Polymer Chemistry Blog

Paper of the month: Direct laser writing of poly(phenylene vinylene) on poly(barrelene)

29 Sep 2020

Bielawski and co-workers report the ROMP of barrelene monomer affording precisely defined fluorescent patterns with micrometer-sized dimensions.

Conjugated polymers have attracted considerable attention owing to their abilities to form films and exhibit high electrical conductivities and as such they have found use in a range of electronic and optical applications. Amongst the various types of polymers, poly(phenylene vinylene) (PPV) is an excellent candidate due to its low optical band gap, large nonlinear optical response, and emissive properties. However, this material is typically intractable and thus challenging to process. To overcome this, Bielawski and co-workers designed a new approach to PPV was through the ring-opening metathesis polymerization (ROMP) of “barrelene” (bicyclo[2.2.2]octa-2,5,7-triene). The monomer was characterized for the first time by X-ray diffraction analysis of a coordination complex. Barrelene was subsequently homopolymerized and copolymerized with norbornene. The solubility of barrelene homopolymers was found to depend on the cis to trans ratio of alkene in its backbone. Both the homo and copolymers were transformed to PPV by undergoing spontaneous dehydrogenation under air. The materials were analyzed by a range of spectroscopic techniques. Importantly, direct laser writing of the barrelene-containing copolymers was also demonstrated resulting in thermal aromatization within a few seconds affording precisely defined fluorescent patterns with micrometer-sized dimensions. An intrinsic advantage of this development is that the monomer can be potentially incorporated into different macromolecular scaffolds and at varying compositions. Owing to this unique characteristic, the authors envision that their designed strategy would enable the synthesis of a broad range of materials for use in laser machining and contemporary lithography applications.

Tips/comments directly from the authors:

1) The solubility of poly(barrelene) is dependent on the cis-to-trans ratio of the exocyclic olefins in the polymer backbone. Polymers with relatively high cis olefin contents appear to be more soluble than their trans isomers.

2) The resolution of the patterns created by direct laser writing appear to be inversely proportional to the barrelene content of the copolymer used and may be enhanced further by increasing the transparency of the films.

3) Poly(barrelene) oxidizes in air (slow) or upon laser irradiation (fast). A convenient way to monitor the oxidation reaction is through fluorescence spectroscopy. The starting material is non-emissive whereas the poly(phenylene vinylene) product emits a fluorescent green color upon excitation.

4) Because barrelene is strained, copolymerization with other monomers used in ring-opening metathesis polymerization methodologies can be expected which, in turn, may expand the utility of the direct laser writing technique.

Citation to the paper: Direct laser writing of poly(phenylene vinylene) on poly(barrelene), Polym. Chem., 2020, 11, 5437-5443, DOI: 10.1039/d0py00869a

Link to the paper:

https://pubs.rsc.org/en/content/articlepdf/2020/py/d0py00869a

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.

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Paper of the month: Single-chain crosslinked polymers via the transesterification of folded polymers: from efficient synthesis to crystallinity control

01 Sep 2020

By Athina Anastasaki, Web Writer.

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

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Polymer Chemistry Author of the Month: Urara Hasegawa

28 Aug 2020

By Simon Harrisson.

Urara Hasegawa received her B.S. and M.Eng. in Applied Chemistry from Waseda University (Japan). She earned her Ph.D. in Biomedical Science from Tokyo Medical and Dental University (Japan) under the supervision of Professor Kazunari Akiyoshi. Then, she worked as a postdoctoral fellow in the lab of Professor Jeffrey Hubbell at École Polytechnique Fédérale de Lausanne (EPFL) (Switzerland). She joined the Department of Applied Chemistry at Osaka University (Japan) as an assistant professor in 2011, and then moved to the Department of Chemical Engineering at Kansas State University in 2017. In 2020, she joined the Department of Materials Science and Engineering at Pennsylvania State University. Her research focuses on development of polymeric nanomaterials for controlled delivery of drugs and bioactive signaling molecules. She has published more than 45 peer-review papers and has received several awards including a NSF CAREER award in 2020.

