Ke Zhang

Gels Based on Cyclic Polymers

Cyclic poly(5-hydroxy-1-cyclooctene) (PACOE) was synthesized by ring-expansion metathesis polymerization (REMP), and thiol-ene chemistry was used to cross-link the internal double bonds in the PACOE backbone. This created a novel network material (gels formed from cyclic polymers) with unique structural units, where the cyclic PACOE main chains, which serve as secondary topological cross-linkages, were connected by primary intermolecular chemical cross-linkages. The resulting properties were notably different from those of traditional chemically cross-linked linear PACOE gels, whose gel fraction (GF) and modulus (G) increased while the swelling ratio (Q) decreased with increasing initial polymer concentration in the gel precursor solution (C(0)). For the gels formed from cyclic polymers, however, the GF, Q, and G all simultaneously increased as C(0) increased at the higher range. Furthermore, at the same preparation state (same C(0)), the swelling ability and the maximum strain at break of the gels formed from cyclic polymers were always greater than those of the gels formed from linear polymers, and these differences became more pronounced as C(0) increased.

Universal cyclic polymer templates.

Two unique molecular templates for generating polymeric materials with a cyclic molecular architecture were developed by combining ring-expansion metathesis polymerization and click chemistry. These two universal cyclic polymers were used in three examples to demonstrate the wide range of potential materials enabled. They include functional cyclic polymers, cyclic polymer brushes, and cyclic gels.

Metallo-Supramolecular Cyclic Polymers

Cyclic brush polymers represent an exciting new macromolecular topology. For the first time, this new topology has been combined with metallo-supramolecular interactions to construct novel cyclic brush polymers. Here, ring-expansion metathesis polymerization was used to synthesize a universal cyclic template with a polynorbornene backbone, which was further modified with the metal-chelating synthon terpyridine. The terpyridine side chains served as the key supramolecular unit for the creation of cyclic polymer brushes and gels. This metallo-supramolecular functionality allowed direct visualization of the cyclic brush polymers by transmission electron microscopy for the first time. This demonstration should open a new area in which supramolecular interactions are used to build an array of novel cyclic brush copolymers as well as other cyclic-polymer-based architectures generating new materials.

Controlling the Compositional Chemistry in Single Nanoparticles for Functional Hollow Carbon Nanospheres

Hollow carbon nanostructures have inspired numerous interests in areas such as energy conversion/storage, biomedicine, catalysis, and adsorption. Unfortunately, their synthesis mainly relies on template-based routes, which include tedious operating procedures and showed inadequate capability to build complex architectures. Here, by looking into the inner structure of single polymeric nanospheres, we identified the complicated compositional chemistry underneath their uniform shape, and confirmed that nanoparticles themselves stand for an effective and versatile synthetic platform for functional hollow carbon architectures. Using the formation of 3-aminophenol/formaldehyde resin as an example, we were able to tune its growth kinetics by controlling the molecular/environmental variables, forming resin nanospheres with designated styles of inner constitutional inhomogeneity. We confirmed that this intraparticle difference could be well exploited to create a large variety of hollow carbon architectures with desirable structural characters for their applications; for example, high-capacity anode for potassium-ion battery has been demonstrated with the multishelled hollow carbon nanospheres.

Mesostructured Spheres of Organic/Inorganic Hybrid from Gelable Block Copolymers and Arched Nano-objects Thereof

Mesostructured microspheres formed by aerosol-assisted self-assembly of a gelable block copolymer, poly(3-(triethoxysilyl)propyl methacrylate)- block-polystyrene (PTEPM -b-PS), were studied by a combination of small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). When the copolymer composition was changed, the spheres with different internal patterns, such as onion-like lamella and curved cylinder, were obtained. Through a self-gelation process of PTEPM domains, novel organic/inorganic hybrid spheres with an internal tunable patterned structure were prepared. Since only PTEMP domains were cross-linked, the hybrid spheres could be further disintegrated by dispersion in a good solvent of PS. As a result, novel organic/inorganic hybrid nano-objects such as arched plates and cylinders were prepared.

