Archive for the ‘Paper of the Month’ Category

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

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

Professor Athina AnastasakiDr. 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: Synthesis of lipase–polymer conjugates by Cu(0)-mediated reversible deactivation radical polymerization: polymerization vs. degradation

Graphic imageZhang 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.

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Paper of the month: Pulsed-addition ring-opening metathesis polymerization with functional enyne reagents

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

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Paper of the month: Synthesis of block copolymers using poly(methyl methacrylate) with unsaturated chain end through kinetic studies

Graphical abstract for the article c9py01367a

The use of a polymethylmethacrylate (PMMA) containing an unsaturated chain end as a macroinitiator during reversible complexation mediated polymerization has been previously reported by Goto and coworkers. Typically, such macroinitiators can also be used as macromonomers to generate branched polymers via propagation. In this work, Goto and co-workers elegantly demonstrate that the occurrence of addition-fragmentation chain transfer and propagation strongly depends on the temperature during the polymerization of styrene. Through carefully monitoring the kinetics of the polymerization of styrene, the authors discovered that propagation is predominant below 60 ̊C, consistent with previous reports. However, upon elevating the temperature (e.g. 120 ̊C), addition-fragmentation chain transfer dominates instead. This discovery then allowed access to the efficient synthesis of block copolymers with PMMA and polystyrene at high temperatures. Importantly, addition-fragmentation chain transfer was also predominant over propagation during the polymerizations of acrylonitrile and acrylates yielding well-defined block copolymers. PMMAs with different molecular weights were also investigated and the polymerization was controlled utilizing iodine transfer polymerization for styrene and reversible complexation mediated polymerization for the other monomers. Such an approach is highly advantageous due to the ease of the operation and it is expected to be a practical alternative for efficient block copolymer synthesis.

Tips/comments directly from the authors:

  1. The proper purification of polymers and the careful NMR analysis were important for obtaining the accurate kinetic data. The kinetic study provided a useful idea enabling the synthesis of block copolymers of PMMA with polystyrene (PSt).
  2. Block copolymers of PMMA with PSt, polyacrylonitrile, and polyacrylates are accessible. Relatively high monomer conversions are achievable.
  3. Not only the isolated alkyl iodide but also the alkyl iodide in situ generated from iodine (I2) and azo compound can effectively be used as the initiating dormant species. The in situ method is less expensive and robust and hence can be a practically attractive

Read the full article now for FREE until 10th January!

Synthesis of block copolymers using poly(methyl methacrylate) with unsaturated chain end through kinetic studies, Polym. Chem., 2019, 10, 5617-5625, DOI: 10.1039/c9py01367a

 

About the web writer

Professor Athina AnastasakiDr. 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: Ab initio RAFT emulsion polymerization mediated by small cationic RAFT agents to form polymers with low molar mass dispersity

Graphical abstract

 

Reversible addition fragmentation chain transfer (RAFT) polymerization has revolutionized the field of polymer chemistry providing access to a wide range of materials with controlled molecular weight, functionality, end-group fidelity and dispersity. In their current contribution, the groups of Moad, Keddie and Fellows joined forces to report a range of low molar mass cationic RAFT agents that allow for predictable molecular weight and dispersity in ab initio emulsion polymerization. In particular, upon utilizing the protonated RAFT agent ((((cyanomethyl)thio)carbonothioyl)(methyl)amino)pyridin1-ium toluenesulfonate and the analogous methyl-quaternized RAFT agents, 4-((((cyanomethyl)thio) carbonothioyl)(methyl)amino)-1-methylpyridin-1-ium dodecyl sulfate, styrene could be efficiently polymerized yielding polystyrene with narrow molecular weight distributions (Đm 1.2–1.4). The authors attribute the success of ab initio emulsion polymerization with the former RAFT agent to the hydrophilicity of the pyridinium group which allows for the predominant partition of the water-soluble RAFT agent into the aqueous phase.  The RAFT agent also gives minimal retardation. In addition, by employing 4-((((cyanomethyl)thio) carbonothioyl)(methyl)amino)-1-methylpyridin-1-ium dodecyl sulfate, a “surfactant-free” RAFT emulsion can be achieved producing a low Đm  polystyrene although the RAFT end-group was lost upon isolating the polymer. Additional preliminary experiments were also performed demonstrating that this class of RAFT agents can be broadly applicable in ab initio emulsion polymerization of a range of other more-activated monomers including acrylates and methacrylates producing low dispersity polymers while the polymerization of less activated monomers such as vinyl acetate showed good control over the molecular weight, albeit broader molecular weight distributions. The authors are currently investigating such systems to establish their full utility in emulsion polymerization and develop robust and scalable conditions for the formation of block copolymers.