What was your inspiration in becoming a polymer chemist?

When I was an undergraduate student, I had the opportunity to learn about cell sheet engineering developed by Professor Teruo Okano at Tokyo Women’s Medical University. They used surfaces coated with poly(N-isopropyl acrylamide) (PNIPAM) as a temperature-responsive tissue culture plate, which enables to harvest cultured cells by lowering temperature below the lower critical solution temperature of PNIPAM. This technology solved the problems associated with conventional techniques requiring a proteolytic enzyme to detach cells. I was excited to see how synthetic polymers can be used to manipulate living cells. Since then, I have developed a strong interest in polymer chemistry that can contribute to the advance of biomedical technologies.

What was the motivation behind your most recent Polymer Chemistry article?

I’m particularly interested in gaseous signaling molecules (gasotransmitters) such as nitric oxide (NO), carbon monoxide (CO) and hydrogen sulfide (H₂S), which are generated ubiquitously in our body and serve as essential signaling regulators in many physiological and pathological processes. One of the challenges in gasotransmitter research is the lack of delivery technologies, which enable the delivery of a known amount of gasotransmitter for a specific period of time to target cells and tissues. So far, significant efforts have been made to develop small gas donor compounds that decompose to generate gasotransmitters under physiological conditions. However, the use of these compounds is limited due to the fast and uncontrolled gas release rate and toxic side effects of the donor compounds and/or their decomposition byproducts.

In the past years, my research group has been focused on developing polymeric micelles for delivery of NO, CO and H₂S to cells. We have been successful in showing the advantages of using polymeric micelles for gasotransmitter delivery by slowing down release rate, reducing toxic side effects and improving therapeutic efficacy. In the recent Polymer Chemistry article, we demonstrated that H₂S release rate from polymeric micelles can be modulated by controlling micellar stability, which significantly affects proangiogenic activity of these H₂S-releasing micelles. The developed technology will be used as a delivery tool to enhance the fundamental understanding of H₂S biology, which will lead to development of innovative approaches for the prevention and treatment of a variety of diseases.

Which polymer scientist are you most inspired by?

Although there are so many remarkable researchers in the field of polymeric biomaterials science, Professor Helmut Ringsdorf at University of Mainz and Professor Jindřich Kopeček at University of Utah are the researchers who I’m inspired by. They developed the concept of “polymer-drug conjugate” in the mid 1970’s to improve solubility and blood circulation time of drugs as well as confer targeting capability. Another researcher who influenced my work is Professor Kazunori Kataoka at University of Tokyo. He is the pioneer of polymeric micelles for drug delivery.

How do you spend your spare time?
I enjoy spending time with my cats. I like to see them sleeping in the most cozy and comfortable spots in the house and hear them purring when I’m patting them. I also love visiting different places and enjoy local foods and cultures. I recently found a new hobby: Origami (Japanese style paper folding). This is a good way to relax and refresh my mind.

What profession would you choose if you weren’t a scientist?

I still would like to choose a job related to science. A graphic designer for scientific illustration could be a profession I would be interested in. I feel graphics are a powerful tool to explain the essence and concepts of research and increase impact of new technologies and scientific findings. I love to draw and would enjoy contributing to science even if I were not a scientist.

