ReviewAmphiphilic gradient copolymers: Synthesis, self-assembly, and applications
Graphical abstract
Introduction
A unique type of copolymers named as gradient copolymer has attracted increasing attention of many investigators, due to its distinctive molecular chain structure. Unlike block copolymers which have an abrupt transition in monomer composition, gradient copolymers vary their monomer composition gradually along molecular chains. Now, gradient copolymers have amazing applications in many fields [1], such as compatibilizers of immiscible polymer blends [2], [3], [4], [5], stabilizers of emulsions or dispersions [6], and thermoplastic elastomers [7]. Amphiphilic gradient copolymer, a noticeable one in gradient copolymers, consists of hydrophilic units and hydrophobic units, so it can self-assemble to form various aggregates in selective solvents. Amphiphilic gradient copolymers have vast potential applications in biomedical and pharmaceutical fields [8], [9], [10].
Here, we review the latest developments in the preparation and self-assembly of amphiphilic gradient copolymers. This paper is divided into four parts. The first part is an introduction of synthetic methods of amphiphilic gradient copolymers. The second part is about how amphiphilic gradient copolymers self-assemble. The third part describes stimuli-responsive properties of gradient copolymer aggregates, such as distinctive thermosensitivity and pH-sensitivity. The last part introduces the applications of amphiphilic gradient copolymers, especially in the biological and medical fields.
Section snippets
Synthesis methods of amphiphilic gradient copolymers
In order to obtain a well-defined gradient copolymer, the vast majority of propagating chains must be initiated simultaneously and keep survival during the polymerization process [11]. Controllable/living polymerization can satisfy these requirements. Moreover, two different monomer feeding approaches can be used to control a suitable gradient sequence, which are ‘spontaneous’ batch feeding method and ‘forced’ semi-batch feeding method [1].
Self-assembly methods and morphologies of amphiphilic gradient copolymers
Like amphiphilic block copolymers, amphiphilic gradient copolymers can self-assemble into various aggregates in solutions under appropriate condition.
Stimuli-responsive property of amphiphilic gradient copolymer aggregates
The morphology of self-assembled polymer aggregates in solution is affected by environmental stimuli, such as temperature, pH and ionic concentration. Unlike block copolymers, gradient copolymers have impurity in the composition distribution of the assembled aggregates. Namely, hydrophilic units exist in the hydrophobic micellar core domain and the hydrophilic micellar shell domain contains certain hydrophobic units. When environmental conditions change, this impurity nature will endow gradient
Applications of amphiphilic gradient copolymers
The application research of amphiphilic gradient copolymers mainly focus on the encapsulation and the controlled release of bioactive molecules. Amphiphilic gradient copolymers can be used to fabricate nanocarrier in biocompatible medium. Stable spherical nanostructures formed by gradient copolymers of 2-methyl-2-oxazoline and 2-phenyl-2-oxazoline (MPOx) could deliver DNA chains [9], and the stable MPOx/DNA complexes had potential applications in the biomedical field. In other works [10], [29],
Conclusion
Amphiphilic gradient copolymers and their self-assembly properties are highlighted in this review. Due to the unique gradient composition profile, the self-assembly behavior of amphiphilic gradient copolymers is different from that of block copolymers and random copolymers. However, how to utilize their aggregates with distinctive stimuli-responsive properties has not got enough attention in application fields. For instance, the advantages of gradient copolymer nanocarriers in drug
Acknowledgements
This work is supported by National Natural Science Foundation of China (Grant No. 50803048 and 50703030) and the Fundamental Research Funds for the Central Universities (WUT: 2016-zy-002).
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