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Determination of E/Z and meso/racemic end-group stereochemistry in the anionic polymerization of methyl methacrylate in tetrahydrofuran
Ist Teil von
Macromolecules, 1991-03, Vol.24 (2), p.353-359
Ort / Verlag
Washington, DC: American Chemical Society
Erscheinungsjahr
1991
Link zum Volltext
Quelle
Alma/SFX Local Collection
Beschreibungen/Notizen
Methyl methacrylate (MMA) was polymerized anionically in THF at --70 deg C in the presence of Li exp + , Na exp + , K exp + , Cs exp + , and cryptated K exp + cations. The living enolates were converted to silylketene acetals through addition of ClSi(CH sub 3 ) sub 3 . The exp 13 C NMR signals of the end groups show a split with respect to E/Z stereochemistry. A further split into triads of meso/racemic (m/r) stereochemistry is observed. The peaks are assigned and the corresponding fractions of stereoisomers calculated. The fraction of E end groups increases from 0.3% for Li exp + to 95% for Cs exp + on a sigmoidal curve, which has its inflection for K exp + . For the same cation the meso fraction of the main chain and the deviation from Bernoullian statistics show a maximum too. This coincidence is consistent with the following assumptions: (1) transition-state monomer approach to the living end is s-cis for Li exp + and Na exp + (leading to Z chain ends) due to coordination with the counterion and s-trans for Cs exp + and cryptated K exp + (leading to E chain ends) due to steric reasons; (2) tacticity is determined by the E/Z stereoisomerism of the chain end and possibly also by the stereochemistry of the preceding diad; (3) with increasing radius of the counterion, addition of monomer to E isomers results in an increasing fraction of racemic diads, while the Z isomers give a decreasing amount of racemic diads. Spectra, Graphs. 24 ref.--AA