In step 2, we have the propagation step. Here, we're going to say the monomer anion reacts with the monomer molecule and forms a new anion through a head to tail addition. So, here is our head, which is negatively charged carbon. It'll come in and attack the tail of this monomer molecule, causing this pi bond to move here to this carbon. So at the end, what do we have?

This lone pair is used to make this bond now to our CH₂. That CH₂ is still connected to CH, which is connected to our nitrile. The pi bond moved to this carbon, so now it's negatively charged. And remember, that nitrile group is an electron withdrawing group. It'll help to stabilize the carbanion that we just created.

Now that we've seen the propagation step, click on the next video and let's take a look at the termination step.