Polyisoprene stereoisomerism

The reader should note that stereoisomerism does not exist if the substituents X and Y in the monomer 4-14 are identical. Thus there are no configurational isomers of polyethylene, polyisobutene, or polyfvinylidene chloride). It should also be clear that 1,2-poly-butadiene (reaction 4-3) and the 1,2- and 3,4-isomers of polyisoprene can exist as isotactic, syndiotactic. and atactic configurational isomers. The number of possible structures of polymers of conjugated dienes can be seen to be quite large when the possibility of head-to-head and head-to-tail isomerism is also taken into account. 

What term does describe the polymerization of isoprene to cis-1,4-polyisoprene Here we quote further from Eliel (1962). Stereoselective, in contrast, simply means that, of two (or more) possible stereoisomeric products in a reaction, one is produced in predominance (usually in great predominance) over all the others. An example is the 1,4 addition of chlorine to butadiene reported by Mislow and Heilman (1951). The product is only ira is-l,4-dichloro-2-butene (IX). Another example of a ster-... 

Another type of stereoisomerism arises in the case of poly-1,4-dienes because carbon-carbon double bonds are rigid and do not allow rotation. The substituent groups on the double-bonded carbons may be either on the same side of the chain (cis) or on the opposite sides (trans), as shown in Figure 3.6 for 1,4 polyisoprene. 

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