Overview

In this Lesson, we will learn about two additional chain growth polymerization mechanisms: cationic and anionic. These ionic polymerization mechanisms have several similarities to the radical chain growth polymerization in terms of the general mechanism (initiation, propagation, termination). An important distinction is that there will be a charged active center in ionic polymerization (while the radical active center is neutral). This charge is what primarily causes any differences we observe between ionic and radical chain growth. The different charge between cationic (positive charge) and anionic (negative charge) further leads to even more variation among ionic polymerization. Try to compare and contrast the three chain growth mechanisms as you go through the lesson to put in perspective and context how the various pathways are related to monomer chemistry, polymer skeletal structure, and reaction mechanism.

Learning Outcomes

By the end of this lesson, you should be able to:

• Predict if a monomer with polymerize via anionic or cationic pathways
• Compare and contrast ionic polymerization with free radical polymerization
• Draw the arrow pushing mechanism for cationic, anionic, and living anionic polymerization
• Describe the characteristics of living anionic polymerization
• Explain how the solvent and counter ions affect ionic polymerization rates

Lesson Checklist

Please refer to the Canvas Calendar for specific timeframes.

Questions?

If you have questions, please feel free to post them to the General Questions and Discussion forum. While you are there, feel free to post your own responses if you, too, are able to help a classmate.

In Lesson 6, we will continue to talk about chain polymerizations, but we will be exploring new reaction mechanisms: ionic polymerization. In contrast to free radical polymerization, in which the reactive species was a radical (which is neutral), in ionic polymerizations, the active center will be either an anion or a cation, thus carrying charge. Because of the charge associated with ionic polymerization, stabilization of that charge on the active center is going to be a much more critical factor to consider when deciding whether monomers will tend to polymerize with anionic or cationic pathways.

Figure 6.1 - Examples of monomers that can be polymerized by cationic pathways

Source: Lauren Zarzar

Think back to organic chemistry, where you learned about carbocations and carbanions. How were the carbocations and carbanions best stabilized? By substituents that could help distribute or delocalize charge! A carbocation is stabilized by substituents that donate electron density, since the carbocation is electron deficient. On the other hand, carbanions are best stabilized by substituents that withdraw electrons, since carbanions are electron rich. In most cases, a monomer will have a preference for whether it’s easier to polymerize by anionic or cationic pathways, depending on which active center intermediate is more stable. There are a few monomers, such as styrene, which distribute the charge so well that they can be polymerized by both pathways.

Let’s refresh your memory on electron withdrawing and electron donating groups:

PROBLEM

Do you expect acrylonitrile to proceed via anionic or cationic polymerization?

Acrylonitrile

1. Cationic
2. Anionic
3. Neither, free radical only

Click here for the answer.

ANSWER

B. Anionic

Because –CN is strongly electron withdrawing.

Before diving into the specifics of both anionic and cationic polymerization, here is a general comparison of ionic polymerization with free radical polymerization:

• A propagating chain active center is accompanied by a counter ion (opposite charge) in ionic polymerization, but not with free radical polymerization.
• The counter ion can affect stereochemistry and rate of polymerization in ionic polymerization, which is not a consideration for free radical polymerization.
• The polarity of the solvent is more important in ionic polymerization than in free radical polymerization, because now we have charge to stabilize at the active center.
• Termination can’t occur by reaction between active centers because they are the same charge, whereas in free radical polymerization it is common to have termination by combination of radicals.

Let’s first consider cationic polymerization in depth. Cationic polymerization goes through initiation, propagation, chain transfer, and termination, much like free radical polymerization. But cationic polymerization is of course characterized by having an active center that is a positively charged cation. Thus, we look for monomers that have substituents to help stabilize that positive charge and which are electron donating. Example monomers that can be polymerized by cationic polymerization are:

Figure 6.2 - Examples of monomers that can be polymerized by cationic pathways

Source: Lauren Zarzar

Initiation:

Cationic active centers are created by reaction of the monomer with an electrophile. Examples of good initiators are H₂SO₄ or HClO₄; BF₃, AlCl₃, SnCl₃ with ‘co-catalyst’ (water or organic halide, makes for more stable counter ion). The general steps of initiation are shown in Figure 6.3. As typical, there are two steps in the initiation process.

Figure 6.3 - Showing the steps of initiation for cationic polymerization.

Source: Lauren Zarzar

R⁺ is an electrophile and A^- is the counter-ion. Note that a double headed arrow is used to show the reaction mechanism because we are pushing two electrons (as compared to with reactions in free radical polymerization, where our single headed arrows signified the movement of individual electrons).

Propagation:

Propagation usually takes place by head to tail addition because of the stability of the carbocation and steric considerations. Again, note the double headed arrows in Figure 6.3 and that the counter-ion is always present.

Figure 6.4 - Propagation in cationic polymerization

Source: Lauren Zarzar

Termination and chain transfer:

Unlike in free radical polymerization, termination by two propagating chains reacting is not possible, because they have the same charge. Instead, termination occurs usually by ion pair rearrangement, releasing H⁺ and generating a terminal C=C as shown in Figure 6.4. Even though the terminated chain still has a double bond which in principle can continue to polymerize, due to the substitution on that terminal carbon, sterics will reduce the reactivity.

Figure 6.5 - Termination by ion pair rearrangement during cationic polymerization

Source: Lauren Zarzar

Chain transfer can also occur, similarly resulting in a terminated chain with an end C=C. Again, due to the substitution on the terminal carbon of the terminated chain, even though there is a double bond which can in principle continue to react, sterics will prevent reactivity. The chain that now contains the active center will continue to polymerize. Similar to free radical polymerization, chain transfer to polymer can affect the skeletal structure. Also like free radical polymerization, chain transfer to solvent can still occur and would reduce the degree of polymerization.

Figure 6.6 - Chain transfer during cationic polymerization

Source: Lauren Zarzar

Solvent and Counter-ion effects:

Because we now have charge at our active center and have a counter-ion associated with our active center, we have to take into account effects of solvent and that counter-ion that we didn’t have to consider for free radical polymerization. As follows from our general understanding of how added steric bulk prevents reactions, then it’s no surprise that “freer” carbocationic active centers propagate faster (higher k_p) than if associated closely with a counter ion. Polar solvents will help to separate ion pairs. Not only can propagation occur faster, but more ion pair separation will also reduce ion pair rearrangement and rate of termination.

Figure 6.7 - Effect of counter-ion association with active enter on rate of propagation

Source: Lauren Zarzar

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