Chemical Communication between Organometallic Single-Chain Polymer Nanoparticles.

Chemical communication between macromolecules was studied by observing the controlled single chain collapse that ensues the exchange of a metal cross-linker between two polymer chains. The rhodium (I) organometallic cross-linker transfer from a low molecular weight collapsed polybutadiene to a larger polymer was followed using size exclusion chromatography. The increased effective molarity in the larger polymer seems to be the driving force for the metal migration. Thus, we demonstrate here a strategy for transferring a molecular signal that induces chain collapse of a polymer chain based on non-covalent interactions, mimicking biological behaviors reminiscent of signal transductions in proteins.

Mechanical Unfolding and Thermal Refolding of Single-Chain Nanoparticles Using Ligand-Metal Bonds.

Covalent macromolecules tend to fragment under mechanical stress through the mechanochemical scission of covalent bonds in the backbone. However, linear polymers that have been intramolecularly collapsed by covalent bonds show greater mechanochemical stability compared to other thermoplastics. Here, rhodium-π bonds are used for intramolecular collapse in order to show that mechanical stress can be removed from the polymer backbone and focused on weaker intramolecular cross-links, leading to polymer unfolding instead of mechanochemical events at the backbone. Moreover, given rhodium-π bonds form spontaneously, by changing the time interval between ultrasound pulses, we demonstrate that entropic spring effects can lead to polymer refolding and reformation of the previously cleaved metal-ligand bonds, effectively repairing the intramolecular noncovalent cross-links. These findings provide the first example of an intramolecular repairing mechanism in synthetic molecules in solution, allowing for restoration of chemical bonds after mechanochemical events.

The effect of intramolecular cross links on the mechanochemical fragmentation of polymers in solution.

Addition of intramolecular cross-links to linear polymers significantly improves their resistance to mechanochemical fragmentation, and hence the physical properties of polymer solutions are maintained under shear. However, while fragmentation is suppressed, mechanochemistry of chemical bonds still occurs. In linear polymers, the rate of mechanochemistry has been shown to increase linearly with the degree of polymerisation. Here, we report a systematic study of the mechanochemical fragmentation of a series of polymers with increasing polymer length, linear and intramolecularly collapsed, in order to understand the correlation between destructing and non-damaging mechanochemical events. By comparing the trends of the fragmentation kinetic rate vs. the degree of polymerisation, the effect of intramolecular collapse on fundamental mechanochemistry parameters such as the limiting molecular weight and stabilisation effect can be further understood.

Intramolecular Cross-Linking: Addressing Mechanochemistry with a Bioinspired Approach.

Many of the attractive properties in polymers are a consequence of their high molecular weight and therefore, scission of chains due to mechanochemistry leads to deterioration in properties and performance. Intramolecular cross-links are systematically added to linear chains, slowing down mechanochemical degradation to the point where the chains become virtually invincible to shear in solution. Our approach mimics the immunoglobulin-like domains of Titin, whose structure directs mechanical force towards the scission of sacrificial intramolecular hydrogen bonds, absorbing mechanical energy while unfolding. The kinetics of the mechanochemical reactions supports this hypothesis, as the polymer properties are maintained while high rates of mechanochemistry are observed. Our results demonstrate that polymers with intramolecular cross-links can be used to make solutions which, even under severe shear, maintain key properties such as viscosity.