The balanced chemical equation for the formation of polypropylene, a type of plastic, from its monomer, propylene, is:

n CH2=CHCH3 → (CH2-CHCH3)n

In this equation, "n" represents the number of propylene molecules that polymerize to form a single chain of polypropylene. The reactants are propylene molecules (CH2=CHCH3), and the product is a polymer chain of polypropylene ((CH2-CHCH3)n).