Synthesis of miktoarm star amphiphilic block copolymers via combination of NMRP and ATRP and investigation on self-assembly behaviors

Constrained thermoresponsive polymers - new insights into fundamentals and applications

In the last decades, numerous stimuli-responsive polymers have been developed and investigated regarding their switching properties. In particular, thermoresponsive polymers, which form a miscibility gap with the ambient solvent with a lower or upper critical demixing point depending on the temperature, have been intensively studied in solution. For the application of such polymers in novel sensors, drug delivery systems or as multifunctional coatings, they typically have to be transferred into specific arrangements, such as micelles, polymer films or grafted nanoparticles. However, it turns out that the thermodynamic concept for the phase transition of free polymer chains fails, when thermoresponsive polymers are assembled into such sterically confined architectures. Whereas many published studies focus on synthetic aspects as well as individual applications of thermoresponsive polymers, the underlying structure-property relationships governing the thermoresponse of sterically constrained assemblies, are still poorly understood. Furthermore, the clear majority of publications deals with polymers that exhibit a lower critical solution temperature (LCST) behavior, with PNIPAAM as their main representative. In contrast, for polymer arrangements with an upper critical solution temperature (UCST), there is only limited knowledge about preparation, application and precise physical understanding of the phase transition. This review article provides an overview about the current knowledge of thermoresponsive polymers with limited mobility focusing on UCST behavior and the possibilities for influencing their thermoresponsive switching characteristics. It comprises star polymers, micelles as well as polymer chains grafted to flat substrates and particulate inorganic surfaces. The elaboration of the physicochemical interplay between the architecture of the polymer assembly and the resulting thermoresponsive switching behavior will be in the foreground of this consideration.

Folate-Targeted Anticancer Drug Delivery via a Combination Strategy of a Micelle Complex and Reducible Conjugation

Conjugation of various active targeting ligands to the surface of nanocarriers to realize specific recognition by the corresponding receptors localized on the membrane of the cancer cells has provided a powerful means toward enhanced cancer therapy. Folic acid (FA) is one of the most used targeting ligands due to the overexpressed FA receptors in many cancer cell lines. However, conjugation of hydrophobic FA to the surface of nanocarriers usually alters the hydrophilic/hydrophobic balance of the stabilized nanoparticles, leading to their thermodynamic instability and subsequent formation of aggregates, which apparently compromises the in vivo long circulation and minimized side effects of nanocarriers. The currently leading strategy to overcome this issue is to incorporate a protecting hydrophilic stealth that can be deshielded to expose the targeting ligand at the desired tumor site, which generally involves multistep chemical modifications, conjugations, and purifications. To develop a simple alternative toward FA-mediated enhanced anticancer drug delivery, a combination strategy of micelle complex and reducible conjugation was reported in this study. FA was first conjugated to the terminus of the hydrophilic block of a reduction-sensitive miktoarm star-shaped amphiphilic copolymer, PCL₃-SS-POEGMA₁, with the previously optimized star structure by click coupling via a reducible disulfide link. The resulting PCL₃-SS-POEGMA₁-SS-FA was further mixed with the parent PCL₃-SS-POEGMA₁ to afford a micelle complex with both reducibly conjugated and relatively low amount of FA-targeting ligands toward excellent FA-mediated targeted drug delivery without compromised salt stability in vitro and in vivo. Therefore, the combined strategy developed herein provides a simple and powerful means to promote FA-mediated anticancer drug delivery.

Nano-Star-Shaped Polymers for Drug Delivery Applications

With the advancement of polymer engineering, complex star-shaped polymer architectures can be synthesized with ease, bringing about a host of unique properties and applications. The polymer arms can be functionalized with different chemical groups to fine-tune the response behavior or be endowed with targeting ligands or stimuli responsive moieties to control its physicochemical behavior and self-organization in solution. Rheological properties of these solutions can be modulated, which also facilitates the control of the diffusion of the drug from these star-based nanocarriers. However, these star-shaped polymers designed for drug delivery are still in a very early stage of development. Due to the sheer diversity of macromolecules that can take on the star architectures and the various combinations of functional groups that can be cross-linked together, there remain many structure-property relationships which have yet to be fully established. This review aims to provide an introductory perspective on the basic synthetic methods of star-shaped polymers, the properties which can be controlled by the unique architecture, and also recent advances in drug delivery applications related to these star candidates.

