Balance reaction equation for stabilised PAN cylisation reaction

I am dealing with polymer chemistry and trying to balance one of these reaction steps. I have read a lot about the cyclisation reaction step but it is not clear to me how to balance these equations or what happens between the *first step* A) and B1) *cyclised form* of polyacrylonitril? 


[![reactions](https://i.stack.imgur.com/eW6ED.png)](https://i.stack.imgur.com/eW6ED.png)


By looking at the image and counting the H- and C-atoms, I can tell that polyacrylonitril in the first step has 13 H-atoms (the same amount of C-atoms like the cyclised structure) while the cyclised structure gives 15 H-atoms. Can someone help me to balance or comment on this or what is missing in this part?


P.s.: I know that it is an increasing polymer chain but I just would like to truncate it to this step. 


