Isoprene

Isoprene is the monomer of natural rubber. It is an unsaturated molecule, so it has carbon–carbon double bonds, and many isoprene units join to build the long chain called poly(isoprene). Malay: isoprena · Chinese: 异戊二烯.

Because isoprene is unsaturated, its molecules can add to one another, and natural rubber forms when thousands of them link into one long poly(isoprene) chain. The chain still contains carbon–carbon double bonds along its length, and these leftover double bonds are important: they are the exact places where sulfur atoms attach during vulcanisation to form cross-links. So the double bonds in isoprene do two jobs, they let the monomer polymerise, and they later allow the rubber to be vulcanised. Knowing isoprene as the rubber monomer connects the start and the end of the natural-rubber story.

Example. In the rubber tree, isoprene units are joined into poly(isoprene), which is carried as particles in latex. When this rubber is later heated with sulfur, the sulfur reacts at the double bonds that came from the isoprene units, forming the cross-links of vulcanised rubber.

Confusion to avoid. Do not confuse isoprene with poly(isoprene). Isoprene is the small monomer, a single unsaturated molecule; poly(isoprene) is the large polymer, the long chain of many isoprene units that makes up natural rubber. The monomer is what you start with; the polymer is what you build.

A useful point for the exam is that the same double bonds explain two stages at once: without them isoprene could not add into a polymer, and without them the finished rubber could not be vulcanised. Being able to trace the double bond from monomer to cross-link shows the examiner you understand the whole rubber pathway.

Isoprene is the key that links natural rubber back to the idea of a monomer, and it reappears in questions on rubber and vulcanisation in SPM Chemistry.