Read Urara’s full article now for FREE until 8 October

Hydrogen sulfide-releasing micelles for promoting angiogenesis

Jerry J. Y. Chen, André J. van der Vlies and Urara Hasegawa

Hydrogen sulfide (H₂S), an important gaseous signalling molecule in the human body, has been shown to be involved in many physiological processes such as angiogenesis. Since the biological activities of H₂S are known to be significantly affected by the dose and exposure duration, the development of H₂S delivery systems that enable control of H₂S release is critical for exploring its therapeutic potential. Here, we prepared polymeric micelles with different H₂S release profiles, which were prepared from amphiphilic block copolymers consisting of a hydrophilic poly(N-acryloyl morpholine) segment and a hydrophobic segment containing H₂S-releasing anethole dithiolethione (ADT) groups. The thermodynamic stability of the micelles was modulated by altering the ADT content of the polymers. The micelles with higher thermodynamic stability showed significantly slower H₂S release. Furthermore, the sustained H₂S release from the micelles enhanced migration and tube formation in human umbilical vein cells (HUVECs) and induced vascularlization in the in ovo chick chorioallantoic membrane (CAM) assay.

About the Webwriter

Simon Harrisson is a Chargé de Recherche at the Centre National de la Recherche Scientifique (CNRS), based at the Laboratoire de la Chimie des Polymères Organiques (LCPO) in Bordeaux, France. His research seeks to apply a fundamental understanding of polymerization kinetics and mechanisms to the development of new materials. He is an Advisory Board member for Polymer Chemistry. Follow him on Twitter @polyharrisson

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Paper of the month: Sequential and alternating RAFT single unit monomer insertion: model trimers as the guide for discrete oligomer synthesis

14 Aug 2020

By Athina Anastasaki, Web Writer.

Xu and co-workers utilize model trimers as a guide for discrete oligomer synthesis.

Biomacromolecules such as DNA and proteins exhibit perfect sequence precision which allows them to fulfill a number of biological functions. A long-lasting challenge in polymer chemistry is to mimic these biopolymers through synthetic analogues. In particular, single unit monomer insertion technique has emerged as a powerful tool to synthesize sequence-defined polymers with perfect uniformity. Key to this approach is the alternating addition of electron-donor and acceptor monomers which can be utilized to prepare long polymer chains through sequential monomer radical additions occurring one unit at a time. Despite notable progress in the last decades, such alternative and sequential monomer additions often produce complex radical reaction kinetics which makes the formation of diverse polymer sequences challenging. To this end, simplifying reaction processes and establishing simple reaction kinetics is essential to bring a rapid and reliable synthesis. In this work, Xu and co-workers describe a methodology to address this challenge by employing model trimers as a guide for the synthesis of sequence-defined polymers. Central to the design is the sequential and alternating PET-RAFT SUMI technology which enables the acquisition of full kinetic data, thus providing a very useful insight over both reaction rates and yields. Four different families of α,β-disubstituted vinyl monomers (N-phenylmaleimide (PMI), fumaronitrile (FCN) and dimethyl fumarate (DMF) and indene (Ind)) were employed to prepare nine model trimers. These model compounds were subsequently utilized to guide the synthesis of longer discrete polymers (pentamers with diverse monomer sequences) through multiple insertions yielding materials with high isolated yields. The authors’ findings were supported by nuclear magnetic resonance and mass-spectrometry which were used to establish reaction rate and product purity respectively. The authors anticipate that their method can also be applied to other vinyl polymers and different RAFT initiation systems. Such monodispersed materials with perfect sequence control are expected to find use in a range of applications.

Tips/comments directly from the authors:

1) The use of automated flash chromatography can effectively simplify the SUMI product purification and allows for a more efficient and reproducible synthesis.

2) The online-NMR spectroscopy is an effective technique to monitor RAFT agent and monomer conversion in RAFT SUMI.

3) There are several diastereoisomers for each of SUMI products that would show different polarities in column chromatography and complicated NMR spectra. Careful implementation and thoughtful data analysis are required.

4) The characterization of long chain oligomers (more than four monomer units) is quite challenging. Only mass spectrometry is available for the structure confirmation. Therefore, the model trimers are very important to guide the synthesis of long chain oligomers. All triad sequences in long chain oligomers can be found in the model trimers.

5). The established model trimers and kinetics data could also provide experimental and theoretical guidance for the synthesis of alternating polymers and investigation of mechanism and kinetics of radical copolymerization.

