shielding of carbon 3 in the transition state. However, all such proposals remain speculative.
LiCH2 CH C 9813_C006.fm Page 171 Tuesday, June 12, 2007 12:35 PM
172 Polymers: Chemistry and Physics of Modern Materials
6.10 SUMMARY
We can now summarize a few important points dealt with so far. Three factors influence stereoregularity during chain propagation:
1. Steric factors, which force the unit into a spatial arrangement determined by the size and position of the substituents already in the chain.
2. Polar factors because solvents that allow contact ion pairs favor isotactic placements, but pairs separated by heavy solvation (free ions) lead to syndiotactic structures.
3. Coordination because if the end group of a growing chain has a planar (sp2) configuration with no established parity, such as that found in free-radical or free-ion propagation, then the configuration of this unit is established only during the course of addition of an incoming monomer.
Normally, this will result in a syndiotactic placement with respect to the penultimate unit. Otherwise, coordination occurs between the gegen-ion, the incoming polar monomer, and the end or penultimate unit.
For polar monomers, the soluble catalysts can produce isotactic structures, but for nonpolar monomers, the homogeneous catalysts lead mainly to atactic or syn-diotactic polymers, and a heterogeneous catalyst is required for isotactic placements to occur. These will be discussed in the next chapter.
PROBLEMS
1. Draw the structures of the polymers generated by 1,4 addition via anionic polymerization starting from:
a. Methyl 2-methyl-2,4-hexadienoate (MMHd) b. Methyl 2,4-dimethyl-2,4-pentadienoate (MDMPd) c. Hydrogenation of poly(MDMPd) and poly(MMHd)
What is the difference between these two structures? What type of co-polymer is produced? (Hirabayashi et al., 2000)
2. Head-to-head polymers cannot be made directly, and a number of indirect approaches have been developed for their synthesis. For each of the methods in the following list, write down a synthetic scheme and name the polymer that is formed:
a. 1,4-Polymerization of 2,3-dimethylbutadiene-1,3 (anionic) followed by hydrogenation
b. Cationic polymerization of hexadiene-2,4 followed by hydrogenation c. Free-radical polymerization of 2,3-diphenylbutadiene-1,3 followed by
hydrogenation
d. Alternating polymerization of maleic anhydride and ethylene followed by esterification with methanol
e. Chlorination of cis-1,4-polybutadiene
9813_C006.fm Page 172 Tuesday, June 12, 2007 12:35 PM
Polymer Stereochemistry 173
3. Which of the following monomers may be prepared in different isomeric forms?
a. Vinylidene fluoride b. Vinyl chloride c. Ethyl acrylate d. Cyclopentene
4. Draw all possible configurational and stereo isomers resulting from poly-merization of chloroprene.
5. Explain why for vinyl monomers of type CHR=CHR′ two diisotactic isomers are possible but only one disyndiotactic structure is possible.
6. Draw a section of a poly(2-pentene) chain in the threo-diisotactic config-uration.
7. Draw structures of isotactic and syndiotactic polyacetaldehyde.
8. Explain why polytetrafluoroethylene (PTFE) (van der Waals diameter of fluorine atoms = 270 pm) chains adopt a helical conformation in the crystal.
9. Consider the low-temperature cationic polymerization of vinyl ethers and discuss the effect of solvent on polymer stereochemistry.
REFERENCES
Bawn, C.E.H. and Ledwith, A., Q. Rev., 16, 361, 1962.
Hirabayashi, T., Yamamoto, H., Kojima, T., Takasu, A., and Inai, Y., Macromolecules, 33, 4304, 2000.
Natta, G. and Corradini, P., Rubber Chem. Technol., 33, 703, 1960.
Vogl, O., J. Macromol. Sci. Chem., A21, 1725, 1984.
BIBLIOGRAPHY
Cooper, W., Stereospecific polymerization, in Progress in High Polymers, Vol. I, Academic Press, 1961.
Goodman, M., Concepts of polymer stereochemistry, Topics in Stereochemistry, Vol. 2, Wiley-Interscience, 1967.
Ketley, A.D., The Stereochemistry of Macromolecules, Vol. I–III, Edward Arnold, 1968.
Natta, G., Precisely constructed polymers, Sci. Am., 205, 33, 1961.
Schildknecht, G.E., Stereoregular polymers, in Encyclopedia of Chemistry, Reinhold Publish-ing, 1966.
Seymour, R.B., Introduction to Polymer Chemistry, McGraw-Hill, 1971, chap. 6.
9813_C006.fm Page 173 Tuesday, June 12, 2007 12:35 PM
9813_C006.fm Page 174 Tuesday, June 12, 2007 12:35 PM
175