Polymerization is a very important chemical process that is used to produce all sorts of plastic products for everyday use... e.g.: nylon, paint, synthetic rubber and even chewing gum! The process really involves the joining of small molecules to form long chains that keep repeating.
PVA glue contains polyvinyl acetate (also known as PVA), a chemical which is made up of carbon, hydrogen and oxygen atoms. PVA reacts with the 'sodium tetraborate' in the borax solution, forming giant chains of cross-linked carbon atoms.