Author Archive

Paper of the month: A Diels–Alder reaction between cyanates and cyclopentadienone-derivatives – a new class of crosslinkable oligomers

Hyperbranched polyphenylenes and cyanate esters are two unique classes of materials that possess complementary properties. On one side, polyphenylenes are good insulators with remarkable solubility owing to their dense packing and the strongly twisted structure hinder π-conjugation respectively. Cyanate esters are also well renowned for their thermal stability as thermosetting materials. To combine these properties, Voit and co-workers investigated the copolymerisation of the two monomers 3,3′-(1,4-phenylene)bis(2,4,5-triphenylcyclo-pentadienone) and 2,2-bis(4-cyanatophenyl) propane through a Diels-Alder cycloaddition where carbon monoxide is released as a side product. The polymerisation was followed by UV/Vis spectroscopy and the structure of the oligomers could be further investigated by in-depth NMR studies. Importantly, the catenation proved to be completely statistical and independent of the temperature of the polymerization while the obtained oligomers can be cured via a trimerisation reaction of the terminal OCN-groups. Finally, the polymerisation and crosslinking reaction kinetics were also studied and upon crosslinking the resins exhibit high thermal resistance and transparency as well as a high refractive index. Thus, the resulting materials simultaneously possess the strengths of polyphenylene polymers while retaining the curing potential of the cyanate esters but at only the tenth of the activation energy of pure cyanate monomers, lowering the risk factors during handling. As the authors elegantly conclude, materials with such unique characteristics may find application in integrated optics.

10.1039/C8PY01374H

Tips/comments directly from the authors:

  1. The Diels-Alder cycloaddition with these substrates requires high temperatures. However, under these conditions the trimerisation reaction of cyanate esters also takes place. To avoid the premature crosslinking of the system while maintaining the cyanate ester termination a special protocol was developed. During the Diels-Alder reaction the ratios where adjusted to obtain a oligomer terminated with cyclopentadienone groups and only 15 minutes prior to the end of the reaction one additional equiv. of cyanate ester was added.
  2. The cyclopentadienone possesses a deep purple color while the polymer is colorless. Therefore, UV/Vis spectroscopy can be a powerful tool to track the reaction, but a simple look inside the reaction vial already gives indications on the state of the reaction.
  3. While cyclopentadienone monomers are sometimes challenging to synthesize there is a wide variety of commercial cyanate ester monomers and prepolymers allowing for a high degree of tunability of the resulting resin without changing the cyclopentadienone unit.
  4. Different to fully phenylene-based systems which are difficult to analyze by 13C NMR spectroscopy, the reaction with cyanate results in pyridine and cyanurate structures that can be well identified thus improving the structural characterization of such oligomers.

Read the full paper for FREE until 1st April 2019!

A Diels–Alder reaction between cyanates and cyclopentadienone-derivatives – a new class of crosslinkable oligomers, Polym. Chem., 2019, 10, 698-704

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 star thermoresponsive amphiphilic block copolymer nano-assemblies and the effect of topology on their thermoresponse

c8py01617h

Thermoresponsive polymers can be used in a wide range of applications ranging from drug delivery to bioengineering owing to their unique capability of undergoing a soluble-to-insoluble transition in response to an external thermal stimulus. Amphiphilic block copolymers that contain a thermoresponsive block can self-assemble into core-corona nanoassemblies where the core consists of the hydrophobic block and the corona is formed by the thermoresponsive block. Zhang, Han and co-workers were interested in studying the dependence of a thermoresponsive phase transition on the topology structure. To achieve this, reversible addition-fragmentation chain transfer (RAFT) dispersion polymerization was employed to synthesize well-defined multi-arm star block copolymer nanoassemblies via polymerization-induced self-assembly. The block copolymer was designed to have the first part consisting of the thermoresponsive poly(N-isopropylacrylamide) (PNIPAM) and the second block being the hydrophobic polystyrene (PS). By carefully modifying the number of arms (n=1, 2, 3 and 4), the degree of polymerization of the hydrophobic block and the polymerization conditions, (PNIPAM-b-PS) nanoassemblies with similar degree of polymerization and chain density, albeit different topology structure, were obtained. A range of characterization techniques were subsequently employed to comparatively study the responsiveness of these materials including turbidity analysis, dynamic light scattering, variable-temperature 1H NMR and rheological analysis. The authors found that the topology of the tethered PNIPAM chains had a significant influence on their thermoresponsive phase transition which decreased upon increasing the number of arms. This can be attributed to the inter-and intra-particle chain entanglement in the synthesized star nanoassemblies. It can thus be concluded that the topology of the thermoresponsive polymers can significantly affect their thermoresponsive and should be taken into account when designing the synthesis of such materials.

