Vulcanisation of rubber template

Schematic radical picture of sulfur vulcanisation of natural rubber: heat opens the S₈ ring, a sulfanyl radical takes an allylic hydrogen from cis-1,4-polyisoprene and the allylic radical bonds to sulfur; the other end of the sulfur chain does the same on a second chain, so Sₓ bridges cross-link the chains.

Mechanism step by step

Sulfur bridges join neighbouring cis-polyisoprene chains at allylic carbons (schematic radical picture)

1. S₈ opens – Heat breaks one S–S bond: a sulfur diradical chain
2. Allylic H abstraction – A sulfanyl radical takes an allylic H (the C–H next to C=C is weakest)
3. C–S bond – The allylic radical bonds to sulfur; the other end of the Sₓ chain joins a second rubber chain the same way

Key points

• Natural rubber (cis-1,4-polyisoprene) is soft and sticky; Sₓ cross-links stop the chains sliding past each other, so vulcanised rubber springs back and does not dissolve.
• The cross-links are at allylic carbons and most C=C bonds survive; a few % sulfur gives elastic rubber, 30–40% gives hard ebonite.
• Accelerators (sulfenamides, MBT) with ZnO and stearic acid cut curing from hours to minutes; whether the real steps are radical or polar is still debated, so this scheme is simplified.
• Goodyear (1839) discovered it; cross-linked rubber is a thermoset, which makes tyres hard to recycle.

Drawn with 80 atoms, 11 arrows, 11 shapes – every one editable in MolDraw.

Sources

• Odian, Principles of Polymerization, 4th ed., Wiley 2004, ch. 9 Textbook
• Wade, Organic Chemistry, 8th ed., Pearson 2013, ch. 26 Textbook