Citation to the paper: Sequential and alternating RAFT single unit monomer insertion: model trimers as the guide for discrete oligomer synthesis, Polym. Chem., 2020, 11, 4557-4567, DOI: 10.1039/d0py00390e

Link to the paper:

https://pubs.rsc.org/en/content/articlepdf/2020/py/d0py00390e

Read more papers from our Pioneering Investigators 2021 collection here!

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Polymer Chemistry Author of the Month: Trang N. T. Phan

17 Jul 2020

By Simon Harrisson.

Trang N. T. Phan is an Associate Professor in the Institute of Radical Chemistry at Aix-Marseille University in France. She completed her undergraduate studies with a Masters in chemistry and then joined the PhD program in Polymers and Organic Chemistry at University of Lille 1. She received her PhD in 2000 under the supervision of Pr. M. Morcellet. During her PhD at the Macromolecular Chemistry Lab, she worked on the synthesis, characterization and water purification application of sorbents based on silica gel functionalized with β-cyclodextrin based polymers. During 2000-2002, she worked as a postdoctoral researcher at the Materials and Interfaces Chemistry Lab of University of Franche-Comté, first in the European project SILACOR and then in a project with BASF on the adsorption of polyelectrolytes on zinc oxide nanoparticles. In 2002, she joined Pr Denis Bertin’s group, firstly as a temporary lecturer and then as a postdoctoral researcher. In September 2004, she became a full associate professor at Aix-Marseille University in the group of Didier Gigmes. In 2014, she received with her colleagues (R. Bouchet and D. Gigmes) the International Prize EDF PULSE Science and Electricity. Her current research interests are in the design and the synthesis of advanced functional polymers via controlled polymerization techniques for specific applications such as solid polymer electrolytes, polymers blend compatibilizers and structure-directing agents.

What was your inspiration in becoming a polymer chemist?

During my Bachelors course work and internship project, I started learning about polymer chemistry. I was fascinated by the “magic” of the transformation of small molecules (monomers) to polymers which are useful functional materials. From that moment, I decided to learn more about how chemistry can help to design and synthesize (co)polymers with precise structure, functionality and composition responding to desired properties and specific applications.

What was the motivation behind your most recent Polymer Chemistry article?

Radical ring-opening polymerization (rROP) of cyclic monomers is an attractive method to synthesize functional polymers bearing both heteroatoms in the backbone and functional groups in the side chain. In addition, the potential applicability of rROPs for copolymerizations with vinyl monomers is a highly attractive feature. Among different cyclic monomers, vinyl cyclopropane (VCP) derivatives were the most promising compounds since their rROPs led to low-shrinkage materials with attractive applications for dental adhesives and composites. However, functional groups in the side chain of VCP polymers are usually limited to halogens, esters and nitrile functions that restrict the development of new functional VCP polymers by post-polymerization modification. During our investigation of new VCP derivatives, we have achieved a straightforward pathway for the synthesis of VCP bearing azlactone functionality. The azlactone group can react via rapid and efficient ring-opening reactions with different nucleophilic species such as primary amines, hydroxyls, and thiols. We expect that the new VCP-azlactone (co)polymers could serve widely as a reactive platform for the introduction of chemical and biological functionality.

Which polymer scientist are you most inspired by?

I am greatly interested by the work being undertaken in the group of Pr. David Mecerreyes since their research allies polymer chemistry, supramolecular chemistry and nanomaterials science to synthesize innovative polymeric materials.

How do you spend your spare time?
I like cooking and experimenting with new recipes by combining western and oriental flavors. After all, cooking is similar to chemistry. I also like hiking and reading.

What profession would you choose if you weren’t a scientist?

I’d be a Scuba dive master practicing somewhere in the warm waters of tropical seas.

