Polymers are large complex substances that arise from the joining of smaller simpler units called Monomers. A monomer is joined together by a covalent bond, to form a polymer molecule. However, this process usually accompanies the loss of a water molecule due to the hydrogen (H+) and hydroxyll (OH-) ions that is contributed by each monomer unit. Hence, this process is called a DEHYDRATION REACTION.
For example, glucose moleculea as monomer units combine together in a dehydration reaction to form larger polymers such as starch, glucose etc.
