Kinetics and mechanism of the polymerization of cyclic acetals, 5. Spectroscopic studies of the polymer end-groups in poly(1,3-dioxolane)

Polymerization of 1,3-dioxolane ( 1 ) was initiated in CH₃NO₂ and CH₂Cl₂ solvents with benzoylium hexafluoroantimonate (C₆H₅CO⁺SbF), terminated with sodium ethanolate (C₂H₅ONa), and the endgroups were determined by using UV spectrophotometry and ¹H NMR methods. Polymers with high polymerization degrees DP _n were obtained (M?_n up to 3·10⁵); DP _n calculated for living polymerization conditions (DP _n=([ 1 ]₀?[ 1 ]_e)/[C₆H₅CO⁺SbF]₀; i.e. one molecule of initiator gives one macromolecule) agree well with DP _n measured by osmometry and DP _n found from ¹H NMR and UV methods assuming one respective end- group per macromolecule. ¹H NMR studies were performed on polymers from perdeuterated 1 (1,3-dioxolane-d₆). These polymers, after purification, were shown to bear benzoyloxy and ethoxy end-groups: documentclass{article}pagestyle{empty}begin{document}${rm C}_6 {rm H}_5 hbox{---} {rm COrlap{--} (O} hbox{---} {rm CD}_2 {rm CD}_2 {rm OCD}_2 {rm rlap{--} )}_n {rm OCH}_2 {rm CH}_3$end{document} Similar results were obtained for polymers initiated with triethyloxonium hexafluoroantimonate ((C₂H₅)₃O⁺SbF), and terminated with triphenylphosphine ((C₆H₅)₃P). These results indicate that initiation with stable oxocarbenium or oxonium ions leads to predominantly linear macromolecules, while initiation with perchloric acid (according to the data reported by Plesch) leads to polymers of low molecular weight and claimed to be mostly cyclic after killing with C₂H₅ONa.