Tips/comments directly from the authors:

There are two significant challenges in implementing successful ab initio emulsion polymerization in a high throughput platform such as the Chemspeed®

  1. Devising a protocol for vortexing/agitating so as to form, and then maintain, a stable latex. The protocol reported was the end-result of many experiments.
  2. Degassing the reaction medium. RAFT polymerization can be successfully carried out in non-degassed media.  However, for good reproducibility, optimal dispersity, high end group fidelity and acceptable polymerization rates, degassing remains important.  In conducting experiments on the Chemspeed®, it is important to make sure the media to be dispensed by the robot are degassed, and that all of the solvent lines, and the solvent used to prime and wash the syringe needles are degassed.

Read the full article for FREE until 6th December!

Ab initio RAFT emulsion polymerization mediated by small cationic RAFT agents to form polymers with low molar mass dispersity, Polym. Chem., 2019, 10, 5044-5051, DOI: 10.1039/C9PY00893D

 

About the Web Writer

Professor Athina AnastasakiDr. 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: Benchtop flow-NMR for rapid online monitoring of RAFT and free radical polymerisation in batch and continuous reactors

Graphical abstract

To precisely engineer macromolecular materials, close monitoring of the polymerization progress is required. Therefore, real-time online monitoring provides polymer chemists the opportunity to accurately observe and optimize their reactions. To this end, Warren and co-workers utilized benchtop flow-nuclear magnetic resonance (NMR) as a very convenient and powerful tool for real-time monitoring of polymers synthesized either by controlled radical polymerization or free radical polymerization protocols. In particular, reversible addition-fragmentation chain-transfer (RAFT) polymerization was employed to polymerize acrylamides giving very high conversions in less than 10 minutes and the kinetic profile of this reaction was efficiently captured. In a second example where RAFT dispersion polymerization was monitored. In spite of the rapid polymerization rates, high temporal resolution enabled the previse determination of the onset of rate acceleration usually observed for polymerization induced self-assembly (PISA) systems. In addition to the monitoring of the aforementioned complex systems, the free radical polymerization of methyl methacrylate was also studied. In this case, the linear semi-logarithmic plot indicated the expected pseudo-first order kinetics. The results discussed here demonstrate the power of using benchtop NMR spectrometers for online flow applications where both controlled and free radical polymerizations can be employed. It is the author’s opinion that the lower price of these instruments will improve access to NMR spectroscopy while the reduced sample preparation/time taken for analysis will increase research output.

Tips/comments directly from the authors:

  1. Despite the reduced field strength, detailed polymerization kinetics comparable to traditional ‘high field’ NMR can be obtained since the vinyl protons are easily resolved.
  2. Flow-NMR is a powerful tool to improve time-resolution and reduce lab workload but must be used with care – e.g. flow rate and sample cell geometry must be optimized.
  3. Hydrogenated solvents can be used with lower-field instruments, but solvent selection is important: minimising any potential solvent overlap is key to reliable data.
  4. Spectral corrections such as to the phase and baseline are crucial for reliable data – especially if using an automated system.

 

Read the full article now for FREE until 8th November!

