Solutions of methyl a-lithioisobutyrate (MIBLD in THF added with various amounts of lithium chloride (LiCl) have been analyzed by 'Li NMR spectroscopy in a wide range of temperatures. Although below ca. 205 K tetramers of MIBLi have been reported to be in equilibrium with dimers, 1/1 and 2/1 LiCl/MIBLi mixed complexes instantaneously form upon the addition of 1 and 2 mol equiv of LiC1, respectively. Further addition of LiCl more likely promotes the formation of a less stable 3/1 complex. Both 1/1 and 211 adducts are stable between -80 "C and at least +12 "C. From 13C NMR spectroscopy, it appears that the addition of an equimolar amount of LiCl to the MIBLi solution in THF is at the origin of a ca. 7.0 ppm upfield shift of the metalated a-carbon compared to the tetrameric MIBLi species and a ca. 1.0 ppm downfield shift with respect to the dimeric MIBLi. However, the charge distribution over the carbanion does not change anymore when the amount of LiCl exceeds 1 mol equiv. In contrast to nonligated MIBLi, the OR group has a considerable rotational freedom in LiC1-modified MIBLi. A comparison of the structure and dynamics of the LiC1-modified MIBLi species in THF at 197 K with the characteristic features of the anionic polymerization of MMA under the same conditions suggests that the formation of one type of LiC1-complexed species has a decisive effect on the control of the living polymerization of MMA. This conclusion is in agreement with a kinetic study carried out by Muller for the same system.
IntroductionThe industrial importance of a large range of poly[alkyl (meth)acrylatel -based products' has raised a steadily increasing interest for the precisely-controlled polymerization of (meth)acrylic ester monomers.Although the anionic polymerization of hydrocarbon monomers such as styrene and dienes is a well-controlled process, one of the polar monomers is still a difficult challenge, because of the occurrence of side termination reactiom2 A research team from Du Pont Co. has, however, demonstrated that the nucleophile-assisted group-transfer polymerization (GTP) reaction of silylketene acetals can be turned into an efficient living polymerization process for alkyl (meth)a~rylates.~ Although that "living" polymerization process is now available to the molecular engineering of alkyl (meth)acrylate based polymers and block copolymers, it is still quite a problem to copolymerize these compounds with comonomers missing a carbonyl conjugated group.In an attempt to modify and hopefully improve that situation, a literature scan has been performed in order to evaluate the possible control of the anionic polymerization of (meth)acrylates. The main conclusion is that most of the positive (if not completely successful) attempts are all related to the use of ligands interacting with the growing ion pairs, which could be divided into two main classes:* inorganic or organometallic p-bonded additives such as alkali and alkaline-earth metal salts, alkyl metal, and metal alkoxides, on the one hand, cation a-bonding organic ligands, ...
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