Cyclic Polymer with Alternating Monomer Sequence

Cyclic polymers with alternating monomer sequence are synthesized for the first time based on the ring-closure strategy. Well-defined telechelic alternating polymers are synthesized by reversible addition-fragmentation chain transfer polymerization by copolymerizing the electron acceptor monomer of N-benzylmaleimide and donor monomer of styrene with a feed ratio of 1 between them. The corresponding cyclic alternating polymers are then produced by the UV-induced Diels-Alder click reaction to ring-close the linear alternating polymer precursors under highly diluted reaction solution.

Synthesis, Thermal Properties, and Thermoresponsive Behaviors of Cyclic Poly(2-(dimethylamino)ethyl Methacrylate)s

This study aims at physicochemical properties of thermo- and pH/CO2 -responsive cyclic homopolymers. Three examples of cyclic poly(2-(dimethylamino)ethyl methacrylate)s (PDMAs) are synthesized by combining the reversible addition-fragmentation chain transfer process and the Diels-Alder ring-closure reaction. After cyclization, the glass transition temperature significantly increases (ΔTg = 51.8-59.7 °C) due to the different configurational entropy and end groups, and the maximum decomposition temperature to lose the pendent groups is drastically decreased from 309 to 278 °C. Effects of polymerization degree, polymer concentration, additive of NaCl, and pH/CO2 on lower critical solution temperature behaviors of PDMA aqueous solutions are investigated. The cloud points (Tc ) of ring PDMAs are usually higher than their linear precursors, and the ΔTc values obtained under a fixed condition can reach up to 20.7 °C, revealing the crucial role of the topology effect. This study paves the way for unique properties and applications of smart cyclic polymers and their derivatives.

Cyclic polymers based on UV-induced strain promoted azide-alkyne cycloaddition reaction

A unique method was developed for the preparation of cyclic polymers based on the combination of atom transfer radical polymerization (ATRP) and UV-induced strain promoted azide–alkyne cycloaddition (SPAAC) reaction. By virtue of a cyclopropenone-masked dibenzocyclooctyne functionalized ATRP initiator (I-1), well-defined telechelic polystyrene (PS) was synthesized to have a cyclopropenone-masked dibenzocyclooctyne at one polymer chain end and a bromo group at the other. The single electron transfer-nitroxide radical coupling reaction was then used to modify the bromo end group to azide, resulting in the corresponding linear PS precursor. Under UV irradiation on its highly diluted solution, the dibenzocyclooctyne end group was quantitatively released from cyclopropenone-masked dibenzocyclooctyne, which intramolecularly reacted with the azide end group in situ to ring-close the linear PS precursor and produce the corresponding cyclic PS based on the SPAAC click reaction.

Aggregation-induced emission block copolymers based on ring-opening metathesis polymerization

Aggregation-induced emission (AIE) amphiphilic block copolymers were developed based on the living ring-opening metathesis polymerization for the first time. By virtue of the block copolymer self-assembly in selective solvents, water soluble fluorescent nano-objects were then prepared with various structures including spherical micelles, cylindrical micelles, and vesicles. This method represents a facile and efficient way to prepare the well-defined AIE polymers and the varied fluorescent nano-objects with controlled structures and functionalities.

Simple, clean preparation method for cross-linked α-cyclodextrin nanoparticles via inclusion complexation.

A simple, clean method was presented in this letter to prepare cross-linked α-cyclodextrin (α-CD) nanoparticles with a low dispersion. The nanoparticles were synthesized in water by cross-linking the inclusion complex of α-CDs and poly(ethylene glycol) (PEG). The structure of the nanoparticles was characterized by (1)H NMR, nuclear overhauser enhancement spectroscopy (NOESY), and wide-angle X-ray diffraction (XRD). Spherical morphology was observed by scanning electron microscopy (SEM) for these nanoparticles. Their average hydrodynamic radius was determined to be 67 nm by dynamic light scattering (DLS). Small guest molecules could be included in the cross-linked α-CD nanoparticles, and anticancer drug cisplatin was used to evaluate the drug release behavior.