 

This paper is FREE to read and download until 27th February!

 

Synthesis of star thermoresponsive amphiphilic block copolymer nano-assemblies and the effect of topology on their thermoresponse, Polym. Chem., 2019, 10, 403-411, DOI: 10.1039/C8PY01617H

 

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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Paper of the month: Synthesis of sheet-coil-helix and coil-sheet-helix triblock copolymers by combining ROMP with palladium-mediated isocyanide polymerization

c8py01361f

Natural proteins are comprised of distinct secondary structure elements such as sheets, helices and coils. It is the combination of these diverse topologies that allow proteins to fulfil their functions. Owing to the properties of these structures, synthetic analogues of these materials are also of great interest to the polymer chemistry community. However, a covalent system where sheet, helix, and coil blocks are combined in a linear system has not been yet realized. Towards this direction, Weck, Elacqua and co-workers developed a new methodology to covalently link three distinct structures together with high fidelity without compromising the control over sequence. This was achieved by combining sequential ring-opening metathesis polymerization (ROMP) of sheet- and coil-forming monomers with palladium-mediated isocyanide polymerization of covalent coil-sheet-helix (ABC) and sheetcoil helix (BAC) domains. After polymerizing the initial sheet of coil-forming monomer through ROMP, the second monomer is introduced and subsequently polymerized yielding to a diblock comprised of sheet and coil structures. ROMP is then terminated by a special transfer agent that contains an isocyanide polymerization initiator. The telechelic diblock copolymers containing both coil-sheet and sheet-coil blocks, can serve as macroinitiators for the polymerization of a P-helix forming monomer. As such, this combination of sequential copolymerization and macroinitiation enables three different polymer chains to be linked covalently. Importantly, throughout the triblock copolymer synthesis, all individual blocks retained their secondary structures as evidenced by circular dicroism and fluorescence spectroscopies. The authors are confident that this work can be extended to the formation of a diverse array of tri- and multiblock copolymers enabling a range of new applications.

Tips/comments directly from the authors:

1. When synthesizing a topologically-diverse block copolymer, oftentimes it is necessary to use different polymerization techniques. If so, prudent selection and design of polymer backbone is key. First, select the class(es) of monomers you intend to employ. This will inform the type of polymerization method required, and subsequently, the initiator to be designed.

2. Ring-opening metathesis polymerization (ROMP) is a widely-used controllable polymerization method that allows for one to not only control molecular weight, but also is amenable to an iterative or tandem ROMP, which is desirable for sequence-controlled block copolymers

3. Performing 31P NMR spectroscopy after each step of block copolymer synthesis, especially before the final step to create the helical block, is crucial. It ensures that only one palladium species is present throughout.

4. Isocyanide polymerization mediated by palladium(II) is a robust technique; there is high functional group tolerance when synthesizing the initiator, which allows for the engineering of multipurpose catalysts like the one featured in this manuscript.

5. Topologically-diverse polymer backbones, such as sheets, helices, and coils, garner much interest from a biomimetic standpoint in the synthetic community. Judicious choice of polymer backbones, as well as block lengths, can inform characterization techniques, such as circular dichroism, fluorescence, and X-ray scattering to gain insights into topology.

6. We are available for any questions and to troubleshoot any issues you may have – please contact mw125@nyu.edu or eze31@psu.edu.

 

Synthesis of sheet-coil-helix and coil-sheet-helix triblock copolymers by combining ROMP with palladium-mediated isocyanide polymerization, Polym. Chem., 2018, 9, 5655-5659, DOI: 10.1039/C8PY01361F

 

About the webwriter

Dr Athina AnastasakiDr. Athina Anastasaki is a Web Writer for Polymer Chemistry. She is currently an Assistant Professor at ETH Materials Department.

