Polymer such as rubber, suberin, melanin and lignin. Naturally occurring polymers such as cotton, starch and rubber were familiar materials for years before synthetic polymers such as polyethene and perspex appeared on the market. Many commercially important polymers are synthesized by chemical modification of naturally occurring polymers. Prominent examples include the reaction of nitric acid and cellulose to form nitrocellulose and the formation of vulcanized rubber by heating natural rubber in the presence of sulfur. Ways in which polymers can be modified include oxidation, cross-linking and endcapping. Especially in the production of polymers the gas separation by membranes has acquired increasing
Silicone rubber cured by a platinum-catalyzed cure system, a condensation cure system, a peroxide cure system, or an oxime cure system. For the platinum-catalyzed cure system, the curing process can be accelerated by adding heat or pressure. In a platinum-based silicone cure system, also called an "addition" system (because the key reaction-building polymer is an "Addition reaction"), two different chemical groups (a silicone hydride and a vinyl) react in the presence of platinum (which serves as the catalyst). In this reaction, an ethyl group [C(H2)- C(H2)] is formed and there are no byproducts. Two separate components must be mixed to catalyze the