DETERMINATION OF E/Z AND MESO RACEMIC END-GROUP STEREOCHEMISTRY IN THE ANIONIC-POLYMERIZATION OF METHYL-METHACRYLATE IN TETRAHYDROFURAN

Methyl methacrylate (MMA) was polymerized anionically in THF at -70-degrees-C in the presence enolates were converted to silylketene acetals through addition of CISi(CH3)3. The C-13 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+ to 95% for Cs+ on a sigmoidal curve, which has its inflection for K+. 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+ and Na+ (leading to Z chain ends) due to coordination with the counterion and s-trans for Cs+ and cryptated K+ (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.