Polymer Chemistry Blog

Audureau et al. report the synthesis of UCST-thermoresponsive diblock copolymers using reversible addition-fragmentation chain transfer (RAFT) polymerization in aqueous media.

Thermoresponsive polymers have attracted widespread interest in both fundamental research and industrial applications due to their special ability to change properties upon cooling or heating. Polymers exhibiting upper critical solution temperature (UCST) are soluble in a solvent above the UCST but precipitate from the same solvent when cooling below its critical temperature. In the large UCST polymer family, the statistical copolymer poly(acrylamide-co-acrylonitrile) (P(AAm-co-AN)) has gained increasing interest and has been used to prepare nanoparticles for drug delivery, cancer theranostics/chemotherapy and photoacoustic imaging. However, a method for scalable synthesis of thermoresponsive P(AAm-co-AN) block copolymer in water and in-situ self-assembly of the resulting copolymers into nanoparticles remains elusive. In this work, Rieger, Stoffelbach and co-workers employed polymerization-induced self-assembly technique (PISA) to synthesize, for the first time in water, well-defined P(AAm-co-AN) block copolymers which self-assembled into nanoparticles. Importantly, the rare worm-like morphology was successfully obtained, which paves the way for developing better cancer drug delivery systems since nanoworms have distinct and advantageous properties when compared to their spherical counterparts such as long circulation time, high accumulation in tumour and deep tumour penetration. Furthermore, an interesting worm-to-sphere morphological transition was observed upon heating the nanoworms solution. This is in contrast to previous reports where a worm-to-sphere transition was only demonstrated upon cooling and therefore, offers a new promising strategy to design novel smart nanoparticles for various applications.

Tips/comments directly from the authors:

1)  The thermoresponsive properties of the copolymers crucially depend on the molar fraction of acrylonitrile (F_AN) in the P(AAm-co-AN) block, tunable by the initial AN fraction (f_AN) in the monomer feed. As AN is volatile, a closed Schlenk system should be used to avoid monomer evaporation during polymerization and produce polymers with predictable properties.

2) P(AAm-co-AN) statistical copolymers exhibited a typical UCST-type thermal transition for acrylonitrile molar fractions (F_AN) ranging from 0.3 to 0.5.

3) In addition to F_AN, the presence of a hydrophilic PDMAc block and the DP_n of the polymer blocks also impact the thermoresponsiveness.

Citation to the paper: Thermoresponsive properties of poly(acrylamide-co-acrylonitrile)-based diblock copolymers synthesized (by PISA) in water), Polym. Chem., 2020, 11, 5998-6008, DOI: 10.1039/D0PY00895H. Link to the paper here.

More papers on PISA can be found at our themed collection here!

About the web writer:

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

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!

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.

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:

1. 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.
2. PEG-b-PVAc block copolymers are obtained in very short times.
3. This system provides an interesting medium for investigating the impact of several parameters on the morphologies obtained through PISA processes.
4. 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!

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.

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.

Read this article for FREE until 15th July!

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.

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.

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:

1. 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.
2. The typical self-assembled morphologies, such as the spherical, worm-like and vesicular micelles, can also be captured in the LAP PISA system.
3. 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.
4. The molecular weights of each block and solids content have important influence on the LAP PISA process.
5. 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.

Read this article for FREE until 11th June!

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

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.

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:

1. This organocatalyzed-ATRP system is easy to operate. It does not need to undergo freeze-pump-thaw cycles.
2. 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.
3. Due to the poor solubility of THS in water, aqueous media cannot be used as a solvent for this organocatalyzed-ATRP.
4. When polymers with high molecular weights were synthesized by this system, the molecular weights were often lower than the theoretic values.
5. 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.

Read this article for FREE until 12th May!

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

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.

Zhang and co-workers report aqueous polymerization-induced self-assembly by atom transfer radical polymerization to generate protein-based nanoassemblies.

Polymerization-induced self-assembly (PISA) has opened the way for the in-situ formation of a wide range of nanoparticles with applications ranging from the material to the biomedical field. However, the vast majority of reports focus on utilizing reversible addition-fragmentation chain transfer polymerization as the main methodology while atom transfer radical polymerization (ATRP) is very rarely combined with PISA, mostly due to the limitations of ATRP in water. Zhang and co-workers utilized Cu(0) reversible deactivation radical polymerization by exploiting the disproportionation of CuBr/ligand in aqueous media generating both Cu(0) particles and Cu(II) deactivator. Upon modifying Candida Antarctica lipase B (CALB), it was used as a macroinitiator for both hydrophilic and hydrophobic monomers generating well-defined protein-based nanoassemblies. A range of acrylamide and acrylate based monomers were successfully polymerized under mild reaction conditions (e.g. room temperature) via he “grafting from” strategy. When hydrophilic monomers were selected, water-soluble conjugates could be obtained in a facile manner while by polymerizing more hydrophobic monomers yields spherical nanoparticles, consistent to a traditional PISA formulation. Importantly, it was also found that they hydrolysis of the ester bonds can be very significant in the presence of lipase-based macroinitiators, which will catalyze the hydrolysis of poly(acrylate) to poly(acrylic acid). The versatility of the reported methodology combined with the use of mild reaction conditions may find applications in enzyme immobilization and nanoreactors.

