Sulfur vulcanization been uncertainly as to whether it proceeds in a radical or ionic manner. The reactive sites, often referred to as 'cure sites', are unsaturated groups such as alkenes and allyls. During vulcanization, sulfur bridges are formed between these sites, crosslinking the polymer. These bridges may consist one or several sulfur atoms. Both the extent of crosslinking and the number of sulfur atoms in the crosslinks strongly influences the physical properties of the rubber produced: Sulfur, by itself, is a slow vulcanizing agent and does not vulcanize synthetic polyolefins. Even with natural rubber, large amounts of sulfur as well as high
Rubber elasticity Joule published the first careful measurements of the temperature increase accompanying extension, confirming the theoretical predictions of Lord Kelvin. It was not until 1838 that the American inventor Charles Goodyear found that its properties could be immensely improved by adding a few percent sulphur. The short sulfur chains produced chemical cross-links. Before it is cross-linked, the liquid natural rubber consists of very long linear chains, containing thousands of isoprene backbone units, connected head-to-tail. Every chain follows a random path through the liquid and is in contact with thousands of other nearby chains. When heated, a cross-linker molecule (such as sulfur