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Paper of the month: Choosing the ideal photoinitiator for free radical photopolymerizations: predictions based on simulations using established data

C8PY01195H

A major requirement to synthesize polymeric materials via photopolymerizations is the efficiency of the photoinitiator. This is because the amount of initiated polymer chain is crucial for the outcome of the radical polymerization. Many factors have been reported to influence this efficiency including the UV-Vis absorption properties, the irradiation wavelengths, the dissociation quantum yields and the reactivity of primary radicals towards monomers. To enable “on demand” access to a wide range of photoinitiators performance, Gescheidt and co-workers developed a tool for predicting the initiator efficiency of various type 1 (α-cleavage) photoinitiators. To achieve this, a systematic analysis of the photoinitiator performance revealed the interplay of absorption properties, dissociation quantum yields, light intensities, irradiation wavelengths and kinetics.  It is important to note that important side reactions such as oxygen quenching have also been taken into account. The author’s simulations demonstrate that under ideal conditions any photoinitiator will function in an almost perfect way. However, the oxygen quenching mechanism combined with the subtleness of the photo-physical properties of the photoinitiator differentiates a well-suited photoinitiator from a less useful one. The authors conclude that a balanced combination of the intrinsic photoinitiator characteristics (i.e. absorption spectrum of initiator, quantum yields of dissociation, rate constants for primary radicals towards monomers), the major side reactions such as oxygen quenching, and the emission properties of the utilized light source (i.e. irradiation wavelengths, light intensity) is crucial to achieve the optimal initiation performance. As the authors nicely note in their conclusion, such a tool is not only useful for experienced researchers but it can also serve as an educational guide. The corresponding kinetic scheme is freely available on the author’s website and can be adjusted by any user.

 

Tips/comments directly from the authors:

1. A single property is not sufficient to reasonably classify a photoinitiator. A subtly interplay between absorbance, quantum yield, kinetics and the character of the light source is what matters.

2. The choice of a suitable photoinitiator strongly depends on the desired application (e.g. bulk polymerization or coatings). Here, the number of initiating radicals directs the efficiency of the polymerization and its robustness vs. oxygen inhibition.

3. The optical density of the formulation combined with the wavelength of irradiation control the thickness of the cured layer. It is crucial to be aware of the exact properties of the used light source (emission bands, intensity), since this drastically influences the amount of generated radicals and the initiation rate.

4. The Simulations rely on experimental parameters, which are straightforwardly determined (e.g. rate constants (Polym. Chem., 2018, 9, 38–47) and quantum yields (Photochem. Photobiol. Sci., 2018, 17, 660–669)) and provide an overall picture of a complex process such as the initiation of a photo-induced polymerization.

5. We are happy to answer any questions or remarks concerning our initiation model – please contact g.gescheidt-demner@tugraz.at.

 

FREE to read until 24th December

 

Choosing the ideal photoinitiator for free radical photopolymerizations: predictions based on simulations using established data, Polym. Chem., 2018, 9, 5107-5115, DOI: 10.1039/C8PY01195H

 

About the web writer

Dr Athina AnastasakiDr. Athina Anastasaki is a Web Writer for Polymer Chemistry. She is currently a Global Marie Curie Fellow working alongside Professor Craig Hawker at the University of California, Santa Barbara (UCSB). In January 2019, she will join the ETH Materials Department as an Assistant Professor to establish her independent group.

 

 

 

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Paper of the month: Thermoresponsive hybrid double-crosslinked networks using magnetic iron oxide nanoparticles as crossing points

Double-network hydrogels (DN gels) consist of two interpenetrating networks that are bound together by covalent and reversible interactions and can resist high deformation by reorganizing their structure. Magnetic hybrid hydrogels in particular have attracted considerable attention owing to the possibility of triggering the properties of the hydrogels with an external magnetic field. Fontaine, Montembault and co-workers further contributed to this field by developing a novel class of thermoresponsive hybrid double-crosslinked polymer networks materials that can rearrange and rebuild upon triggering a Diels-Alder (DA) reaction. Central to this approach is the use of iron oxide nanoparticles as the nano-crosslinkers and difuran-functionalized poly(ethylene oxide) as the diene partner for the thermoreversible DA reaction. The thermoreversibility of the network was confirmed by 1H nuclear magnetic resonance (NMR) spectroscopy and rheological studies showing a fast gel/liquid state transition upon heating the sample. Importantly, the rheological properties of 3D networks with and without iron oxide nanoparticles were compared. The studies conclude that the presence of iron oxide nanoparticles in the network ensured that a gel-like structure was maintained after the retro DA reaction. These characteristics were attributed to the establishment of a secondary network through covalently integrated iron oxide nanoparticles. The unique combination of the Diels-Alder reaction with iron oxide nanoparticles to generate new reversible reticulated networks can pave the way for further applications mediated by magnetic hyperthermia stimuli.

