Steric Control in Anionic Polymerization

Use of hydrocarbon solvents has an advantage in polymerizations of conjugated dienes, because they yield some steric control over monomer placement. This is true of both tacticity and geometric isomerism. As stated earlier, the insertions can be 1,2 3,4 or 1,4. Furthermore, the 1,4-placements can be cis or trans. Lithium and organolithium initiators in hydrocarbon solvents can yield polyisoprene, for instance, which is 90% cw-1,4 in structure. The same reaction in polar solvents, however, yields polymers that are mostly 1,2 and 3,4, or trans-lA in structure. There is still no mechanism that fully explains steric control in polymerization of dienes. 

High cis-1,4 polyisoprene forms with lithium or alkyllithium initiators in nonpolar solvents, because propagation takes place through essentially covalent or intimate ion-pair lithium to carix)n bonds. An intermediate pseudo-six-membered ring is believed to form during the addition of the diene  

O Driscoll, Yonezawa, and Higashimura proposed a mechanism for steric control. In isoprene polymerization the terminal charges are complexed with the metal cations. These cations are close to the active centers through the occupied zr-orbitals of the chain ends and the unoccupied p-orbitals of the lithium ions, hi the transition state the monomers are conqilexed with the cations in die same way. The lithium cations are assumed to be in hybridized tetrahedral sp configurations with four vacant orbitals. The chain ends are presumed to be allylic and the diene monomers are bidentate. During the propagation steps both the monomers and chain ends complex with the same counterions  

In hydrocarbon solvents, the complexes are tight and the rotations of the C2-C3 bonds are steric ally hindered by the methyl groups. This constrains the 1,4-additions to ci -configurations. In polar solvents, however, like tetrahydrofuran, the complexes are loose and thermodynamically favored trans additions take place. 

It was observed, however, that the polymerizations of 2,3-dimettwlbutadiene with organo-lithium initiators in nonpolar solvents result in high trans-1,4 structures. This appears to contradict the above-proposed mechanism. 

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Many refinements were introduced into the various proposed explanations of steric control in anionic polymerizations over the last twenty years. Two important features of these mechanisms are (1) coordinations of the chain ends with the counterions, and (2) counterion solvations. 

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