Anionic Polymerization

Maurice Morton Institute of Polymer Science The University of Akron, Akron, OH

Living anionic polymerization, especially using alkyllithium initiators, has been demonstrated to be a convenient and useful method to make well-defined polymers with low degrees of compositional heterogeneity and with control of the major structural variables that affect polymer properties.1,2 Living polymerizations are chainreaction polymerizations that proceed in the absence of the kinetic steps of chain termination and chain transfer. For a living polymerization, one initiator molecule generates one polymer molecule; thus, it is possible to calculate and control the number average molecular weight (Mn) of the final polymer via the stoichiometry of the reaction using the following relationship.

Mn = g of monomer consumed/moles of initiator

Given a comparable or faster rate of initiation relative to propagation, it is possible to obtain narrow molecular weight distribution polymers, i.e., Mw/Mn ≤ 1.1.3 Due to the absence of termination and transfer steps, the product after complete monomer consumption is a reactive, polymeric organolithium compound. The living nature of alkyllithium-initiated anionic polymerizations using suitable monomers provides versatile methods for the preparation of well-defined block copolymers by sequential addition of monomers,4 chain-end functionalized polymers by reaction of the living chain ends with appropriate monomers and/or electrophilic terminating agents5,6 and branched polymers by linking reactions with multi-functional linking agents.7

The monomers that can be polymerized anionically are classified into two categories: (a) unsaturated monomers with one or more double bonds, such as vinyl (e.g., styrenes, vinylpyridines, alkyl methacrylates), dienes (e.g., isoprene, 1,3-butadiene) and carbonyl-type monomers (e.g., formaldehyde); and (b) heterocyclic monomers (e.g., epoxides, thiiranes, lactones, lactams, and siloxanes). In the case of vinyl monomers, the presence of electronwithdrawing substituents (e.g., X, Y) in the double bond is generally required to stabilize the negative charge that develops in the transition state as shown below.

Organolithium Initiators

Of all alkali metals, lithium is unique in that it exhibits the highest electronegativity, the smallest covalent and ionic bond radii, along with low-lying, unoccupied p-orbitals available for bonding.^8,9 Organolithium compounds are unique among organoalkali compounds in exhibiting properties characteristic of both covalent and ionic compounds. Thus, they are aggregated in solution, in the solid state and in the gas phase, and they are generally soluble in hydrocarbon solution. In general, the initiation of anionic polymerization of styrene and diene monomers is effected with alkyllithium compounds such as sec-butyllithium (195596) and n-butyllithium (186171, 230707, 230715, 302104, and 302120 in hydrocarbon solution. Under these conditions, the unique ability of organolithium compounds to effect 1,4-enchainment of 1,3- dienes is achieved.^1,10 The concentrations of solutions of active organolithium compounds can be determined using the Gilman double titration method.¹¹

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Experimental Methods

Due to the reactivity of organolithium compounds and other carbanionic species toward impurities such as oxygen, moisture or carbon dioxide,¹² it is necessary to exclude these contaminants from the reaction environment by the use of an inert gas atmosphere^13,14 or high vacuum techniques.^15–17

High Vacuum Techniques

The use of high vacuum techniques provides the most effective experimental method to exclude impurities from the reaction system.^15–17 In order to attain high vacuum, the combination of a mechanical pump and an oil diffusion pump (Z220418 ) is used in conjunction with a two-stage glass manifold as shown in Figure 1.

Figure 1.The typical construction of a glass high vacuum line for anionic polymerization.^15–17

In order to achieve the desired levels of purity for controlled anionic polymerization, all monomers, reactants, and solvents should be purified, dried, and degassed, preferably on the vacuum line. Solvents are distilled directly into the requisite glass reactors (Figure 2) via D followed by flame sealing from the vacuum line. Ampules B, E, and F contain monomers or functionalizing agents. Ampule C contains a terminating agent such as degassed methanol. Ampule A is equipped with a degassed methanol tube, and it is used to remove a base sample of the living polymer.

Figure 2. General set-up for a glass polymerization reactor.

