Mechanism of Anionic Polymerization

Anionic polymerization is a type of chain growth polymerization in which an anionic initiator transfers a charge to a vinyl monomer which then becomes reactive. Each reactive monomer goes on to react with other monomers to form a linear polymer.

Let RMe be an organometalic compound such as butyl lithium (LiC₄H₉) that initiates polymerization and M a monomer, then the individual steps can be represented as follows:

RMe ⇔ Me⁺ + R^-

R^- + M → RM^-

RM^- + M → RM₂^-

RM_n-1^- + M → RM_n^-

There is no termination step in these reactions unless contaminations such as water, alcohol or carbon dioxide are present or deliberately added to terminate the reactions (carbanion quenching). This is only the case, if all starting materials are thoroughly purified and if the reaction is carried out in vaccum or inert atmosphere. Under these conditions, very high molecular weight polymers are formed by anionic polymerization. Furthermore, if chain transfer is absent, anionic techniques result in unusual narrow molecular weight distribution because the rate of polymerization is approximately the same for all active chains and chain growth of all growing chains is initiated at about the same time when the "catalyst"¹ is added to the monomer. The polymerization only stops when all monomers are consumed. For this reason, this type of polymerization is often called anionic "living" polymerization. The efficiency of the catalyst is usually very high because each anion initiates polymerization, and the active carbanions will not react with each other due to strong Colomb repulsion of the carbanions. Thus 

[In₀] ≈ ∑_x [RM_x^-]

where [In₀] and [RM_x^-] denote the initial initiator concentration and the concentration of the active mer units, respectively. The reaction rate depends on the nature of the organometallic initiator and vinyl monomer. It also depends on the polarity of the solvent. In polar solvents, free solvated ion-pairs form which promote direct addition of the anion to the monomer as shown above. In a homogenous system, the rate of initiation is usually proportional to the concentration of the catalyst (complex) [R^-] and monomer [M] and the kinetic rate of polymerization is assumed to be independent of chain length, that is, it is directly proportional to the total concentration of growing chains, ∑ [RM_x] and monomer, [M]. If one assumes that the kinetic rate constant of initiation is much larger than that of propagtion, k_i » k_r, then the rate of monomer consumption is given by

d[M] / dt = -k_i [M] [R^-] - k_r [M] ∑_x [RM_x]
              ≈ - k_r [M] ∑_x [RM_x] = k_r [M] [In₀]

Separation of the variables and integration of this equation yields

[M](t) =[M₀] exp(-k_r [In₀] t)

which implies instantaneous initiation of all chains uppon addition of catalyst. This is only true in first approximation and only if addition of monomers is relative slow due to steric hindrance, ion pair aggregate formation etc. In some other cases, the propagation step can be much faster than the initiation step, as it is the case of styrene monomers in aromatic solvents initiated with butyllithium where ion pairs of growing chains do not form large aggregates. In general, the rate of propagation increases with solvent polarity and with the degree of ion pair dissociation. The propagation rate also depends on the structure of the monomer. For example, alkyl substitution on the α-carbon decreases the propagation rate due to the destabilizing effect of the alkyl group (increased electron density) and steric interference with solvation of the growing chain ends and with the addition of monomers whereas electron withdrawing groups stabilize the negative charge on the α-carbon atom of the carbanion through resonance or induction.

Notes & Further Readings

1. The initiators are often called catalysts. However, initiators are consumed in the reaction while catalysts are regenerated after the completion of the reaction.

2. A. Ravve, Principles of Polymer Chemistry, 3rd Edition 2012
3. Paul J. Flory, Principles of Polymer Chemistry, 1st Edition 1953