Benchtop flow-NMR for rapid online monitoring of RAFT and free radical polymerisation in batch and continuous reactorsPolym. Chem., 2019, 10, 4774-4778, DOI: 10.1039/C9PY00982E

 

About the web writer

Professor Athina AnastasakiDr. 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: Engineering mannosylated nanogels with membrane-disrupting properties

Graphical image 10.1039/C9PY00492K

Engineering mannosylated nanomaterials with various functionalities can significantly contribute to the development of more effective vaccines or cancer immunotherapeutics that target immune cell subsets that express the mannose receptor. With this in mind, De Geest’s group aimed at equipping mannosylated nanogels with membrane-destabilizing properties that are responsive to the acidic pH found in intracellular vesicles, such as endosomes, but are shielded when the nanogels are intact in neutral pH. In particular, membrane destabilizing tertiary amine moieties were successfully introduced in the core of the nanogels. Subsequently and via using a pH-sensitive ketal-based crosslinker, the membrane-destabilizing properties only become activated upon pH-triggered disassembly of the nanogels into soluble unimers. In order to achieve this, the effect of tertiary amine modification of mannosylated block copolymers with N,N-dimethylamine (DMAEA) and N,N-diisopropylamine (DiPAEA) was initially evaluated. Both block copolymers showed strong haemolytic activity and the DiPAE block copolymers demonstrated an activity only at acidic endosomal pH values. To silence the membrane destabilizing activity and render the nanogels non-cytotoxic at high concentration, cross-linking of the block copolymers into nanogels was conducted. Interestingly, when a pH degradable ketal cross-linker was used, the nanogels could regain their activity by exposing them to mild acidic pH. As the authors nicely conclude, such synthetic mannosylated materials may hold promise for cytoplasmic delivery of non-membrane permeable therapeutic macromolecules.

Tips/comments directly from the authors:

 

  1. Dendritic cells and macrophages reside in peripheral tissue, lymphoid organs and sites of inflammation and tumor tissue. They are a primary therapeutic target.
  2. The use of tetraacetylated carbohydrate monomers allows for straightforward polymerization and work-up in organic media. Deacetylation is easily performed in a final step and yields hydrophilic glyconanogels.
  3. The use of a pentafluorophenyl activated ester hydrophobic polymer bock allows for self-assembly in aprotic polar solvents. This is ideal for successive post-modification steps without facing hydrolysis as a side reaction.
  4. Diisopropylamine motifs are highly efficient in destabilizing lipid membranes at acidic pH, presumably through hydrophobic interaction with phospholipid membranes.

 

Read this article for FREE until the 15th October!

Engineering mannosylated nanogels with membrane-disrupting properties Polym. Chem., 2019, 10, 4297-4307, DOI: 10.1039/C9PY00492K

About the Web Writer

Dr. AthinProfessor Athina Anastasakia 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: Bottom-up design of model network elastomers and hydrogels from precise star polymers

Synthetic polymer networks have attracted considerable attention owing to their exceptional mechanical properties including high resilience and toughness. Such materials are typically based on multi-arm poly(ethylene glycol) (PEG) which is a commercially available compound. However, PEG networks suffer from restricted access to higher molecular weight which limits the network dimensions. In addition, the crystalline nature of PEG does not allow for a comprehensive understanding of the mechanical behaviour in bulk network elastomers. To overcome this challenge, Walther and co-workers introduced a new class of high molecular weight star polymer building blocks for the construction of model network elastomers and hydrogels with tuneable mechanical properties. To achieve this, triethylene glycol methyl ether acrylate was successfully polymerized via light-inducted atom transfer radical polymerization and Cu(0)-wire reversible deactivation radical polymerization, yielding well-defined polymers with narrow molecular weight distributions and high end-group fidelity. Upon synthesis, functional motifs were introduced within the polymer through either post-polymerization modification of the bromine end-groups or the use of a fluorescent star initiator. In particular, the introduction of norbornene end-groups allowed for the subsequent crosslinking of the materials in presence of a photo-radical initiator. This allowed access to thermally reversible model network hydrogels based on dynamic supramolecular bonds. Overall, this work enables the simultaneous study of the mechanical behaviour of bulk network elastomers and swollen hydrogens with the same network topology. As the authors elude in their conclusions, by elegantly exploiting precision polymer chemistry, our understanding of architecture control can be enhanced leading to the rational design of functional mechanical network materials.

