Polymerization of methylmethacrylate by organometallic compounds—VIII. Initiation by n-, i-, s- or t-butylmagnesium compounds—evidence for an eneidic pseudoanionic mechanism
✍ Scribed by P.E.M. Allen; B.O. Bateup
- Book ID
- 103072586
- Publisher
- Elsevier Science
- Year
- 1978
- Tongue
- English
- Weight
- 812 KB
- Volume
- 14
- Category
- Article
- ISSN
- 0014-3057
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✦ Synopsis
The molar-mass distributions of PMMA initiated by various n-, s-, i-and t-butylmagnesium compounds were investigated. In most cases the distributions were polymodal. In some cases the different peaks were shown to have different chain configurations. The position of the peaks of the distribution was independent of THF concentration in the solvent mixture, but the relative amount of polymer in each peak changed, as did the steric triad composition of the whole sample. It is postulated that the molar-mass distribution and stereospecificity of the polymer are controlled by an eneidic mechanism in which persistent, independent growth-centres of different reactivity and stereospecificity are formed in the initiation step. The character of the polymer produced is primarily determined by conditions prevailing in the initiation stage: structure of the butyl group; presence or absence of halide: solvent composition and temperature.
The concentrations of the various growth sites (down to the pM range, with the more reactive) and their reactivities have been calculated, subject to certain assumptions concerning the mechanism. Some conclusions about the structures of some of them can be drawn, but, in general, no direct evidence is obtainable.
In cases where the mechanism was examined in detail, the propagation reaction was most easily explained in terms of the formation of a monomer-complex at the growth site which proceeds through a four-centre orientation to insert in a covalent Mg--C bond. The mechanism is probably more accurately classified as pseudoanionic rather than anionic.