b8aeretmoegncdgdbsureodtrans,ds,trane 14-10 Polymerization of Coniugated Dienes:Rubber 2。 for mers 。 CH-C-CCH-CHCHCH Ee器d 1.4-Paly mereatiom of cme ageaeoiymertaton eicrubbers are derived from poly-1,3-dines Crono-Bekin --1. nt olyns oub rn 股2a8 S8pememeanse3seng6wengeatpmc3io8etes 9 9 However, the photochemical ring closure of trans,cis,trans￾octatriene is conrotatory. The stereocontrol of these and many other electrocyclic transformations is governed by the symmetry properties of the relevant π molecular orbitals and is summarized by the Woodward-Hoffmann rules: 14-10 Polymerization of Conjugated Dienes: Rubber 1,3-Butadiene can form cross-linked polymers. 1,2-Butadiene can be polymerized at C1 and C4 to yield polyethenylethene (polyvinylethylene) or at C1 and C4 to yield trans-polybutadiene, cis-polybutadiene or a mixed polymer. Because the product of butadiene polymerization is unsaturated, irradiation or radical initiators can cause additional linkages between individual polymer chains. These substances are called cross-linked polymers. Cross-linking generally increases the density and hardness of polymeric materials. In addition, cross-linking increases the elasticity of polymeric materials, a property characteristic of rubbers. Synthetic rubbers are derived from poly-1,3-dienes. 2-Methyl-1,3-butadiene (isoprene), when polymerized using a Ziegler-Natta catalyst, results in a synthetic rubber called polyisoprene of almost 100% Z configuration. 2-Chloro-1,3-butadiene can be polymerized into an elastic, heat- and oxygen-resistant polymer having trans double bonds called neoprene. The elasticity of polyisoprene-like polymers can be increased by a process called vulcanization, in which the polymer is treated with hot elemental sulfur, which causes additional crosslinks. Copolymers of 1,3-butadiene with other alkenes result in polymeric materials having a wide range of physical properties. Polyisoprene is the basis of natural rubber. Natural rubber results from the polymerization of 3-methyl-2- butenyl pyrophosphate: