Condensation Mechanism

PET is a Condensation Polymer - a copolymer

Condensation reactions in organic are when two molecules join together to make a combined/bigger molecule. The process also splits out a small molecule such as water during the process. PET does this. The scheme is a bit different than an addition polymer in that there are two monomer units - which is why it is called a copolymer. Below is the quick and dirty idea behind how this works. A and B are the two monomers and C is the little condensate molecule which will be water for us.

1. A + B → A-B + C
2. A-B + A → A-B-A + C
3. A-B-A + B → A-B-A-B + C
4. A-B-A-B + A → A-B-A-B-A + C
5. and so on ...→ -(-A-B-)_n- + nC

So the chain increases one unit at a time but alternates A-B-A-B- as it grows. Below is the more detailed mechanism for the specific case of making PET. There is no initiation step because a radical is not needed. The two monomers are reactive with each other.

Overall Process for a Single Link

Let me show you the two reactants and the water that will split out. This is NOT the mechanism but it DOES show you the start and finish. The mechanism is actually quite involved and is shown further down as a 5-step mechanism. All polyesters form this way, not just PET.

The atoms that split out as water are highlighted in red. Note how the oxygen and one hydrogen comes from the carboxylic acid and the other hydrogen comes from the hydroxyl group on the glycol. As you will see below, there is a lot that happens to facilitate this entire process. Realize that the process above continues to cycle - the ends of the growing polymer chain are the reactive functional groups that keep coupling and the chain grows. The by-product of water can be easily removed as steam or condensate.

Here is the Whole Big-Ass Mechanism

This is so long that I just made it totally a vertical scroll. The five steps are numbered. The whole reaction is done with an acid catalyst. This means that a proton (H⁺) enters the mechanism at the start, but is released at the end so that it can continue catalyzing cycle after cycle. This mechanism is really for any carboxylic acid that combines with an alcohol to make an ester. It's just with PET, there are two active ends of the reactants and therefore just keeps reacting with itself lengthening the polymer chain.