Living Anionic Polymerization

Living anionic polymerization is one of the most reliable and versatile methods to synthesize polymers with well-defined structure, very low poldispersity, and extremely high molecular weight.¹ This method also allows for sequential growth of block-copolymers because the growth-active chain ends remain active ("living") even though all monomers have been consumed. In addition, the living chain ends can be converted to various functional groups such as carboxylic acid, thiol, and hydroxyl groups yielding telechelic polymers.²

The very narrow molecular weight (MW) distribution results from the absence of termination and transfer reactions and from the almost instantaneous initiation which is typically much faster than propagation. For this reason, all polymer chains propagate at the same time and with the same rate and, thus, reach the same length at the same time. Under these conditions, the number of polymers and their number average MW depend only on the initial initiator concentration [I₀], the initial monomer concentration [M₀] and fractional conversion p. In the simplest case, the number-average degree of polymerization, ⟨X_n⟩, is the ratio of consumed monomer concentration, p [M₀], and concentration of living chain ends, [I₀]:

⟨X_n⟩ = p [M₀] / [I₀]

This is the case, when all initiator molecules [I] are converted into propagating chain ends and when only one initiator molecule is needed per polymer molecule to start living anionic chain growth.

To produce high molecular weight polymers, long lifetimes of the propagating chain ends are required which can be achieved by solvation and aggregation of the propagating carbanions, i.e. by choosing a suitable solvent. Furthermore, termination, chain transfer, and other side reactions have to be avoided. This typically requires relatively low or moderate temperatures.^3,4

A controlled living polymerization can be initiated by a number of compounds including alkali metals and their salts, bases, and organometalic derivatives such as alkyl, aryl, alkoxy, amino, and cyano. One of the most popular and effective initiators are alkyl lithium compounds which are frequently used to polymerize diene-styrene copolymers.¹¹ Chain growth is initiated by addition of alkyl lithium to the double bond of a monomer:

which is followed by anionic chain propagation:

Alkyl lithiums are not suitable for polymerization of vinyl monomers with strongly electron-withdrawing side groups because they have a strong tendency to attack the side group. In the case of methyl methacrylate (MMA), they will attack both the vinyl and carbonyl group:⁵

The result is a copolymerization of these two monomers (alkyl isopropenyl ketone and MMA), and not a homopolymerization of MMA. Thus, less reactive initiators must be used to avoid attack of the carbonyl group.^4,6 Some suitable initiators are metal compounds that have electron withdrawing and bulky substituents. Two examples are phenyl substituted organometal compounds such as triphenyl lithium and tertiary metal alkoxides.⁷ Both steric hindrance and charge distribution over the substituents lowers the reactivity of the anion and thus reduces the propensity of the initiator to attack the carbonyl group.

Despite the many advantages, some major disadvantages exist, making anionic polymerization a rather expensive and difficult to control polymerization technique which is probably the main reason why it has not gained widespread industrial use. In fact, styrenic block copolymers are the only copolymers that are produced on a large scale via living anionic polymerization.⁸ One major disadvantage is the high reactivity of the nucleophilic chain ends towards impurities such as moisture, oxygen, carbon dioxide and many other electrophilic compounds.^4,10 Water molecules, for example, readily transfer a proton to the carbanion and terminate propagation:

whereas oxygen molecules form peroxy and carboxyl anions. These anions are typically not reactive enough to propagate. Thus, (atmospheric) moisture, oxygen etc. has to be completely removed from the reaction vessel which requires extensive purification of solvents, monomers, and equipment.^1,9-10

Despite these disadvantages, living anionic polymerization remains a very popular polymerization technique in research because it allows for a very high degree of control over molecular parameters such as molecular weight, polydispersity, composition, and chain architecture. In fact, this technique still remains the "gold standard" for the synthesis of well-defined polymers.¹

References & Notes

1. H. Frey and T. Ishizone, Macromol. Chem. Phys., 218, 1700217 (2017) &, 219, 1700567 (2018)
2. D.N. Schulz, A.O. Patil, Functional Polymers, ACS Symposium Series; Washington, DC (1998)
3. Most alkyllithiums are extremely reactive and very unstable in polar solvents which requires the initiation step to be performed at low temperatures (−78 °C).⁴
4. D. Baskaran and A.H.E. Mueller, Anionic Vinyl Polymerization, in Controlled and Living Polymerizations, edited by A.H.E. Muller and K. Matyjaszewski, Wiley-VCH, Weinheim 2009
5. P. Vlcek, L. Lochmann, Prog. Polym. Sci., Vol. 24, pp. 793-873 (1999)
6. M. Tomoi, K. Sekiya, and H. Kakiuchi, Poly. J., Vol. 6, No. 5, pp 438 - 444 (1974
7. J. Baca et al., J. Poly. Sci. Part C, 16, 3865 - 3875 (1968)
8. J.E. Mark, B. Erman and C.M. Roland, The Science and Technology of Rubber, Chapter 13, Thermoplastic Elastomers, 4^th Ed., Elsevier 2013
9. K. Hong et al., Anionic Polymerization in Encyclopedia of Materials: Science and Technology, Editors K.H.J. Buschow et al., Elsevier 2001
10. N. Hadjichristidis, H. Iatrou, S. Pispas, M. Pitsikalis, J. Poly Sci.: Part A: Poly. Chem., 38, 3211-3234 (2000)
11. Lithium compounds are typically the most effective initiators because they have the highest electronegativity and the smallest ionic bond radius among all alkali metals.

November 11, 2019