| Abstract: | Anionic oligomerization of methacrylic esters (methyl (MMA), ethyl (EMA), butyl (n-BMA), tert-butyl (t-BMA) methacrylates) was conducted in a one-step process by mixing, in tetrahydrofuran, the monomer, an alkali metal (sodium) and a deactivating agent (sulfur or tert-butyl alcohol). The formation of alcohol due to a nucleophilic attack of the ester group by the methacrylic carbanion was quantified and related to the monomer structure, temperature and nature of the living ends. It was shown that this side reaction is not limited to the propagation step but can occur during the initiation step. From mass spectra and nuclear magnetic resonance (NMR) spectroscopy of the protonated oligomers (tert-butyl alcohol as deactivating agent), it was concluded that this side reaction led to five-membered cyclic β-keto esters during the initiation step of MMA, EMA and n-BMA monomers only. This implies an intramolecular reaction between one carbanion of the dianionic dimer and the penultimate ester group. It was found that this reaction involves primarily dianionic tail-to-tail dimer, even though head-to-tail dimer was also formed. |
