Synthesis and Ring-Opening Metathesis Polymerization of a Strained <i>trans</i>-Silacycloheptene and Single-Molecule Mechanics of Its Polymer
Herbert Wakefield, Ilia Kevlishvili, Kelsie E. Wentz, Yunxin Yao, Tatiana B. Kouznetsova, Sophia J. Melvin, Em G. Ambrosius, Abraham Herzog‐Arbeitman, Maxime A. Siegler, Jeremiah A. Johnson, Stephen L. Craig, Heather J. Kulik, Rebekka S. Klausen
Abstract
The cis - and trans -isomers of a silacycloheptene were selectively synthesized by the alkylation of a silyl dianion, a novel approach to strained cycloalkenes. The trans -silacycloheptene ( trans -SiCH) was significantly more strained than the cis isomer, as predicted by quantum chemical calculations and confirmed by crystallographic signatures of a twisted alkene. Each isomer exhibited distinct reactivity toward ring-opening metathesis polymerization (ROMP), where only trans -SiCH afforded high-molar-mass polymer under enthalpy-driven ROMP. Hypothesizing that the introduction of silicon might result in increased molecular compliance at large extensions, we compared poly( trans -SiCH) to organic polymers by single-molecule force spectroscopy (SMFS). Force-extension curves from SMFS showed that poly( trans -SiCH) is more easily overstretched than two carbon-based analogues, polycyclooctene and polybutadiene, with stretching constants that agree well with the results of computational simulations.