Polydiene stereochemistry

The homogeneous alkyllithium-initiated polymerization of styrene in hydrocarbon media produces polystyrene with an almost random (i.e., ataaic) microstructure. The racemic diad fraction (PJ was 0.53 for the butyllithium/toluene system. The effects of cotmterion, solvent, and temperature on polystyrene stereochemistry are shown in Table 12. The principal conclusion from these results is that the stereoregularity of polystyrenes prepared by aniortic polymerization is predominantly syndio-tactic (Pr = 0.56-0.74) and that the stereoregularity is surprisingly independent of the nature of the cation, solvent, and temperature, in contrast to the sensitivity of polydiene stereochemistry to these variables. A report on the effect of added alkali metal alkoxides showed that polystyrene stereochemistry can be varied from 64% syndiotactic triads with lithium t-butoxide to 58% isotactic triads with potassium t-butoxide. °... 

In alkyllithium initiated, solution polymerization of dienes, some polymerization conditions affect the configurations more than others. In general, the stereochemistry of polybutadiene and polyisoprene respond to the same variables Thus, solvent has a profound influence on the stereochemistry of polydienes when initiated with alkyllithium. Polymerization of isoprene in nonpolar solvents results largely in cis-unsaturation (70-90 percent) whereas in the case of butadiene, the polymer exhibits about equal amounts of cis- and trans-unsaturation. Aromatic solvents such as toluene tend to increase the 1,2 or 3,4 linkages. Polymers prepared in the presence of active polar compounds such as ethers, tertiary amines or sulfides show increased 1,2 (or 3,4 in the case of isoprene) and trans unsaturation.4. 1P U It appears that the solvent influences the ionic character of the propagating ion pair which in turn determines the stereochemistry. 

Hydrocarbon Solvents One of the most important synthetic and commercial aspects of anionic polymerization is the ability to prepare polydienes [poly(l,3-dienes)] with high 1,4-microstructure using lithium as the counterion in hydrocarbon solutions [3, 156]. The key discovery was reported in 1956 by scientists at the Firestone Tire and Rubber Company that polyisoprene produced by lithium metal-initiated anionic polymerization had a high (>90%) cm-1,4-microstructure similar to natural rubber [47], In general, conjugated 1,3-dienes [CH2=C(R)-CH=CH2] can polymerize to form four constimtional isomeric microstructures as shown below. The stereochemistry of the anionic polymerization of isoprene and... 

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