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Radical ring-opening polymerization of novel azlactone-functionalized vinyl cyclopropanes

Hien The Ho, Véronique Montembault, Marion Rollet, Soioulata Aboudou, Kamel Mabrouk, Sagrario Pascual, Laurent Fontaine, Didier Gigmes and Trang N. T. Phan

Azlactone-functionalized polymers are considered powerful materials for bioconjugation and many other applications. However, the limited number of azlactone monomers available and their multistage syntheses pose major challenges for the preparation of new reactive polymers from these monomers. In this article, we report the synthesis of a new class of azlactone monomers based on vinylcyclopropane (VCP). Furthermore, the (co)polymerization of the azlactone-functionalized VCPs has been successfully demonstrated to provide new azlactone polymers by using free-radical polymerization. The ability of the resulting amine-reactive polymers to be engaged in post-polymerization modifications was demonstrated using dansylcadaverine. These new azlactone-functionalized VCP monomers and polymers are potential candidates for the synthesis of innovative (bio)materials.

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Paper of the month: Poly(ethylene glycol)-b-poly(vinyl acetate) block copolymer particles with various morphologies via RAFT/MADIX aqueous emulsion PISA

02 Jul 2020

By Athina Anastasaki, Web Writer.

D’Agosto, Lansalot and co-workers report the synthesis of nanoparticles with various shapes through the RAFT/MADIX PISA polymerization of vinyl acetate.

Polymerization-induced self-assembly (PISA) is a widely used technique that allows access to the formation of a range of polymeric nanoparticles including spheres, worms and vesicles. Although this methodology has been very successful with dispersion polymerizations, emulsion polymerization systems are mostly limited to the preparation of spherical particles. Poly(vinyl acetate) latexes are obtained by emulsion polymerization and find use in many industrial applications but yet, the preparation of higher ordered morphologies remains challenging. D’Agosto, Lansalot and co-workers were able to circumvent this by conducting the emulsion polymerization of vinyl acetate at higher temperatures anticipating that this would not only lead to much faster reaction kinetics but also to the softening of the polymeric nanoparticles allowing for increased flexibility and rearrangements. Indeed, the aqueous macromolecular design via interchange of xanthate (MADIX)-mediated emulsion polymerization of vinyl acetate from a poly(ethylene glycol) with a xanthate chain-end macro-CTA led to well-controlled polymerizations with high blocking efficiency accompanied with the formation of stable latexes. By judiciously adjusting the targeted degree of polymerization, the authors triggered for the first time the morphological transformation from spherical to higher ordered morphologies and observed the formation of vesicles (with different sizes) as well as worm-like nanoparticles. In particular, the worm-like morphology could alternatively be observed by increasing the solid content from 10 to 15 wt%. The data was supported by very nice cryo-TEM images which depicted all the discussed morphologies. The range of obtained shapes were attributed to the high water solubility of vinyl acetate combined with the low Tg of PVAc. The presented elegant findings enhance our fundamental understanding on emulsion PISA systems where polymerization temperature and solid content significantly affect the resulting morphology.

Tips/comments directly from the authors:

The polymerization takes place above the T_g of the forming PVAc block, which seems to be key for accessing non spherical morphologies in VAc PISA.
PEG-b-PVAc block copolymers are obtained in very short times.
This system provides an interesting medium for investigating the impact of several parameters on the morphologies obtained through PISA processes.
Extension of this strategy to other non-activated monomers, for instance in the copolymerization of vinyl acetate and ethylene, seems accessible.

Citation to the paper: Poly(ethylene glycol)-b-poly(vinyl acetate) block copolymer particles with various morphologies via RAFT/MADIX aqueous emulsion PISA, Polym. Chem., 2020, 11, 3922-3930, DOI: 10.1039/d0py00467g

Link to the paper:

https://pubs.rsc.org/en/content/articlepdf/2020/py/d0py00467g

Read more papers on PISA in our Polymerisation-Induced Self Assembly themed collection here!

