Polymer Chemistry Blog

Kocaarslan et al. employ a heterogeneous near-infrared catalytic system using up-conversion glass (UCG) and g-CN to synthesize (macro)molecules via click chemistry.

The efficiency and accessibility of “click” chemistry, and more specifically the copper-catalyzed azide-alkyne cycloaddition reaction (CuAAC), have been valorized in synthetic macromolecular chemistry via a plethora of approaches. In this synthetic arena, photochemical process have been shown to efficiently achieve the in situ reduction of air-stable copper(II) species to the active copper(I) species catalyzing CuAAC.

Expanding the scope of current CuAAC photocatalysis, Yagci and coworkers developed a “geared photochemistry” approach for NIR induced CuAAC click chemistry using a dual-heterogeneous photocatalyst that generates light emission in upconversion materials combined with graphitic carbon nitride (g-CN). To achieve this, it is shown that Tm³⁺ and Yb³⁺ ion-doped zinc-tellurite glass that absorbs laser irradiation at 975 nm and is capable of emitting blue light at 475 nm, can photocatalytically activate g-CN via an internal light emitting process. For the CuAAC process, this visible light excitation of g-CN in the presence of CuCl₂/PMDETA was proven to generate active copper(Ι) species able to catalyze a CuAAC click reaction between various azide and alkyne compounds. This system was proven efficient in click reactions between macromolecular chains such as azide functional polystyrene (PS-N₃) and alkyne functional poly(ε-caprolactone) (PCL-alkyne) yielding block copolymers with structurally different segments. In the same vein, photoinduced crosslinking could also be achieved upon irradiation of multifunctional click components (such as bisphenol A di(3-azido-2-hydroxy propan-1-ol) ether and 1-(prop-2-yn-1-yloxy)-2,2-bis((prop-2-yn-1yloxy)methyl) butane) with a 875 nm laser in the presence of mesoporous graphitic carbon nitride (mpg-CN) and CuCl₂/PMDETA under open air conditions within 2 hours. Importantly, the heterogeneous catalyst prepared via the combination of graphitic carbon and UCG could be successfully used several times enhancing the applicability of the system.

The interdependent heterogeneous system using UCG in conjunction with g-CN under NIR light presented in this study, offers a highly efficient click methodology for (macro)molecules in synthetic (polymer) chemistry.

Tips/comments directly from the authors:

• Our group’s research activities focus on the development of new photoinitiating systems for macromolecular synthesis. In this line, many photoinitiators acting at wide wavelength range of the electromagnetic spectrum were developed. “Geared photochemistry”, introduced for the first time in this paper, reflects the light-triggered reaction sequence that interdependently proceeds .  In this approach, up-conversion glass absorbs light at NIR region and convert it to visible light. Upon absorption of the emitted visible light graphitic carbon nitride (mpg-CN) in the reaction media creates electron and hole pairs. The copper (II) complex, which has no absorbance at these two wavelengths, is reduced from copper II to copper I by the released electrons. After all this gear-like system, copper I ions catalyze the click reaction between azide and alkyne compounds to form a triazole ring. We are happy to publish this work in an important journal in polymer science, Polymer Chemistry.
• This approach will open new horizons not only for click chemistry, but also for many synthesis procedures involving electron transfer reactions. It should be considered that the change of absorbance with upconversion glasses is important for many light-activated photocatalysts.

Citation to the paper: Geared photochemistry: an interdependent heterogeneous near-infrared catalytic system using up-conversion glass and g-CN for CuAAC chemistry, Polym. Chem., 2022,13, 6393-6399, DOI: 10.1039/D2PY01075E

Link to the paper:

https://pubs.rsc.org/en/content/articlelanding/2022/py/d2py01075e

Kelly Velonia is saddened to hear about the passing of Prof. Yusuf Yagci, an exceptional scientist and person. Condolences to his family and loved ones. The polymer community will certainly miss him.

Dr. Kelly Velonia is an Advisory Board Member and a Web Writer for Polymer Chemistry. She joined the Department of Materials Science and Technology at the University of Crete in 2007. Research in her group focuses on the synthesis and applications of bioconjugates and biopolymers. You can follow Kelly on twitter @KellyVelonia

Atta et al. demonstrates a simplified ROP protocol which operates in the absence of any inert gas and without the need of drying any of the reaction’s reagents.

