Polymer and Chain End Structure in Anionic Diene Polymerization
The microstructure of the polybutadiene and polyisoprene produced by anionic polymerization is correlated to the structure of the allylic anion of the active chain end. The charge distribution over this anion is affected greatly by changing counterions, the use of solvating solvents, and the presence of cation chelating additives.
The polymer cis 1,4/trans 1,4 ratio is determined by several factors. Of importance is the cis or trans structure of the ion formed at the moment of reaction, and the rate at which it will isomerize to its equilibrium structure compared to the rate of addition of the next monomer unit. The relative rates of reaction at the two isomeric chain ends can also be important.
Variations in the ratio of 1,4 structures to vinyl structures in the polymers are more difficult to interpret in terms of active chain end structure. Although in general an increase in charge at the Y position in the allyl ion leads to the larger vinyl contents, very wide and apparently irregular variations occur with change in counterion. Some of the largest increases of vinyl structure appear in polymerizations in the presence of chelating diamines, and the complex series of solvates that form are analyzed by means of data from U.V., N.M.R. and kinetic studies.
KeywordsActive Chain Anionic Polymerization Vinyl Polymer Hydrocarbon Solvent Solvate Dimer
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