Graphical Abstract for c9py00731h

 

Tips/comments directly from the authors:

  1. Water-soluble star polymers with a low Tg and quantitative end-group introduction allow the simultaneous investigation of identical model networks as hydrogels and bulk elastomers.
  2. The monomer triethylene glycol methyl ether acrylate (mTEGA) yields low-Tg, water-soluble polymers. A distinct advantage over other oligo(ethylene glycol) acrylates is the absence of potential diacrylate impurities compromising polymerization control.
  3. Polymerization of mTEGA by photo-induced and Cu0-catalyzed Cu-RDRP from commercial and functional 4-arm initiators yields narrowly dispersed star polymers up to high molecular weights. In order to achieve optimal control with minimal side reactions, a balance in the initiator-to-CuBr2 ratio is necessary.
  4. Cu0-mediated Cu-RDRP is suitable for scale up, and the polymers can be isolated by precipitation into 85/15 diethyl ether/n-pentane followed by salt removal through neutral alumina.
  5. Following end-group transformation with primary amines, both excess amines and bromide salts must be removed. The former is removed through precipitation, the latter by taking the polymer up into a diethyl ether/THF mixture and removing insoluble components.
  6. Constructing hydrogels by photo-crosslinking 4-arm p(mTEGA)-norbornene with a bifunctional thiol is fast (<1 min) with the photo-radical initiator LAP and slower (>30 min) with Irgacure-2959.
  7. Supramolecular hydrogels constructed from 4-arm p(mTEGA)-terpyridine with divalent metal ions are highly dependent on the metal. ZnII yields hydrogels which are dynamic at room temperature, and increasingly so upon heating making them suitable for thermal 3D-printing.

 

Read the full Paper now for FREE until the 31st August! 

Bottom-up design of model network elastomers and hydrogels from precise star polymers, Polym. Chem., 2019, 10, 3740-3750, DOI: 10.1039/c9py00731h

About the web writer
Professor Athina AnastasakiDr. 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: Hierarchical patterns with sub-20 nm pattern fidelity via block copolymer self-assembly and soft nanotransfer printing

Nanotransfer printing is a technique often used to construct complex patterns by employing elastomeric stamps and relying on surface chemistries. This has enabled not only the assembly of complex constructs but also the effective integration of heterogeneous materials. In the current manuscript, Campos and co-workers significantly contributed to this direction by introducing a nanotransfer technique, termed soft pattern-transfer printing, which does not rely on adhesive layers or external stimuli. As a result, a cost and time efficient high throughput processing platform is being developed. To achieve this, representative organic thin films of P3HT homopolymers, self-assembled diblock copolymers and functionalized perylene diimide small molecules were employed as inks for micron-sized array of patterns ranging from squares, lines, polygons and rings. Importantly, hierarchical patterns were obtained through microns-sized arrays of self-assembled block copolymers. In addition, to build layers of complex structures onto the same film, the technique can be repeated through sequential printing. As the authors elude in their conclusion, such high-fidelity pattern transfer work is very promising for potential uses in a number of areas such as the construction of van der Waals heterostructures interfaced with self-assembled block copolymer thin films and the development of platforms to investigate the influence of hierarchical patterning on cell differentiation.

Graphical abstract

Tips/comments directly from the authors:

  1. Solvent-vapor induced self-assembly of diblock copolymer thin films is an attractive approach to achieve long-range microphase segregation.
  2. Achieving solvent-vapor induced self-assembly of diblock copolymer thin films directly on exfoliated materials is particularly challenging because of the macroscopic topographical heterogeneities which disrupt the film integrity.
  3. Moreover, the generation of hierarchical patterns, particularly with one length scale in the nanometer regime, often involves lithographic processes which are difficult to scale.
  4. A simple contact-based approach is presented for transfer of polymeric materials (e.g. self-assembled block copolymers, homopolymers, small molecules), with well-defined edge resolution (<20 nm) and high fidelity of nanoscale pattern transfers.
  5. To avoid warped or cracked transfers, it is critical to handle PDMS stamps with care, avoiding excessive mechanical deformation, and to apply minimal pressure.
  6. Importantly, we show successful transfer of solvent-vapor induced self-assembled diblock copolymer films onto 2D materials (e.g. boron nitride).
  7. The transfer of micron-scale patterns of self-assembled diblock copolymers with nanoscale features yield hierarchical ordering.
  8. Patterns resulting from sequential soft nanotransfer printing resemble Moiré patterns, large-scale interference patterns. Such complex patterns may be used to impart local physical and electronic perturbations.

