Metal-free anionic polymerization of methyl methacrylate.
Me methacrylate was polymd. in THF, using tetrabutylammonium di-Et 2-ethylmalonate (I) and tetrabutylammonium 2-nitropropanate (II) as initiators. Immediate polymn. was obsd. only in cases where the initiators did not contain traces of their precursor mols., the presence of which caused an appreciable inhibition period. This was attributed to the transfer reaction between active species and the precursor mols. Only after their consumption could the major polymn. start. Using very pure initiators, no inhibition period was obsd. Curves from dilatometric measurements showed the existence of an induction period. As usual, this was attributed to an initiation reaction which was slower than the propagation step. It is more clearly marked for I than for II. All polymns. usually went to complete monomer consumption. However, termination reactions have to be taken into account as well. The well-known intramol. cyclization reaction was not obsd. for the monomer. Here, a Hofmann elimination reaction terminates the growing of chains, which is due to the Bu ligands of the ammonium cation, where a b-hydrogen atom can be transferred to the growing chain under the formation of Bu3N and 1-butene. To eliminate this termination step, tetramethylammonium di-Et 2-ethylmalonate was synthesized and used as the initiator. Now transalkylation under the formation of a Me group terminated the satd. chain end and Me2N could be detected as a second termination reaction. Neither of the termination reactions seems to be very fast. The mol. wt. distributions of the synthesized polymers are relatively broad.
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