Naturally occurring polymers like starch, rubber, and cotton were familiar materials for many years before synthetic polymers like perspex and polyethene appeared on the market. Several commercially essential polymers are synthesized by the chemical modification of naturally occurring polymers. Some prominent examples are the reaction of cellulose and nitric acid to produce nitrocellulose and the vulcanized rubber formation by heating natural rubber in the presence of sulfur compounds. Polymers can be modified in cross-linking, end-capping, and oxidation ways.