Lithium amides as initiators of anionic polymerisation of methyl methacrylate

Reaction of Bu nLi with isopropylcyclohexylamine and 2,6-dimethylpiperidine allows isolation of the corresponding lithium amides. Both of these are effective initiators of polymerisation of methyl methacrylate (MMA). A more convenient procedure however is to generate the lithium amides in situ and six secondary amines have been used in this way. Polymerisations in THF at −78°C, both with and without added TMEDA Lewis base, yield polymer quantitatively with a high degree of syndiotacticity. M̄ n values, however, are somewhat larger than expected for clean living systems and likewise molecular weight distributions are rather broad. Similar reactions at 25°C give much lower yields of polymer, somewhat reduced syndiotacticity and similar molecular weight effects. Using diisopropylaminolithium as the initiator in toluene solution at −78°C yields highly isotactic polymer essentially quantitatively. Again the M̄ n and molecular weight distributions are higher than ideal. Adding trace levels of TMEDA to this polymerisation switches the microstructure of the polymer to high levels of syndiotacticity, but the initiator efficiency is reduced at the same time resulting in a very high M̄ n and a broad molecular weight distribution. Addition of varying levels of LiOBu t to the polymerisation in toluene retains the isotactic selectivity of the propagation and improves the initiator efficiency significantly. The results are discussed in the light of current understanding of anionic vinyl polymerisations.