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Polymer Chemistry Author of the Month: Nicholas J. Warren

12 Jun 2020

By Simon Harrisson.

Nick Warren is a University Academic Fellow within the School of Chemical and Process Engineering at the University of Leeds. He was awarded an Masters in Chemistry from the University of Bristol in 2005 following which he conducted two years industrial research. He then moved to the University of Sheffield where he studied for a PhD in Polymer Chemistry within Prof Steve Armes’ research group where he focussed on synthesis of biocompatible block copolymers. Following his PhD, he continued as a postdoctoral researcher in Sheffield working in the area of polymerisation-induced self-assembly (PISA) until 2016, when he moved to Leeds to start his independent research career. His current research aims to exploit the latest advances in polymerisation techniques, combined with new reactor technologies for the design and discovery of controlled-structure polymers.

What was your inspiration in becoming a polymer chemist?

During my undergraduate masters project, I worked on development on combining pH responsive microgels with photo-responsive surfactants. I was fascinated by the ability to use chemical composition as a means of tuning physical characteristics of a material and imparting responsive behaviour. This brought on a specific interest in synthetic polymer chemistry, where there are so many synthetic routes to generating responsive materials. This was the focus of my PhD, where I gained expertise in ATRP and RAFT polymerisation which provided a convenient tool-box allowing me to design and synthesise pH responsive block copolymers.

What was the motivation behind your most recent Polymer Chemistry article?

Continuous-flow techniques are well utilised in small molecule synthesis and are now becoming commonplace in polymer chemistry. In my research group, we aim to use flow-chemistry to conduct polymer synthesis and try and exploit its characteristics to develop new materials, streamline methods for optimising polymerisation processes; or for detailed online monitoring. We have already published some work conducting PISA in flow, which combined my existing expertise in PISA, with my growing interest in reactor technologies, but it became apparent that the relatively long timescales for the reactions meant that there were limited advantages over batch synthesis. We therefore looked to speed up the process, which was relatively straight-forward since our all-acrylamide PISA system was ideally suited to Seb Perrier’s ‘ultrafast’ RAFT technology. By using flow-reactors equipped with online monitoring, we were not only able to synthesise a wide range of PISA nanoparticles on short timescales, but also obtain kinetic data despite the short reaction time.

Which polymer scientist are you most inspired by?

From a synthetic perspective, the work being undertaken in Prof Brent Sumerlin’s group encompasses many of the areas I have a keen interest. I am also inspired by Prof Tanja Junkers’ research, since she is at the forefront of work into applying automation and flow chemistry to polymer synthesis.

How do you spend your spare time?

I now have two children under 3, so I spend most of my time running around after them! We spend quite a lot of time hiking in the Peak District and I also like to cook, which has recently expanded into bread making (to varying degrees of success).

What profession would you choose if you weren’t a scientist?

I’d be a barista with a small coffee shop somewhere sunny.

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And if you are interested in reading more about PISA then check out our recent themed collection here

Rapid production of block copolymer nano-objects via continuous-flow ultrafast RAFT dispersion polymerisation

