Chain Propagation in Ethers

Perhaps the most noteworthy feature of many anionic polymerizations conducted in ethereal solvents is that the propagation reaction is a dual process, i.e., ion pairs and free ions coexist and exhibit distinctly different reactivies 240 24% The free ion concentration is generally limited to 1 % or less of the total active center concentration while the relative reactivities diminish in the sequence free anion S loose (solvent-separated) ion pair tight (contact) ion pair. 

The propagation reaction in these systems is characterized by an expression of the type 240 245  

The poly(styryl)Iithium active center was found 179) to partially dissociate into the free ion in benzene-THF solution when the mole fraction of THF in the solvent mixtures was ca. 50%. Solvents or solvent mixtures of lesser polarity generally do not lead to the formation of significant concentrations of the highly reactive free ions, i.e., the ion pair reaction appears to dominate. 

Morton and co-workers126) were apparently the first to study the polymerization of butadiene and isoprene in THF. Their work was subsequently followed by that of Arest-Yakubovich and Medvedev 245). These combined results indicated that for the 

However, the later work of By water and Worsfold 228) clearly showed that the free-ions do contribute, albeit to a relatively small extent, to the propagation process for isoprene in THF. At 30 °C, the rate constants for the ion pair and free ion were found 228), respectively, to be 0.20 M-1 s-1 and 2.8 x 103 M 1 s 1 while the dissociation constant was 5.0 x 1CT10 M. In diethyl ether at 20 °C, the ion-pair seems, though, to be the sole active center 246). At —20 °C, the ion pair propagation constant in THF is decreased 193) to ca. 7 x 10-3 M-1 s-1. 

However, the later work of By water and Worsfold clearly showed that the free- 

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