Well-defined star polymers for co-delivery of plasmid DNA and imiquimod to dendritic cells

Co-delivery of antigen-encoding plasmid DNA (pDNA) and immune-modulatory molecules has importance in advancing gene-based immunotherapy and vaccines. Here novel star polymer nanocarriers were synthesized for co-delivery of pDNA and imiquimod (IMQ), a poorly soluble small-molecule adjuvant, to dendritic cells. Computational modeling and experimental results revealed that the polymers formed either multimolecular or unimolecular core-shell-type micelles in water, depending on the nature of the outer hydrophilic shell. Micelles loaded with both IMQ and pDNA were able to release IMQ in response to intracellular pH of the endo-lysosome and transfect mouse dendritic cells (DC2.4 line) in vitro. Importantly, IMQ-loaded micelle/pDNA complexes displayed much enhanced transfection efficiency than IMQ-free complexes. These results demonstrate the feasibility of co-delivery of pDNA and IMQ to antigen-presenting cells by multifunctional polymer nanocarriers with potential use in gene-based vaccine approaches.

Opportunities for dual RDRP agents in synthesizing novel polymeric materials

Dual RDRP agents provide access to new polymeric materials by combining ATRP, NMP, and RAFT polymerization without end group transformations.

pH-Responsive polymers

This review summarizes pH-responsive monomers, polymers and their derivative nano- and micro-structures including micelles, cross-linked micelles, microgels and hydrogels.

Star Polymers

Recent advances in controlled/living polymerization techniques and highly efficient coupling chemistries have enabled the facile synthesis of complex polymer architectures with controlled dimensions and functionality. As an example, star polymers consist of many linear polymers fused at a central point with a large number of chain end functionalities. Owing to this exclusive structure, star polymers exhibit some remarkable characteristics and properties unattainable by simple linear polymers. Hence, they constitute a unique class of technologically important nanomaterials that have been utilized or are currently under audition for many applications in life sciences and nanotechnologies. This article first provides a comprehensive summary of synthetic strategies towards star polymers, then reviews the latest developments in the synthesis and characterization methods of star macromolecules, and lastly outlines emerging applications and current commercial use of star-shaped polymers. The aim of this work is to promote star polymer research, generate new avenues of scientific investigation, and provide contemporary perspectives on chemical innovation that may expedite the commercialization of new star nanomaterials. We envision in the not-too-distant future star polymers will play an increasingly important role in materials science and nanotechnology in both academic and industrial settings.

PNIPAM-based heteroarm star-graft quarterpolymers: synthesis, characterization and pH-dependent thermoresponsiveness in aqueous media

We report the design of PS_n(P2VP-b-PAA-g-PNIPAM)_n heteroarm star-graft quarterpolymers, the thermoresponsiveness of which is strongly dependent on pH ionic strength, and their macromolecular features, e.g. arm number and grafting density.

pH-responsive micelles based on (PCL)2(PDEA-b-PPEGMA)2 miktoarm polymer: controlled synthesis, characterization, and application as anticancer drug carrier

Amphiphilic A2(BC)2 miktoarm star polymers [poly(ϵ-caprolactone)]2-[poly(2-(diethylamino)ethyl methacrylate)-b- poly(poly(ethylene glycol) methyl ether methacrylate)]2 [(PCL)2(PDEA-b-PPEGMA)2] were developed by a combination of ring opening polymerization (ROP) and continuous activators regenerated by electron transfer atom transfer radical polymerization (ARGET ATRP). The critical micelle concentration (CMC) values were extremely low (0.0024 to 0.0043 mg/mL), depending on the architecture of the polymers. The self-assembled empty and doxorubicin (DOX)-loaded micelles were spherical in morphologies, and the average sizes were about 63 and 110 nm. The release of DOX at pH 5.0 was much faster than that at pH 6.5 and pH 7.4. Moreover, DOX-loaded micelles could effectively inhibit the growth of cancer cells HepG2 with IC50 of 2.0 μg/mL. Intracellular uptake demonstrated that DOX was delivered into the cells effectively after the cells were incubated with DOX-loaded micelles. Therefore, the pH-sensitive (PCL)2(PDEA-b-PPEGMA)2 micelles could be a prospective candidate as anticancer drug carrier for hydrophobic drugs with sustained release behavior.

Synthesis of polyethylene and polystyrene miktoarm star copolymers using an “in–out” strategy

Miktoarm star copolymers having multiple polyethylene and polystyrene arms joined at the crosslinked polydivinylbenzene core were synthesized using an “in–out” strategy with the combined Pd-catalyzed ethylene “living” polymerization and atom transfer radical polymerization.