Polyisoprene tactic forms

As described in Chapters 2 and 3, the monomer can be inserted into the polyisoprene chain potentially in nine different ways. These are the three tactic forms of the 1,2-adducts, two 1,4-adducts, cis and transy and three tactic forms of 3,4-adducts. In addition, there is some possibility of head-to-head and tail-to-tail insertion, though the common addition is head-to-tail. Table 5.8 presents the various microstructuies that can be obtained in polymerizations of isoprene with different catalysts. 

The principles apphed in the previous section to essentially polar monomers can be extended to the stereoregnlar polymerization of dienes by alkali metals and metal alkyls. We have already seen that the cis-trans isomerism presents a variety of possible structures for the polydiene to adopt and complicates the preparation of a sample containing only one form rather than a mixture. Thus polyisoprene may contain units in the 1,2 or 3,4, or cis-1,4 or trans-1,4 configuration without even considering the tacticity of the 1,2 or 3,4 monomer sequences in the chain. 

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