Lithium diisopropylamide as initiator for the anionic polymerization of methacrylates

The anionic polymerization of a series of methacrylates, i.e. methyl methacrylate (MMA), tert-butyl methacrylate (tBuMA), (dimethylamino)ethyl methacrylate (DMAEMA), and glycidyl methacrylate (GMA), has been investigated in THF at -78 degrees C, by using both lithium diisopropylamide (LDA) and alpha-lithioethyl isobutyrate (Li-EIB) complexed with LiCl as initiators. Compared to ligated Li-EIB, which is a model for the propagating enolate in the methacrylate anionic polymerization, ligated LDA has quite a comparable initiation efficiency(>90%) and gives rise to polymers with a predictable molecular weight and a narrow molecular weight distribution (MWD <1.1). The effect of LiCl on the MWD of poly-GMA. is less pronounced compared to the other polymethacrylates. An already narrow MWD in the absence of LiCl might be accounted for by an intramolecular coordination of the ion pair by the oxirane group of the (ante)penultimate unit(s) of growing poly GMA. As a direct consequence of the well-controlled anionic polymerization of these methacrylates, pairs of them have been polymerized in a sequential way with formation of well-defined block copolymers. Actually, there is no restriction on the addition order of the comonomers, so that both types of triblocks (ABA and BAB) can be successfully synthesized. Size exclusion chromatography supports that block copolymers are not contaminated by homopolymers, at least in detectable amounts.