Ring Opening Polymerization (ROP) is arguably one of the most popular methodologies to synthesize biodegradable materials such as polycaprolactone (PCL) and poly (lactic acid) (PLA). However, a major drawback of this approach which severely limits its applicability is that it typically operates under completely moisture-free conditions, as water is well-known to deactivate the catalyst and terminate the propagating chains. To avoid water contamination, highly specialized equipment (e.g., Schlenk lines or glove boxes) as well as anhydrous reagents have to be employed which makes the process particularly tedious for both experts and non-experts. To overcome this, Gormley and co-workers have developed two elegant and simple methods that allow for the facile synthesis of PCL through ROP in a laboratory oven and without using any inert gas or dry reagents. In the first technique, a vacuum oven was employed to evaporate water from a traditional ROP reaction with stannous octoate as the catalyst while in the second approach titanium isopropoxide was utilized to simultaneously quench residual water and catalyze ROP. Impressively, and despite the simplicity of those methodologies, a range of chain lengths could be synthesized (degree of polymerization 25-500) with relatively good control over the molecular weight distributions of PCL (Đ < 1.5 for all cases). It is highlighted that a large excess of water impurities (750 ppm) could be tolerated by both methods yielding well-defined polymers at quantitative conversions. This work represents a great example of a simplified ROP which operates in the absence of complicated reactions set ups and can be performed in any laboratory. As the authors also remark, targeting even higher molecular weights or achieving even lower dispersity values will be the next challenge to address and we very much look forward to the next developments by the Gormley group.

Tips/comments directly from the authors:

• The rational goal of this work is to enable the ROP reaction in an oven without inert gas environment and without drying or purifying the reagents.
• The most exciting aspect of this work is to enable non-experts to synthesize custom polymers.
• TTIP plays multiple roles in this ROP reaction. It not only initiates and catalyzes the polymerization reaction but also eliminates water from the reaction medium.
• It is important for the audience that we should perform this experiment with minimal mixing time (within 1-5 sec) as water present in the air can contaminate CL.
• The purity of CL can be easily checked by TTIP. A precipitate of TiO₂ was formed when the water content of CL was above 750 ppm, and a cloudy solution was observed.

Citation to the paper: Ring opening polymerization of ε-caprolactone through water, Polym. Chem., 2021,12, 159-164, DOI: 10.1039/D0PY01481H

Link to the paper: https://pubs.rsc.org/en/content/articlepdf/2021/py/d0py01481h 

Dr. Athina Anastasaki is an Editorial Board Member and a Web Writer for Polymer Chemistry. Since January 2019, she joined the Materials Department of ETH Zurich as an Assistant Professor to establish her independent research group.

Yan et al. develop new enzyme-responsive polymeric micelles with potential applications in cancer therapy.

One of the most exciting and fast-growing topics in polymer chemistry is the synthesis of amphiphilic copolymers that can self-assemble into nanoparticles. Hydrophobic compounds such as cancer drugs can be encapsulated in the core of these self-assembled nanoparticles, thus protecting them from degradation or unwanted interactions with healthy cells. In addition, advances in polymer end-group functionalization allow the conjugation of special ligands on the nanoparticle surface which are responsible for directing the nanoparticles to cancer cells. Upon reaching the tumours (or being taken up by cancer cells), the nanoparticles must release the encapsulated drugs in order to kill the cancer cells. This drug release step requires the use of stimuli-responsive smart polymers that can switch from hydrophobic to hydrophilic upon exposure to stimuli. Temperature, pH, and enzyme-responsive polymers are therefore developed to release drugs on-demand. In this work, Zhao and co-workers further advance the field by synthesizing new fluorescent nanoparticles which can release a cancer drug (doxorubicin) while simultaneously turning off the fluorescent signal when the drug is released. This was achieved by efficiently coupling a tetraphenylethene moiety onto poly(acrylic acid). The hydrophobic property of the tetraphenylethene moiety induces the self-assembly of the resulting diblock copolymers into fluorescent nanoparticles via an aggregation-induced self-assembly mechanism. Upon exposure of the fluorescent nanoparticles to esterase, this enzyme can hydrolyze the ester bond between the tetraphenylethene side chain and the polymer backbone. The enzyme-catalyzed hydrolysis reaction turns the hydrophobic block back to the water-soluble poly(acrylic acid) block and therefore, disassembles the nanoparticles and also turns the fluorescent signal off. The diblock copolymer has poly(ethylene glycol) as the corona-forming block which possesses negligible toxicity to healthy cells. Therefore, this new copolymer is very promising for drug delivery applications, especially when monitoring the drug release is essential.

