1,4-cis
1,4-trans
1,2-vinyl
3,4-vinyl
Conjugated dienes (butadiene, isoprene, etc.) are a special case of vinyl polymerization. When the propagation goes 1,4, a double bond ends up in the backbone. Of course, this double bond exhibits cis and trans isomers. However, propagation can involve only one of the two bonds in the monomer. For isoprene, the two double bonds are not the same, so two different vinyl-type repeat units result (1,2 and 3,4). The possible isomers form in varying amounts, depending on the method of polymerization:
| Reaction Conditions | 1,4-cis |
1,4-trans |
1,2-vinyl |
3,4-vinyl |
| Free radical, emulsion | 22% | 65% | 6% | 7% |
| Cationic in CHCl3 | 0% | 90% | 4% | 6% |
| Anionic, BuLi in pentane | 93% | 0% | 0% | 7% |
| Coordination, VCl3/AlEt3 | 0% | 99% | 0% | 0% |
| Coordination, TiCl4/AlEt3 + amine | 100% | 0% | 0% | 0% |
Obviously, the distribution of isomers varies considerably with polymerization conditions. The remarkable thing is that coordination polymerization is capable of producing pure 1,4-cis or 1,4-trans isomers with only minor modifications to the initiator.
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