Sam Parkinson, Stephen T. Knox, Richard A. Bourne and Nicholas J. Warren

Ultrafast RAFT polymerisation is exploited under dispersion polymerisation conditions for the synthesis of poly(dimethylacrylamide)-b-poly(diacetoneacrylamide) (PDMAm_x–b-PDAAm_y) diblock copolymer nanoparticles. This process is conducted within continuous-flow reactors, which are well suited to fast reactions and can easily dissipate exotherms making the process potentially scalable. Transient kinetic profiles obtained in-line via low-field flow nuclear magnetic resonance spectroscopy (flow-NMR) confirmed the rapid rate of polymerisation whilst still maintaining pseudo first order kinetics. Gel permeation chromatography (GPC) reported molar mass dispersities, Đ < 1.3 for a series of PDMAm_x–b-PDAAm_y diblock copolymers (x = 46, or 113; y = 50, 75, 100, 150 and 200) confirming control over molecular weight was maintained. Particle characterisation by dynamic light scattering (DLS) and transmission electron microscopy (TEM) indicated successful preparation of spheres and a majority worm phase at 90 °C but the formation of vesicular morphologies was only possible at 70 °C. To maintain the rapid rate of reaction at this lower temperature, initiator concentration was increased which was also required to overcome the gradual ingress of oxygen into the PFA tubing which was quenching the reaction at low radical concentrations. Ill-defined morphologies observed at PDAAm DPs close to the worm-vesicle boundary, combined with a peak in molar mass dispersity suggested poor mixing prevented an efficient morphological transition for these samples. However, by targeting higher PDAAm DPs, the additional monomer present during the transition plasticises the chains to facilitate formation of vesicles at PDAAm DPs of ≥300.

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Paper of the month: A general method to greatly enhance ultrasound-responsiveness for common polymeric assemblies

04 Jun 2020

By Athina Anastasaki, Web Writer.

Dou and co-workers report a new way to improve ultrasound responsiveness in polymeric self-assemblies.

Polymer assemblies or nanoparticles hold great potential to improve diagnosis and treatment of diseases by encapsulating chemotherapeutic or imaging agents with masked toxicity and triggerring release at target sites. To release encapsulated agents, polymer assemblies are often composed of specific stimuli-responsive polymers that can change their properties upon response to external stimuli such as pH, temperature, light, redox, magnetic, and ultrasound. However, this approach limits the components of polymer nanoparticles to stimuli-responsive polymers. In this work, Chen and co-workers elegantly crosslink a common non-responsive diblock copolymer using an ultrasound-responsive crosslinker, followed by the preparation of polymer assemblies that can dissociate under gentle ultrasound treatment. In particular, the photodimerization of coumarin groups under UV irradiation (365 nm) triggered the crosslinking, and a subsequent ultrasound treatment (5 min treatment by the ultrasound of 20-25 kHz at 32.5 W) dissociated the resultant polymer nanoparticles. Interestingly, this strategy could be successfully applied to not only spherical micelles but also worms and vesicles. The use of ultrasound-responsive crosslinker reported in this work paves the way for synthesizing ultrasound-responsive polymer nanoparticles from any block copolymer (not limited to a few ultrasound-responsive copolymers), thus representing a major step forward in the synthesis of smart polymer nanoparticles for biological science and technology.

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Citation to the paper: A general method to greatly enhance ultrasound-responsiveness for common polymeric assemblies, Polym. Chem., 2020, 11, 3296-3304, DOI: 10.1039/d0py00254b

You can read the paper here.

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Paper of the month: A polymerization-induced self-assembly process for all-styrenic nano-objects using the living anionic polymerization mechanism

30 Apr 2020

By Athina Anastasaki, Web Writer.

Wang and co-workers report an anionic polymerization combined with polymerization-induced self-assembly.

Polymerization-induced self-assembly (PISA) is arguably one of the most versatile and robust self-assembly methodologies and has been extensively evolved over the last decade to produce nanomaterials of various shapes. However, the vast majority of reported PISA methods employ a controlled radical polymerization strategy such as reversible addition–fragmentation chain transfer (RAFT) polymerization while low activated monomers such as styrenics are not frequently utilized. In this work, Wang and co-workers elegantly combine living anionic polymerization (LAP) with PISA to afford the facile and quantitative synthesis of spherical and worm-like nanoparticles. In particular, poly(p-tert-butylstyrene)-b-polystyrene was used as a model diblock copolymer and the polymerization was performed in heptane, a good solvent for the first block and a poorer solvent for the polystyrene segment. This formulation allowed the first monomer to polymerize in a homogenous system while the formation of the second block was performed under heterogeneous conditions. Importantly, all diblock copolymers synthesized exhibited narrow molecular weight distributions thus demonstrating excellent control over the polymerization. By adjusting the solid content and the molecular weight of each block, the authors were able to attain spheres, vesicles and worms at relatively high purity. To increase reproducibility, the authors also constructed a detailed phase diagram, where the exact location of each morphology was shown. Overall, it was demonstrated that LAP can be successfully combined with PISA therefore expanding PISA formulations beyond controlled radical polymerization.

