Cationic Polymerization

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