Non‐ergodic effect of LiCl on the anionic polymerization of styrene in ether‐solvents. Specific solvation of Li+ ions in THF solution and prevention of this effect by LiCl

Abstract

AbstractIn this work conductance measurements were performed on polystyryllithium PStLi in tetrahydrofuran (THF) in the concentration range of 10−3 mol dm−3 at various temperatures between −60°C and 20°C. The comparison with the other alkali salts shows that in these solutions Li+ gives specific interactions with partial electronic charge transfer from the solvent molecules, presumably of the formula LiS4+. A quantitative treatment shows that at 25°C the extrapolated stabilization factor KS is larger than 50000 but rapidly drops for the heavier alkali ions: 3000 for Na+, 200 for K+ and negligible for Cs+. Surprisingly, such a stabilization is not observed for LiCl, although the ionic radii of the anions are quite comparable. The conductances κ at given concentration C of the electrolyte are 100 times smaller. Furthermore the curves of κ2 versus C exhibit in this case an important curvature whereas they are practically linear for PStLi. The absence of specific solvation for LiCl seems thus to be accompanied by the formation of triple ions. Due to the symmetry of the electrolyte the formation of both triple anions ClLiCl− and cations LiClLi+ has to be considered. Moreover, the concentrations of these ions is then always much smaller than that of the neutral dimer LiClLiCl, even if the extent of dimerization of LiCl remains small. The triple ions therefore appear as related to the dissociation of the dimer. This means that through the intermediate formation of the neutral dimer the couples triple ion ‐ counterfoil perpetually exchange an LiCl entity in the course of time: Li+ + ClLiCl−⇋ LiClLiCl ⇋ LiClLi+ + Cl−. Only in the dimer the central LiCl is in possession of the (negative) energy of the insertion bond. In the solution this bond can be attributed neither to ClLiCl− nor to LiClLi+. These entities have to be considered as transient ones during the life‐time of which the energy of the insertion bond is transferred to the medium or vice‐versa and which possess the energy of the insertion bond only during half of their life‐time. The energy of such entities is thus not unambiguously defined in the ensemble at a given time and the ergodic principle does not hold. Such transient species cannot be specifically solvated by the solvent molecules because this would prevent the necessary passage through the dimer form. It is therefore the dimerization of LiCl which opposes itself to the formation of LiS4+ in the THF solutions. Quantitatively the problem can be treated by a thermodynamics based not on ensemble fractions but on time fractions. One considers that a given LiCl can only give solvated ions during a fraction ξC° of the time and that during the remaining fraction it participates in a dissociation process which passes through the formation of non‐ergodic triple ions and neutral dimers. (1‐ξC°)/ξC° is equal to KaC3/2/[(1 + KS)Kdo]½ where Ka is the non‐ergodic equilibrium constant governing the formation of dimers and higher aggregates and Kdo the dissociation constant of LiCl in non‐solvated ions. This non‐ergodic treatment also allows to describe quantitatively the strange conductometric behaviour of ternary solutions of LiCl and PStLi in THF. The addition of amounts of LiCl in a mole ratio of 7/1 to a given solution of PStLi increases unexpectedly in a very spectacular way the conductivity and provokes the appearance of a non‐linear term in the κ2 versus concentration function. In fact this behaviour is due to the replacement of a LiCl entity in the dimers by a PStLi molecule, yielding mixed dimers LiClLiStP. The displaced LiCl molecules are again susceptible of ergodic dissociation and specific solvation of the Li+ ions which originate from this dissociation. Thus for the LiCl entities the time fraction ξC increases. Moreover, at higher concentrations the dissociation of the mixed dimers leads to an important formation of non‐ergodic triple anions: Li+ + ClLiStP− ⇋ LiClLiStP ⇋ LiClLi+ + StP− where the entity LiCl constantly jumps in the course of time from an StP− to an Li+ and vice‐versa.