Anionic Polymerization

We now move onto anionic polymerization where we have a negative charge at our active center. To stabilize our active center, we then want to have electron withdrawing substituents on the active center carbon in order to help delocalize the excess charge.

Initiation:

Anionic polymerizations are often initiated by strong nucleophilic initiators, such as potassium amide. As we often see, there are two steps for initiation:

Figure 6.8 - Initiation of anionic polymerization of styrene using potassium amide.

Source: Lauren Zarzar

Styrene is one of the monomer most commonly polymerized by anionic polymerization.

Propagation:

Again, looks similar to what we have seen before for propagation but with an anion at the active center. Similarly to initiation, we are showing the propagation for polymerization of styrene.

Figure 6.9 - Propagation for polymerization of styrene

Source: Lauren Zarzar

Termination and Chain Transfer:

Like other chain polymerizations we have seen, we can get chain transfer to solvent. However, for anionic polymerization, we do not see ion pair rearrangement, and there is no formal termination. Why? Well, it would require transfer of a hydride (H^-), rather than an H⁺ which we saw for cationic polymerization; this is unfavorable. Therefore, there is no formal pathway for termination. In order to terminate the polymerization, special molecules would need to be added to cap the polymer with non-reactive functional groups. In the absence of any termination, we call this kind of anionic polymerization a living polymerization.

Kinetics of Living Anionic Polymerization:

It is interesting to compare the kinetics of living polymerization with the free radical polymerization kinetics to see the effect that removing chain transfer and termination have on the kinetics. When carried out in a polar solvent, it is generally true for these anionic polymerizations that k_i is much greater than k_p since pretty much all our initiator is in the active form very early on. So, the total concentration of carbanionic active centers equals the concentration of initiator used initially ${\left[ I \right]}_0$. And we can write the rate of propagation:

$$R_p=-\frac{d\left[ M \right]}{dt}=k_p{\left[ I \right]}_0\left[ M \right]$$

where $K_p{\left[ I \right]}_0$ is a constant, so we see there is only dependence on ${\left[ M \right]}_0$, concentration of monomer. We can also easily describe the degree of polymerization:

$${\overline{x}}_n=\frac{molesmonomerconsumed}{molesofpolymerproduced}=\frac{cK{\left[ M \right]}_0}{{\left[ I \right]}_0}$$

where c=fractional conversion of monomer and K equals the number of active centers generated per initiator (often, this is 1 for initiators like KNH₂ but there are some initiators that generate 2 active carbanions per initiator molecule, which case K=2). We find from this equation that degree of polymerization increases linearly with monomer conversion (which is very different than for free radical polymerization!)

Because the mechanisms for living polymerizations without terminations are quite different from step or chain mechanisms, the derivation for the molar mass distribution is different. (we will not go into depth here). For living polymerization, the molar mass dispersity is given below and a plot of the distributions for various values of ${\overline{x}}_n$ is given in Figure 6.10 below (textbook Figure 5.1). Notice that the distributions are relatively narrow, which is different from what we have seen previously for step polymerization and free radical polymerization.

$$\frac{{\overline{M}}_w}{{\overline{M}}_n}=1+\frac{1}{{\overline{x}}_n}$$

Figure 6.10 - Weight-fraction Poisson distribution w_x of chain lengths for values of x̄_n in a polymerization without termination. For comparison, the dotted line shows the weight-fraction most probable distribution of chain lengths when ${\overline{x}}_n$ =50 (See Sections 3.1.3.2 and 4.3.8). Thus, polymerization without termination produces far narrower distributions of chain length than either step polymerization or conventional free-radical polymerization.

Source: Derived from Introduction to Polymers Figure 5.1