c8py01006d

Tips/comments directly from the authors:

1. The strategy used for the synthesis of difuran-functionalized diphosphonic acid terminated poly(ethylene oxide) by a combination of Kabachnik-Fields reaction and “click” copper-catalyzed 1,3-dipolar cycloaddition is a versatile method and poly(ethylene oxide) backbone can be replaced by a wide range of polymers that may bring new properties.

2. The formation of a 3D network via the Diels-Alder (DA) reaction with a trismaleimide is thermoreversible, with a faster retro-DA (rDA) reaction rate compared to the DA reaction, leading to an easier destruction of the 3D network than its formation.

3. The presence of phosphonic acid groups is needed to allow the crosslinking through interactions with the iron oxide nanoparticles and the formation of the 3D double-crosslinked network.

4. The 3D network is preserved even after the rDA reaction, demonstrating that the iron oxide nanoparticles serve as crossing points through strong bidendate Fe-O-P bonds. Furthermore, a gel-like structure is maintained at least in the limit of the percolation threshold.

5. The viscoelastic properties of the double-crosslinked gels demonstrates that the double-crosslinking leads to stiffer gels.

 

Read the full article for FREE until 26th November!

Thermoresponsive hybrid double-crosslinked networks using magnetic iron oxide nanoparticles as crossing points, Polym. Chem., 2018, 9, 4642-4650, DOI: 10.1039/C8PY01006D

 

About the web writer

Dr Athina AnastasakiDr. Athina Anastasaki is a Web Writer for Polymer Chemistry. She is currently a Global Marie Curie Fellow working alongside Professor Craig Hawker at the University of California, Santa Barbara (UCSB). In January 2019, she will join the ETH Materials Department as an Assistant Professor to establish her independent group.

 

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Paper of the month: A user’s guide to the thiol-thioester exchange in organic media: scope, limitations, and applications in material science

The thiol-thioester exchange is a common reaction in dynamic covalent chemistry. This reaction has been extensively optimized in aqueous media facilitating the widespread use in biochemistry and other bio-related applications. However, the utility of this reaction in material and polymer science is currently underexplored. This is possibly due to the fact that most polymer/material systems require organic media for their respective synthesis. To overcome this barrier and extend the scope and applications of thiol-thioester exchange, Bowman and co-workers explored this dynamic exchange in both small molecule and polymer analogues in a wide range of organic solvents. The effect of the pKa of the thiol and base employed, the electronic character of the thioester, the polarity of the solvent, the effect of the temperature and the nucleophilicity of the catalyst were thoroughly investigated. By judiciously choosing and optimizing all these parameters, the authors were able to tune the thiol-thioester exchange in both small molecules and subsequently network polymers to reduce applied stresses or change shape of the materials following polymerization. All these findings were thoroughly reported and explained by crafting an impressive “user’s guide” that can be useful for a large number of practitioners within the polymer/material community. The authors anticipate that the extremely robust, tuneable and responsive exchange reaction will further enable polymer/material scientists to develop new smart materials and will pave the way for further applications.

thiol-thioester exchange

 

Tips/comments directly from the authors:  

1. When comparing a panel of various thioesters exchanging with various thiols, the authors have found that thermodynamically the acyl group of a thioester prefers to rest on thiols of the highest pKa and will exchange rapidly to achieve this minima.

2. Base catalysts of a higher pKa form more thiolate and therefore promote the thiol-thioester exchange more rapidly in both small molecule systems and in crosslinked polymers, all things held the same. Base catalysts, if employed in larger concentrations, were, however, found to significantly retard the free radical thiol-ene reaction to produce crosslinked polymers. This can be overcome by optimizing the concentration of radical initiator (-photo, -thermal, or –redox) with respect to base.

3. Nucleophilic catalysts of a higher N-value (see Herbert Mayr’s work for more details on the calculation of this parameter) were found to promote the thiol-thioester exchange more rapidly in both small molecule systems and in crosslinked polymers, all things held the same. Unlike basic catalysts, nucleophilic catalysts did not show similar retardation in the free radical thiol-ene reaction to produce crosslinked polymers.

