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.
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:
| Electron Withdrawing Groups | Electron Donating Groups |
|---|---|
| -CF3 | -NH2 |
| -CN | -OR |
| -NO2 | -OH |
| -COOH (moderate) | -NHR |
| -COOR (moderate) | -OCOR (moderate) |
| -halides (weak) | -Ph (moderate) |
| - | -R (weak) |
PROBLEM
Do you expect acrylonitrile to proceed via anionic or cationic polymerization?
- Cationic
- Anionic
- Neither, free radical only
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.