How do free-radical conditions affect the repeating units formed from isoprene?

When isoprene undergoes polymerization under free-radical conditions, the resulting polymer chains primarily consist of trans-1,4 repeating units. While other structural variations may be present within the chains to a smaller extent, the trans-1,4 configuration predominates when this specific initiation method is employed.1

This outcome differs from other polymerization techniques, such as low-temperature anionic polymerization, which can be used to produce the cis-1,4 polyisoprene found in natural rubber. In industrial applications, these diene polymers are typically subjected to cross-linking or vulcanization to achieve the desired elastomeric properties.1