 

Read this article for free until the 31st July!

Hierarchical patterns with sub-20 nm pattern fidelity via block copolymer self-assembly and soft nanotransfer printing, Polym. Chem., 2019, 10, 3194-3200, DOI: 10.1039/C9PY00335E

 

 

About the Web writer

Professor Athina AnastasakiDr. 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: Transformation of gels via catalyst-free selective RAFT photoactivation

Controlled radical polymerization strategies are often exploited to tailor the properties of functional polymer networks. With the recent developments in external stimuli to regulate polymerization, the use of light has received significant attention as it enables the synthesis of materials with precise spatial, temporal, and sequence control.  In order to design structurally tailored and engineered macromolecular (STEM) networks, Matyjaszewski and co-workers proposed a new, metal-free approach to prepare well-defined networks. To achieve this, the authors selectively activated the fragmentation of trithiocarbonate reversible addition-fragmentation chain-transfer (RAFT) agents by visible light RAFT iniferter photolysis coupled with RAFT addition-fragmentation process. Through this two-step synthesis, different materials could be polymerized yielding compositionally and mechanically differentiated networks. Upon carefully selecting the crosslinker as well as the RAFT inimer, three different types of primary polymethacrylate networks could be generated under green light. The obtained networks were further enriched by the addition of methyl acrylate and dimethylacrylamide under blue light, resulting in soft and stiff gels respectively. Importantly, dynamic mechanical analysis was utilized to characterize the mechanical properties of both the starting and the final materials and to determine their glass transition temperatures. Such STEM networks significantly expand the toolbox of polymer and material science.

c9py00213h-ga[1]

Tips/comments directly from the authors:

 

  1. Structurally tailored and engineered macromolecular (STEM) networks are versatile materials containing latent functional groups accessible for post-synthesis modifications to afford new chemical and material properties.
  2. The network synthesis and modifications were controlled using dual wavelengths (green and blue). The primary network was synthesized under green light irradiation, and the subsequent modifications were performed under blue light.
  3. Initial network synthesis involves incorporation of two RAFT photoiniferters with similar Z groups (thioalkyl) but different R groups (either a tertiary or secondary carbon radical) to enable activation of one RAFT agent over the other under green light. This is followed by activation of both RAFT agents for secondary modification under blue light.
  4. The n to π* electronic transition at 520 nm affords photolysis of trithiocarbonate with 4-cyanopentanoic acid R-leaving group under green light leading to generation of tertiary carbon radicals promoting polymerization of methacrylates. The second trithiocarbonate RAFT agent with propionic acid R-leaving group is also incorporated into this network during this process as a RAFT methacrylate monomer or dimethacrylate crosslinker.
  5. Selective activation under green light is made possible as the addition of 4-cyanopentanoicacid radical to trithiocarbonate RAFT agent with propionic acid R-leaving group does not lead to fragmentation as radical stabilization energies of tertiary radicals are higher than secondary radicals.
  6. Therefore, the methacrylate/dimethacrylate RAFT agent with propionic acid R-leaving group remains inert under green light and can only be activated under blue light (465 nm) where the n to π* electronic transition lies.
  7. Both RAFT agents (secondary and tertiary leaving groups) are then activated in a second step which involves soaking in a second monomer (acrylate or acrylamides) into the network followed by polymerization under blue light.
  8. Depending on the functionality of the second monomer, the post-modified network can be either softer or stiffer with different responses to polarity (hydrophilicity/hydrophobicity).

Read the full paper now for FREE until 12th July!

Transformation of gels via catalyst-free selective RAFT photoactivation, Polym. Chem., 2019, 10, 2477-2483, DOI: 10.1039/C9PY00213H

About the webwriter

Professor Athina AnastasakiDr. 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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