Schlenk Line and Glove Box Techniques

Schlenk Line and Glove Box Techniques are often suitable for carrying out many living anionic polymerization procedures. Alkyllithithium-initiated polymerizations are somewhat forgiving in the sense that one can add a calculated excess of initiator to clean the reactor/solvent/monomer system of reactive impurities. See Figure 3 for representative Schlek Line Glassware. For representative and the Sigma-Aldrich glass center, visit sigmaaldrich.com/glasssigmaaldrich.com/glass.

Figure 3.Aldrich products a) Z544787 b) Z173053 c) Z174254 d)Z174432 e)Z220418

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Safety Considerations

Vacuum traps should be vented while warming because of the possibility of trapped, liquefied gases. Hydrocarbon solutions of alkyllithium compounds are air- and moisture-sensitive; they should be either handled under an inert atmosphere or using syringes and recommended procedures for handling air-sensitive compounds.¹³
Carbon dioxide extinguishers should not be used because RLi compounds and many other organometallic compounds react with carbon dioxide exothermically. An all-purpose fire extinguisher, or one designed specifically for combustible metals, should be available when working with these organometallic compounds and alkali metals.¹⁸

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Materials

References

1. Hsieh, H. L.; Quirk, R. P. Anionic Polymerization: Principles and Practical Applications; Dekker: New York, 1996.
2. Quirk, R.P. Anionic Polymerization. In Encyclopedia of Polymer Science and Technology Kroschwitz, J. I., Ed.; 3rd ed.; Wiley-Interscience: New York, 2003; Vol. 5, p 111.
3. Fetters, L. J. Monodisperse Polymers. In Encyclopedia of Polymer Science and Engineering Kroschwitz, J. I., Ed.; 2nd ed.; Wiley-Interscience: New York, 1985; Vol. 2, p 478.
4. Hadjichristidis, N.; Pispas, S.; Flouds, G. A. Block Copolymers: Synthetic Strategies, Physical Properties, and Applications; Wiley-Interscience: New York, 2003.
5. Quirk, R. P. Anionic Synthesis of Polymers with Functional Groups. In Comprehensive Polymer Science, First Supplement; Aggarwal, S. L., Russo, S., Eds.; Pergamon Press: Oxford, 1992; p 83.
6. Hirao, A.; Hayashi, M. Acta Polym. 1999, 50, 219.
7. Hadjichristidis, N. et al. Chem. Rev. 2001, 101, 3747.
8. Wardell, J. L. Alkali Metals. In Comprehensive Organometallic Chemistry: The Synthesis, Reactions and Structures of Organometallic Compounds; Wilkinson, G., Gordon, F., Stone, A. Abel, E. W., Eds.; Pergamon Press: Oxford; 1982; Vol. 1, p 43.
9. Sanderson, R. T. Chemical Periodicity; Reinhold: New York; 1960.
10. Bywater, S. In Comprehensive Polymer Science; Eastmond, G. C., Ledwith, A., Russo, S., Sigwalt, P., Eds.; Chain Polymerization I; Pergamon Press: Elmsford, New York, 1989, Vol. 3, p. 433.
11. Gilman, H.; Cartledge, F. K. J. Organomet. Chem. 1964, 2, 447.
12. Wakefield, B. J. The Chemistry of Organolithium Compounds; Pergamon Press: New York; 1974.
13. Shriver, D. F.; Drezdzon, M. A. The manipulation of Air-Sensitive Compounds; Wiley: New York, 1986. (Cat. No. Z558486)
14. Ndoni, S.; et al. Rev. Sci. Instrum. 1995, 66, 1090.
15. Morton, M; Fetters, L. J. Rubber Chem. Technol. 1975, 48, 359.
16. Hadjichristidis, N. et al. J. Polym. Sci., Part A: Polym. Chem. 2000, 38, 3211.
17. Uhrig, D; Mays, J. W. J. Polym. Sci., Part A: Polym. Chem. 2005, 43, 6179.
18. Wietelmann, U.; Bauer, R. J. In Ullmann’s Encyclopedia. Industrial Inorganic Chemicals and Products; Wiley- VCH Verlag: Weinheim, Germany; 1998; Vol. 4, p. 2899.

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