Tips/comments directly from the authors:

1. It is necessary to purify the commercial CuBr as it could be partially oxidized during storage and routine use.
2. Typical Cu(0)-RDRP in water is fast enough to reach full conversion in minutes; however, the polymerizations would be slower when grafting from proteins, possibly due to the low concentration of macroinitiators.
3. Although copper ions were known to be able to denature proteins, CALB still maintained its function after polymerization. The mild reaction conditions such as aqueous system, low reaction temperature (0-25 ℃) and fast polymerization rate (minutes to hours) could be suitable for more sensitive proteins.
4. The degradation of lipase-poly(acrylate) conjugates was fast and gradual disappearance of precipitates could even be visually observed during the dialysis in water. So it is better to quickly purify the conjugates via centrifugation. From another point of view, such conjugates could be potentially used for drug delivery and controlled release.

Citation to the paper: Synthesis of lipase–polymer conjugates by Cu(0)-mediated reversible deactivation radical polymerization: polymerization vs. degradation, Polym. Chem., 2020, 11, 1386-1392, DOI: 10.1039/c9py01462d

Link to the paper:

https://pubs.rsc.org/en/content/articlelanding/2020/py/c9py01462d#!divAbstract

This paper is free to read until 10th April 2020!

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.

Zhang and Gutekunst utilize functional enyne molecules in pulsed-addition ring-opening metathesis polymerization to generate multiple functional polymer chains from a single 3^rd generation Grubbs initiator.

Ring-opening metathesis polymerization (ROMP) has gained popularity within the polymer chemistry community thanks to the invention of functional group-tolerant ruthenium-based initiators. However, remaining challenges in the area include the use of stoichiometric amounts of metal and the difficulty to satisfactory control the end-group functionality. To address these challenges, Gutekunst and Zhang have elegantly employed functional enyne molecules in pulsed-addition ring-opening metathesis polymerization. This led to the generation of multiple functional polymer chains from a single 3rd generation Grubbs initiator. Importantly, all polymers synthesized displayed monomodal molecular weight distributions and very low dispersity values, as characterized by size exclusion chromatography, thus supporting the high efficiency of enyne chain-transfer. Detailed analysis of the molecular weights obtained from each pulse demonstrate that 50% of the ruthenium initiator remains active even after 10 cycles which corresponds to 4% of catalyst death per cycle. This is improved over previous established protocols where 8.5% of catalyst death per cycle was reported. The materials synthesized were further characterized by mass-spectrometry. In particular, matrix assisted laser desorption ionization showed extremely high end-group fidelity obtained using the enyne chemistry with a single polymer distribution and no observable side reactions. Different monomer structures were tested, the vast majority of which were compatible with the developed protocol. Bifunctional enyne molecules can also be used to give heterotelechelic polymers. Last but not least, the possibility of diblock copolymer formation was also examined yielding well-defined block copolymers with low final dispersity values. It is the author’s belief that their user-friendly and catalyst economical method will yield to the facile synthesis of materials with reduced metal contamination thus paving the way for further biomedical and electronic applications.

Tips/comments directly from the authors:

1. An inert atmosphere is important to this protocol, though a glovebox is not needed. All experiments were performed with a standard Schlenk line, and solutions were degassed by simply bubbling with nitrogen gas.
2. Three equivalents of the enyne CTA are used to ensure complete conversion of the Grubbs 3rd generation initiator, but only 1.2 eq is needed for full transfer after a given polymerization cycle of an exo-norbornene imide monomer. This reflects the differences in reactivity between ruthenium alkylidenes and benzylidenes with the enyne CTAs.
3. The exo-Oxanorbornene imide examined in this protocol was also effective but required 2.4 eq of the enyne CTA to recycle the system. This implies that different monomers may have variable reactivities. 1H NMR is very useful to monitor this process, as each of the ruthenium alkylidene/benzylidene species have diagnostic chemical shifts.
4. Small molecule byproducts are formed in each cycle but are inert under the reaction conditions and do not interfere with the polymerization.
5. If any readers are interested in using this approach, feel free to reach out to willgute@gatech.edu with any questions.

Citation to the paper: Pulsed-addition ring-opening metathesis polymerization with functional enyne reagents, Polym. Chem., 2020, 11, 259-264, DOI: 10.1039/c9py00965e

Link to the paper:
https://pubs.rsc.org/en/content/articlepdf/2020/py/c9py00965e