Thermoresponsive Helical Dendronized Poly(phenylacetylene)s: Remarkable Stabilization of Their Helicity via Photo-Dimerization of the Dendritic Pendants

Abstract

Dynamic helical polymers can change their helicity according to external stimuli due to the low helix-inversion barriers, while helicity stabilization for polymers is important for applications in chiral recognition or chiral separations. Here, we present a convenient methodology to stabilize dynamic helical conformations of polymers through intramolecular cross-linking. Thermoresponsive dendronized poly(phenylacetylene)s (PPAs) carrying 3-fold dendritic oligoethylene glycol pendants containing cinnamate moieties were synthesized. These polymers exhibit typical features of dynamic helical structures in different solvents, that is, racemic contracted conformations in less polar organic solvents and predominantly one-handed stretched helical conformations in highly polar solvents. This dynamic helicity can be enhanced through selective solvation by increasing the polarity of the organic solvents or simply via their thermally mediated dehydration in water. However, through photocycloaddition of the cinnamate moieties between the neighboring pendants via UV irradiation, these dendronized PPAs adopt stable helical conformations either below or above their phase transition temperatures in water, and their helical conformations can even be retained in less polar organic solvents. Spectroscopic and atomic force microscopy measurements demonstrate that photocycloaddition between the cinnamate moieties occurs on the individual molecular level, and this is found to be helpful in restraining the photodegradation of the PPA backbones. Molecular dynamics simulations reveal that the spatial orientation of the pendants along the rigid polyene backbone is crucial for the photodimerization of cinnamates within one helix pitch.

Thermo/Light Dual-Responsive Helical Dendronized Poly(phenylacetylene)s

“…Recently, we reported a homologous series of helical dendronized homopolymers carrying dendritic oligoethylene glycol (OEG) pendants with different branching densities. − These polymers exhibited unprecedented lower critical solution temperature (LCST)-type thermoresponsive behavior, and the cloud point temperatures ( T cp s) were mainly dependent on the overall hydrophilicity and branching density of the dendritic OEGs. Interestingly, due to the dynamic nature of the PPA backbones, these polymers exhibited reversible helicity transitions across their thermal phase transitions which could be stabilized through photodimerization among the neighboring pendants . Here we report the first example of helical dendronized copolymers that are responsive to both temperature and photoirradiation.…”

Thermoresponsive Dendronized Poly(phenylacetylene)s via Dynamic Covalent Chemistry Showing Multiple-Responsive Chirality

“…54−57 These dendronized PPAs showed characteristic thermoresponsive properties with sharp phase transitions and small hysteresis, and their phase transition temperatures are dominated by hydrophobicity of the pendanted OEG dendrons. 58 Integration of DCC with PPAs may not only pave a new way to afford dynamic chirality to optically inactive polymers through dynamic covalent linkages but also provide a convenient method to prepare helical polymers with both constitutional dynamic and chirality dynamic features. Here, we report the synthesis and chirality evaluation of a class of thermoresponsive dendronized copoly(phenylacetylene)s (Chart 1).…”

“…Through dendronization with three-fold dendritic oligoethylene glycols (OEGs), we recently reported a series of helical dendronized PPAs through covalent linkages. − These dendronized PPAs showed characteristic thermoresponsive properties with sharp phase transitions and small hysteresis, and their phase transition temperatures are dominated by hydrophobicity of the pendanted OEG dendrons . Integration of DCC with PPAs may not only pave a new way to afford dynamic chirality to optically inactive polymers through dynamic covalent linkages but also provide a convenient method to prepare helical polymers with both constitutional dynamic and chirality dynamic features.…”

Ligand Dissociation of Metal-Complex Photocatalysts toward pH-Photomanipulation in Dynamic Covalent Hydrogels for Printing Reprocessable and Recyclable Devices

“…Such hydrogel polymers with dynamic properties primarily contain reversible covalent bonds capable of exchanging, dissociating, or switching in response to various stimuli, giving rise to constitutional dynamics. − Thus, designing reversible reactions to create dynamic bonding is crucial for preparing high-performance dynamic covalent polymers. Typically, the bonding interactions generated via dynamic chemistries, − like Diels–Alder and reshuffling reactions, transesterification, and photoresponsive dimerization, offer stable polymer networks at normal states and outstanding reversibility under heating and UV-light irradiation. These bonds enable hydrogels to regulate dynamic behaviors rapidly and efficiently.…”