Synthesis and Ring-Opening Metathesis Polymerization of a Strained trans-Silacycloheptene and Single-Molecule Mechanics of Its Polymer

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.

Systematic Investigation of Silicon Substitution on Single Macromolecule Mechanics

“…The overall reduction in f elastic with the increasing Si content suggests an opportunity to tune the chain extension behavior through subtle variations in the monomer design in a way that can eventually be used to probe the connection between the micromechanical properties of single strands and the properties of polymer networks made from those polymers. Furthermore, P5 – P8 had higher elasticity than our previously reported poly­( trans -SiCH) (1.4 × 10 4 pN/ L 0 ) with a silicon incorporation of 43%, which is in agreement with the calculated trends …”

“…Nonetheless, the data are broadly consistent with the computational predictions. First, f elastic of all of the polymers (1.6–3.4 × 10 4 pN per unit L 0 ) is lower than that of the carbon-based polymer polycyclooctene (3.5 × 10 4 pN per unit L 0 ) . Second, the measured f elastic is greater for the low Si content polymers P5 and P6 (2.9 × 10 4 and 3.1 × 10 4 pN/ L 0 , respectively) than for the higher Si content polymers P7 and P8 (2.1 × 10 4 and 1.6 × 10 4 pN/ L 0 , respectively).…”

“… An exception is P3 ( M w / M n = 4.3), which we hypothesize might be due to the formation of cyclic or oligomeric byproducts that could not be separated during polymer precipitation. Given Si’s tendency to lessen the ring strain relative to a carbocycle of the same number of atoms, ,, the ring/chain equilibria in ADMET of Si-based polymers likely follow trends distinctive from all-carbon backbones; indeed, Wagener has characterized the significant formation of a nine-membered macrocycle ( Z / E = 4:1) via postpolymerization backbiting. , However, attempts to conclusively identify cyclic or oligomeric species by MALDI-TOF mass spectrometry were not conclusive due to the challenges in ionization.…”

“…An alternative approach is the polymerization of metallic segments, e.g., oligosilyl chains capped with organic functional groups, which takes advantage of the functional group diversity of organic compounds while giving rise to macromolecules with alternating organic and metallic segments. Recent examples from Klausen et al include Kumada polycondensation of dithienylcyclosilanes, building on foundational examples from Ohshita, , and ring-opening metathesis polymerization (ROMP) of a strained cycloalkene with embedded oligosilanes (Figure a) …”

Solvent Polarity Effects on the Mechanochemistry of Spiropyran Ring Opening

“…The tension is unevenly distributed at the molecular level, and individual strands may experience forces that are large enough to overstretch and even cleave covalent bonds. These bond stretching − and scissile , physical properties of strands are tied to the chemical composition of the polymer, and they are implicated in the high strain behavior and degradation of polymeric materials. , Insights into the molecular responses that accompany macroscopic mechanical behavior can be obtained by embedding mechanophores that generate a constructive response to high forces of tension. Mechanically triggered signals that report stress or damage are particularly useful.…”