Question

Assuming only head-to-tail polymerization and ci sdouble-bond opening occurs, give the structure and stereochemical designation of all stereoregular polymers obtainable in principle by stereospecific polymerization of (a) 2-methyl-1-butene, (b) trans-2-pentene, (c) isoprene, and (d) propyne.

Answer 1

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According to the IUPAC definitions stereospecific polymerization is a “polymerization in which a tactic polymer is formed. However, polymerization in which stereoisomerism present in the monomer is merely retained in the polymer is not to be regarded as stereospecific.” This problem refers to a polymerization process performed on pro-chiral molecules that generate a specific stereoisomer. A regular and stereospecific polymer is a regular polymer whose molecules are units that repeat but only one of the stereoisomers. They form a single sequential arrangement that has a defined configuration at all the possible sites where isomerism is possible (within the chain). Regular polymers that come from head-to-tail reactions can exhibit stereoisomerism.

Case (a): 2-methyl-1-butene

Head-to-tail polymerization means that carbon 1 from one alkene kinks to carbon 2 of the next alkene. Then a cis double-bond opening pushes the substituent to the same side.

Case (b): trans-2-pentene

Case (c): Isoprene

Isoprenoids possess carbon skeletons that may be regarded as built up from isoprene units linked “head to tail”; that is, carbon atom 1 of one unit is bonded to carbon atom 4 of the next unit. Combined with cis addition we get:

Case (d): Propyne