|126. Yakimansky, A.V.; Müller, A.H.E.; Van Beylen, M.V.: Density Functional Theory Study on the Aggregation and Dissociation Behavior of Lithium Chloride in THF and its Interaction with the Active Centers of the Anionic Polymerization of Methyl Methacrylate and Styrene, Macromolecules 33, 5686 (2000)|
The structure of LiCl in tetrahydrofuran (THF) solution and its effect on the structure and stability of active sites of the anionic polymerization of methyl methacrylate (MMA) and styrene (St) was studied using the quantum-chemical density functional theory (DFT) approach. In the case of MMA anionic polymerization, it was found that LiCl forms stable mixed aggregates with ester enolates which model the PMMA living chain ends, thus preventing them from self-aggregation. They may even stabilize more reactive zwitterionic structures of these chain ends. The dissociation of solvated LiCl dimers to form Li+(THF)4 cations is slightly endothermic in THF, while scavenging of Li+(THF)4 by LiCl dimers to produce more stable quintuple cations [(THF)3Li-Cl-Li(THF)2-Cl-Li(THF)3]+ is even exothermic. Therefore, if the concentration of LiCl exceeds a certain threshold value, Li+(THF)4 cations should effectively be scavenged by LiCl dimers. Thus, increasing LiCl concentration below the threshold concentration should lead to an increase in the concentration of free Li+(THF)4 cations. In the anionic polymerization of styrene in the presence of LiCl this results in the suppression of PSt-Li chain end dissociation due to the common ion effect, slowing down the polymerization. Further addition of LiCl above the threshold concentration should decrease the concentration of free Li+(THF)4 cations, leading to enhanced PSt-Li chain end dissociation, thus increasing the polymerization rate, in agreement with kinetic data reported in the literature.