Mechanistic Insights into Mechanochemically Triggered Polymer Degradation

Abstract

Mechanochemistry has been extensively investigated as a novel method to modulate the properties of polymers. Among them, various functional groups that are sensitive to external force, which are so called mechanophores, have been synthesized that are able to change color, release stored length or release small molecules. A group of molecules, gem-dihalocyclopropanes (gDHC) have emerged as versatile mechanophores. This dissertation delves into various gDHC, encompassing their role in the network degradation and the discovery of a new mechanophore being able to release hydrogen fluoride upon the external force. Furthermore, we examined the mechanochemical reactivity with different positions of oxygen substitution on gDHC derivatives. Polymers that amplify a transient, external stimulus into changes in their morphology, physical state, or properties continue to be desirable targets for a range of applications. Therefore, in the first Chapter, we report a polymer comprising an acid-sensitive, hydrolytically unstable enol ether backbone onto which is embedded gem-dichlorocyclopropane (gDCC) mechanophores through a single post-synthetic modification. The gDCC mechanophore releases HCl in response to large forces of tension along the polymer backbone, and the acid subsequently catalyzes polymer deconstruction at the enol ether sites. Pulsed sonication of a 61 kDa PDHF with 77% gDCC on the backbone in THF with 100 mM H2O for 10 min triggers the subsequent degradation of the polymer to a final molecular weight of less than 3 kDa after 24 h standing, whereas controls lacking either the gDCC or the enol ether reach final molecular weights of 38 kDa and 27 kDa, respectively. The process of sonication, along with the presence of water and existence of gDCC on the backbone, significantly accelerates the rate of polymer chain deconstruction. Both acid generation and the resulting triggered polymer deconstruction are translated to bulk, cross-linked polymer networks. Networks formed via thiol-ene cross-linking and subjected to unconstrained quasi-static uniaxial compression dissolve on timescales that are at least three times faster than controls where the mechanophore is not covalently coupled to the network. We anticipate that this concept can be extended to other acid-sensitive polymer networks for the stress-responsive deconstruction of gels and solvent-free elastomers. After the investigation into the degradation capability of gDCC-PDHF embedded in the network, we recognize that while the proton in HCl is useful, the release of additional useful molecules or ions after the external force application could broaden the scope of the application and help us to understand the mechanism of acid release better. Therefore, hydrogen fluoride (HF) is a promising candidate. HF is a versatile reagent for material transformation, with applications in self-immolative polymers, remodeled siloxanes, and degradable polymers. The responsive, in situ generation of HF in materials therefore holds promise for new classes of adaptive material systems. In Chapter 2, the mechanochemically coupled generation of HF from alkoxy-gem-difluorocyclopropane (gDFC) mechanophores derived from difluorocarbene addition to enol ethers is reported. Production of HF involves an initial mechanochemically assisted rearrangement of gDFC mechanophore to α-fluoro allyl ether whose regiochemistry involves preferential migration of fluoride to the alkoxy substituted carbon, and ab initio steered molecular dynamics simulations reproduce the observed selectivity and offer insights into the mechanism. When the alkoxy gDFC mechanophore is derived from poly(dihydrofuran), the α -fluoro allyl ether undergoes subsequent hydrolysis to generate one equivalent of HF and cleave the polymer chain. The hydrolysis is accelerated via acid catalysis, leading to self-amplifying HF generation and concomitant polymer degradation. The mechanically generated HF can be used in combination with fluoride indicators to generate optical response and to degrade poly(norbornene) with embedded HF-cleavable silyl ethers (20 mol%). The alkoxy-gDFC mechanophore thus provides a mechanically coupled mechanism of releasing HF for polymer remodeling pathways that complements previous thermally driven mechanisms. After learning that the alkoxy-substituted gDHCs are useful since they are capable of releasing HCl or HF, I further investigated their mechanochemical reactivity from single molecule force spectroscopy (SMFS). Subsequent findings indicate that the presence of alkoxy substitution on -carbon leads to a significant impact of lowering activation force. Additionally, the specific position of alkoxy influences the force sensitivity. To better understand the effect of alkoxy-substitution of gDHCs, we use single molecule force spectroscopy (SMFS) to study reactivities of two different alkoxy substituents on both gDCC and gDFC. The results reveal that gDHCs with alkoxy-substitution on the main chain exhibit greater force sensitivity compared to those with methoxy substituents as side chains. When methoxy group is substituted on the -carbon of cyclopropane, MeO-gDCC has over 400 pN decrease, and approximately same amount of force decrease for MeO-gDFC. The AkO-gDHCs have lower force than MeO-gDHCs, and about 200 pN lower for AkO-gDCC, and 100 pN lower for AkO-gDFC. Furthermore, the specific location of the oxygen atom also has a distinct influence on trans gDCC which has over 400 pN larger activation force than trans AkO gDCC. However, trans gDFC has approximately same activation force as trans AkO-gDFC. In conclusion, the study has developed a new system of gDHC derivatives based on PDHF, with the demonstration of being able to degrade the network and releasing HCl, and releasing self-amplified HF. The research further reveals that different oxygen substitution positions of gDHC derivatives also influence their mechanochemical reactivities. Through the study, we have acquired a more profound comprehension of gDHCs derivatives, ring-opening mechanisms and HCl and HF release mechanisms.