Diene Polymerization in Nonpolar Media

The polymerization of 1,4-butadiene and related conjugated dienes in polar media was discussed earlier. The major focus of this part of the review will be devoted to the polymerization of 1,3-butadiene and iso-prene in hydrocarbon media, with emphasis on the allylic lithium structure and reactivity. 

The structures of alkyllithium compounds have been studied extensively by various investigators, and their findings are given in Table I (44-49). Data in Table I suggest that the alkyllithium compounds in hydrocarbon medium are of either tetrameric or hexameric nature. The highly branched compounds are tetrameric. 

There are reports in the literature that the higher molecular-weight alkyllithium compounds such as polystyryl- or polyisopropyllithium may be dimeric (41,42). 

Compound Solvent Concentration range n Method11 Reference 

Determination of the absolute rate constant of anionic polymerization has been complicated by the existence of an equilibrium of various associated species. Work on the rate of initiation Kx has been reviewed by By water (55). Investigation of the rate of propagation Kv has involved two kinetic approaches. 

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Thus, it appears that there is no unambiguous mechanistic interpretation for RLi diene polymerization in nonpolar media and more work is needed. Nevertheless, some 13C-NMR data have been obtained that strongly favor some type of interaction between the positive lithium center and the 7r-electrons of the double bond, e.g., the stronger shift centered on the methine (CH2=C) carbon atom. It also appears that the live-end 1,4-structures in hydrocarbon media are in equilibrium with live ends that give rise to 1,2-structures, as alluded to in the earlier discussion. 

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