Tips/comments directly from the authors:

All-styrenic monomers with relatively low activity were firstly introduced into the PISA system and can be completely converted in the LAP PISA system with a rapid polymerization rate.
The typical self-assembled morphologies, such as the spherical, worm-like and vesicular micelles, can also be captured in the LAP PISA system.
Due to the excellent control on the molecular weight and structure of polymers in the LAP process, the nano-objects formed in the LAP PISA process were featuring with uniform sizes and morphologies.
The molecular weights of each block and solids content have important influence on the LAP PISA process.
The LAP PISA process can be performed in a large scale, and the potential industrial application is hoped to be explored for some novel nanomaterials in the future.

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Citation to the paper: A polymerization-induced self-assembly process for all-styrenic nano-objects using the living anionic polymerization mechanism, Polym. Chem., 2020, 11, 2635-2639, DOI: 10.1039/d0py00296h

Link to the paper:

https://pubs.rsc.org/en/content/articlepdf/2020/py/d0py00296h

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Paper of the month: Organocatalyzed atom transfer radical polymerization (ATRP) using triarylsulfonium hexafluorophosphate salt (THS) as a photocatalyst

01 Apr 2020

By Athina Anastasaki, Web Writer.

Lei and co-workers report an inexpensive organocatalyzed atom transfer radical polymerization.

Organocatalyzed atom transfer radical polymerization (ATRP), also referred to as metal-free ATRP, has emerged over the last few years as an alternative to copper mediated ATRP in order to address the issue of metal contamination on the final polymers. In their current contribution, Lei and co-workers introduce triarylsulfonium hexafluorophosphate salt (THS) as an organic and inexpensive photocatalyst for ATRP of methacrylic monomers. The authors demonstrate exceptional temporal control with the polymerization completely stopping during the dark periods. Importantly, by adding sodium hydroxide, a significant acceleration over the polymerization rate was observed reaching relatively high conversions and narrow molecular weight distributions (Đ = 1.26–1.32). Block-copolymers were also possible, thus demonstrating high end-group fidelity. Last but not least, polymer brushes could also be prepared in an efficient manner on silicon wafer by utilizing surface-initiated ATRP in the presence of THS as a photocatalyst. Overall, the presented strategy is particularly attractive owing to the use of inexpensive compounds, the absence of metals and the mild temperatures employed. As the authors remark in the conclusions, such metal-free polymers may find interesting applications in the pharmaceutical, biomedical and food industries.

Tips/comments directly from the authors:

This organocatalyzed-ATRP system is easy to operate. It does not need to undergo freeze-pump-thaw cycles.
Temperature is an important factor for this organocatalyzed-ATRP system. Polymerization rate will be higher in summer and lower in winter unless you use an oil bath to have the temperature fixed.
Due to the poor solubility of THS in water, aqueous media cannot be used as a solvent for this organocatalyzed-ATRP.
When polymers with high molecular weights were synthesized by this system, the molecular weights were often lower than the theoretic values.
In order to more effectively neutralize the free H⁺ generated by the rearrangement of triarylsulfonium hexafluorophosphate salt (THS), the use of powdered sodium hydroxide (NaOH) is a good choice.

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Citation to the paper: Organocatalyzed atom transfer radical polymerization (ATRP) using triarylsulfonium hexafluorophosphate salt (THS) as a photocatalyst, Polym. Chem., 2020, 11, 2222-2229, DOI: 10.1039/c9py01742a

Link to the paper:

https://pubs.rsc.org/en/content/articlepdf/2020/py/c9py01742a

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