MODEL STUDIES OF END CAPPING OF MONOENDED AND BIENDED POLYSTYRENE ANIONS - STEREOISOMERISM AT THE CHAIN ENDS

Model studies of end capping of living polystyryl anions, with and without alpha-methylstyrene at the chain ends, with chlorotrimethylsilane and chloromethyl methyl ether were done. In H-1 NMR spectra multiple peaks for Si(CH3)3 and CH2OCH3 group protons were observed. However, Si(CH3)3 peak patterns for polymers with and without alpha-methylstyrene at the chain ends were quite different. Trimethylsilyl end-group protons and methine or methylene protons of the polystrene chain were not coupled, as determined by decoupling experiments. Various parameters such as initiator, solvent, temperature, and molecular weights did not change the peak pattern. This study indicates that the multiple peaks are due to stereoisomerism at the chain ends. Due to steric factors, selective stereoisomer formation is indicated when alpha-methylstyrene is the terminal unit before end capping. Further, the chain-end tacticities correspond to bulk tacticities and statistical analyses of multiple peaks gave P(r) = 0.55, which is consistent with the reported values for anionically prepared polystyrene. Thus, end-group spectroscopy provides a means of determining bulk tacticity.