4. Due to polar intermediates formed during the thiol-thioester exchange reaction, polar solvents/matrices most effectively promote this dynamic exchange, especially with weaker bases.

5. Due to the low energetic barrier for the thiol-thioester exchange reaction, temperature does not have a great affect on the outcome of the exchange, however, it does improve the kinetics.

6. If the readers have any unanswered questions regarding this reaction, schemes for placing it into a polymer matrix, or other general queries, please direct them to brady.worrell@gmail.com and we’ll get to the bottom of it.

 

Read the full article for FREE until 30th October

 

A user’s guide to the thiol-thioester exchange in organic media: scope, limitations, and applications in material science, Polym. Chem., 2018, 9, 4523-4534, DOI: 10.1039/C8PY01031E

 

About the web writer

AthinaDr. Athina Anastasaki is a Web Writer for Polymer Chemistry. She is currently a Global Marie Curie Fellow working alongside Professor Craig Hawker at the University of California, Santa Barbara (UCSB). In January 2019, she will join the ETH Materials Department as an Assistant Professor to establish her independent group.

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Paper of the month: Copper catalyzed synthesis of conjugated copolymers using direct arylation polymerization

C8PY00913A

Direct arylation polymerization is a useful methodology that allows for the preparation of conjugated polymers. One of the great advantages of this technique is that it eliminates the use of toxic and pyrophoric reagents typically utilized to prepare monomers for other polymerization methods (e.g. Stille, Suzuki, etc.). Importantly, the technique allows the preparation of polymers with undetectable levels of homo-coupling or branching defects leading to the defect free synthesis of a wide range of conjugated polymer architectures such as homopolymers, donor-acceptors copolymers and porous polymers. However, the vast majority of direct arylation polymerization methodologies rely on the use of noble metals such as palladium (Pd), which is a costly, low abundant, relatively toxic, and unsustainable metal. To circumvent this, Thompson and co-workers were inspired by the small molecule copper catalyzed aryl-aryl cross-coupling for various iodinated arenes and electron-deficient heterocycles. To transition from small molecules to conjugated polymers, the group judiciously optimized the reaction conditions including the concentration, the nature of the solvent, the ligand, the temperature and the base employed. Conjugated polymers were then prepared in very high yields (up to 97%) with Mn of up to 10 kDa. NMR spectroscopy was used to characterize the recovered polymer products and confirmed the absence (or minimization) of undesired couplings. This report is the first example of perfectly alternating donor-acceptor conjugated polymers using copper catalyzed direct arylation polymerization thus offering an initial step towards the replacement of toxic metals like Pd. Future work will hope to address milder reaction conditions, lower catalyst loading, and a broader scope.

Tips/comments directly from the authors:  

1. Fresh CuI is recommended. It is recommended to acquire a fresh bottle or a freshly purified stock of CuI and store it under inert gas in a freezer.

2. The strict exclusion of air and moisture is advisable, and so care should be taken to thoroughly sparge the reaction vessel with inert gas before and after the addition of reagents.

3. The selection of base is critical for the polymerization, and the base should be finely ground and dried using a vacuum oven before use. The base can be stored in a desiccator after drying, but should be dispensed quickly to limit contact with moisture

4. After addition of the CuI, the copper-phenanthroline catalyst appears to form instantly. However, stirring at room temperature for a several minutes before heating may help ensure complete coordination of the phenanthroline to copper

 

This article is FREE to read and download until 21st September 2018

 

Copper catalyzed synthesis of conjugated copolymers using direct arylation polymerization  Polym. Chem., 2018, 9, 4120-4124, DOI: 10.1039/C8PY00913A

 

About the Web Writer

AthinaDr. Athina Anastasaki is a Web Writer for Polymer Chemistry. She is currently a Global Marie Curie Fellow working alongside Professor Craig Hawker at the University of California, Santa Barbara (UCSB). In January 2019, she will join the ETH Materials Department as an Assistant Professor to establish her independent group.

