chapter 12.6

Conjugated dienes can be polymerized like simple alkenes.

  • Their polymers are more complex due to double bonds appearing every four carbon atoms, which allows for cis-trans isomers.

  • The polymerization reaction, a 1,4 addition to the diene monomer, can be initiated by either a radical or an acid.

Two molecules of 1,3-butadiene form cis-polybutadiene and trans-polybutadiene. Arrows on 1,3-butadiene molecules depict the movement of electrons and initiator (In) lies on the left side of 1,3-butadiene.

Conjugated dienes polymerize similarly to simple alkenes.

  • The resulting polymers are more intricate, featuring double bonds every four carbon atoms and supporting cis-trans isomerism.

  • This polymerization occurs via 1,4 addition to the diene monomer, initiated by either radicals or acids.

Two molecules of isoprene (2-methyl-1,3-butadiene) form natural rubber (Z) and Gutta-percha (E). Arrows on isoprene molecules depict the electrons' movement and initiator (In) lies on the left side of isoprene.

A variety of synthetic rubbers are commercially produced through diene polymerization.

  • Both cis
    and trans
    polyisoprene can be synthesized, resembling natural rubber.

  • Chloroprene (2-chloro-1,3-butadiene) is polymerized to produce neoprene.

  • Neoprene is an excellent but costly synthetic rubber known for its good weather resistance.

  • It is utilized in products such as industrial hoses and gloves.

Two molecules of chloroprene (2-chloro-1 3-butadiene) form neoprene (Z). Arrows above chloroprene molecules depict the movement of electrons and and initiator (In) lies on the left side of chloroprene.

  • Both natural and synthetic rubbers are initially too soft and tacky for practical use.

  • They are hardened through a process called vulcanization, which involves heating them with elemental sulfur.

  • Vulcanization creates carbon–sulfur bonds, cross-linking the rubber chains.

  • This cross-linking hardens and stiffens the polymer, and the degree of hardening can be adjusted for different applications (e.g., automobile tires vs. bowling balls).

  • Rubber's unique ability to stretch and contract is due to the irregular structure of its polymer chains, caused by double bonds that introduce bends and kinks.

  • These irregularities prevent neighboring chains from nesting together.

  • When stretched, the randomly coiled chains straighten and align but are held by cross-links.

  • Upon release of tension, the polymer reverts to its original random, coiled state.