Citation to the paper: Visible light enabled para-fluoro-thiol ligation, Polym. Chem., 2020, 11, 7704-7713, DOI: 10.1039/D0PY01328E

Link to the paper: https://pubs.rsc.org/en/content/articlepdf/2020/py/d0py01328e

Dr. Athina Anastasaki is an Editorial Board Member and a Web Writer for Polymer Chemistry. Since January 2019, she joined the Materials Department of ETH Zurich as an Assistant Professor to establish her independent research group.

Terashima and co-workers report efficient synthetic systems of single-chain crosslinked polymers.

Crosslinked polymers have emerged as a class of unique materials which find use in a diverse range of applications such as drug delivery, dispersants and coating industries. Typically, those materials are made through a combination of controlled polymerization and crosslinked methods. In this work, Terashima and co-workers prepared a range of single-chain crosslinked polymers with controlled crystallization. This was achieved by the intramolecular transesterification of random copolymers compromising of octadecyl methacrylate, 2-hydroxyethyl methacrylate, and methyl acrylate. Those copolymers were self-folded in organic media (octane was used as the solvent) through the association of the hydroxyl groups to form reverse micelles. Upon synthesis, the micelles were intramolecularly crosslinked by an efficient transesterification of the methyl acrylate units with the hydroxyl groups to produce polymer nanoparticles with pending octadecyl groups. The materials synthesized were thoroughly characterized by a number of techniques including nuclear magnetic resonance, gel permeation chromatography, small angle X-ray scattering and dynamic light scattering. The developed system allowed for the efficient control of the molecular weight of the crosslinked polymers owing to the precise synthesis of the precursors prepared by living radical polymerization. Importantly, the degree of crosslinking was found to control the crystallinity of the products. Last but not least, a relatively high concentration could be used (up to 50 mg ml^-1).  As the authors allude to in their conclusion, their work has paved the way to the production of well-defined polymeric nanoparticles that can be employed for surface coating, painting, optical plastics and cosmetics.

Tips/comments directly from the authors:

1) Intramolecular crosslinking of folded polymers in organic media via transesterification affords the precision and high-throughput synthesis of single-chain crosslinked polymer nanoparticles.

2) The molecular weight of the crosslinked polymers can be controlled as desired at the stage of the synthesis of the precursor polymers by controlled radical polymerization.

3) Transesterification between hydroxyl groups and methyl acrylate units efficiently proceeds within the cores of folded micelles to fix the folded structures in a specific solvent.

4) SEC-MALLS analysis is essential to characterize single-chain crosslinked polymers. Because of the compact structures, the apparent molecular weight of the crosslinked polymers by the general RI detector with PMMA standard calibration turns smaller than that of the non-crosslinked precursor polymers. If the absolute weight-average molecular weight of the crosslinked polymers by the MALLS detector is also close to that of the precursor polymers, you can conclude that the products consist of single chain-crosslinked polymers.

5) Crystallinity of the bulk polymers is controlled by tuning the degree of intramolecular crosslinking. This is an interesting approach to control the thermal and physical properties of solid polymer materials.

Citation to the paper: Single-chain crosslinked polymers via the transesterification of folded polymers: from efficient synthesis to crystallinity control, Polym. Chem., 2020, 11, 5181-5190, doi.org/10.1039/D0PY00758G

Link to the paper: https://pubs.rsc.org/en/content/articlepdf/2020/py/d0py00758g

About the web writer:

Dr. Athina Anastasaki is an Editorial Board Member and a Web Writer for Polymer Chemistry. Since January 2019, she joined the Materials Department of ETH Zurich as an Assistant Professor to establish her independent research group.