Why do microwave ovens make bread rubbery? Why does heating bread (cinnamon buns) in a microwave give it a rubbery texture, when a regular oven doesn't?  What are the chemical or structural changes?
It is simply that the microwave heats primarily the water molecules, causing the bread to steam. A normal oven heats all of the molecules of the bread, and by the time the water is heavily steaming you will have pulled it out of the oven.