POLYISOPRENE

Natural rubber is a stereoregular polymer composed of isoprene units attached in a cis configuration. This arrangement gives the rubber high resilience and strength. Isoprene can be polymerized using free radical initiators, but a random polymer is obtained. As with butadiene, polymerization of isoprene can produce a mixture of isomers. However, because the isoprene molecule is asymmetrical, the addition can occur in 1,2-, 1,4- and 3,4- positions. Six tactic forms are possible from both 1,2- and 3,4- addition and two geometrical isomers from 1,4- addition (cis and trans):

Stereoregular polyisoprene is obtained when Zieglar-Natta catalysts or anionic initiators are used. The most important coordination catalyst is α- TiCl₃ cocatalyzed with aluminum alkyls. The polymerization rate and cis content depends upon Al/Ti ratio, which should be greater than one.

Lower ratios predominantly produce the trans structure. Figure 1.1 shows a process for producing cis-polyisoprene by a solution polymerization.

Figure 1.1. A process for producing 1,4-polyisoprene (>99%) by a continuous solution polymerization.