Multistate Mechanism for Homogeneous Ionic Polymerization. I. The Diastereosequence Distribution

Abstract

A mechanism is proposed to explain the ``stereoblock'' structures which occasionally result from homogeneous anionic polymerizations [cf. J. Am. Chem. Soc. 80, 1768 (1958)]. The mechanism postulates that a growing polymer chain has two reactive states in dynamic equilibrium, both capable of adding monomer, but each with its own stereospecificity. The resulting diastereosequence distribution is calculated and it turns out to be non-Markoffian. It is shown that our ``two-state'' polymerization mechanism yields predictions about the dependence of NMR spectra on polymerization conditions, and as such is capable of experimental test. The case of more than two states is also discussed.