 

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Paper of the month: Graft-modified cellulose nanocrystals as CO2-switchable Pickering emulsifiers

Renewable bio-based colloidal particles in emulsion-based products can be highly desirable as they can replace synthetic, small molecule surfactants. Pickering stabilisers or emulsifiers are solid particles that can be used as insoluble surfactants to stabilise emulsions. For such application, cellulose appears to be a good renewable biopolymer candidate owing to its abundance, sustainability and nontoxicity. Cellulose nanocrystals (CNCs) in particular exhibit little to no cytotoxicity and can thus represent a more sustainable and greener alternative to conventional surfactants. Cunningham and co-workers introduced new properties to cellulose nanocrystals by exploiting their graft modification with switchable poly((diethylamino)ethyl methacrylate) (PDEAMEA) and poly((dimethylamino)ethyl methacrylamide) (DMAPMAm). In this work, the use of well-defined graft modified CNCs with small amounts of grafted CO2-switchable PDMAPMAm and PDEAEMA as stimuli-responsive Pickering stabilisers for the reversible emulsification/demulsification of oil and water is thoroughly investigated. The obtained CNCs contained less than 25 wt% of grafted synthetic polymer and impressively resulted in stable Pickering emulsions with a shelf life up to one month without desulfating the CNCs or the introduction of ionic strength to the system. N2 and CO2 were used as environmentally benign triggers to stabilise the emulsions under N2 and break the emulsions under CO2. Importantly, the emulsification and demulsification were reversible and repeatable and the CNC-based Pickering emulsifier could be easily recovered, thus enabling it to be a potential candidate for oil harvesting applications. Such Pickering emulsifiers are not expected to have significant ecotoxicity compared to other conventional surfactants due to the CNCs and the polymer chains being too large in molecular weight to be bioavailable. The authors conclude that a higher fraction of hydrophobic copolymer in the grafts may further improve their system and enhance the adsorption of graft-modified CNC to the oil droplets and increase the emulsion stability.

C8PY00417J

 

Tips/comments directly from the authors:  

  1. It can be difficult to ‘switch off’ PDMAPMAm at room temperature using N2, meaning deprotonating the tertiary amine groups in water. To ensure sufficient wettability of CNC-g-P(DMAPMAm-co-S) with the oil phase when homogenizing, i.e. a high enough degree of deprotonation, the temperature of the CNC dispersion has to be slightly increased first (~40°C, although this depends by the chain length) before adding the oil phase and preparing the emulsion.

  1. For the synthesis of the materials (Polym. Chem., 2017, 8, 6000–6012), it is important that premade polymers are living and purified from unreacted monomer, BlocBuilder and dead polymer chains. Any nitrogen-containing impurity will artificially increase the nitrogen content obtained from elemental analysis and thus falsify the final value for the amount of grafted polymer, graft density and amino groups per 1 g of CNC. It is thus advisable to repeat the purification/elemental analysis until a constant N value is obtained.

  1. SEM analysis of the grafted CNC (ESI) can be very difficult unless the appropriate conditions are chosen. CNC needs to be coated with conductive material as the polymer and CNC are both non-conductive. The materials were coated with 3nm osmium particles using a standard 30 micron aperture probe at 300V. The current corresponded to 275 pA at 10kV. Low current and low voltage conditions need to be chosen in order to visualize the structures.

 

This paper is FREE to read and download until the 31st August!

 

Graft-modified cellulose nanocrystals as CO2-switchable Pickering emulsifiers, Polym. Chem., 2018, 9, 3864-3872

 

About the web writer
AthinaDr. Athina Anastasaki is a Web Writer for Polymer Chemistry. She is currently a Global Marie Curie Fellow working alongside Professor Craig Hawker at the University of California, Santa Barbara (UCSB). Please visit this link for more information.

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Paper of the month: Enzyme-driven biodegradable nanomotor based on tubular-shaped polymeric vesicles

The potential of nanomachines to mimic biological systems that can continuously move in and between cells to perform a function has attracted significant attention over the past decades. Although the autonomous movement, speed and functionality of various artificial nanomotors has improved over the years, it is interesting to note that the vast majority of them are based on catalytically active metals and harsh metal surfaces while requiring toxic fuel for propulsion. Wilson and co-workers are the first to report a biodegradable nanomotor which can autonomously move in the presence of fuel while carrying a load. To achieve this, Wilson’s group created poly(ethylene glycol)-b-poly(D,L-lactide) (PEG-PDLLA) polymerosomes with 5% wt% functional azide handles by employing ring opening polymerization. The spherical polymerosomes were then transformed to nanotubes by inducing osmotic pressure. The azide handles presented in periphery of the nanotubes were converted into COOH groups using strain-promoted alkyne-azide cycloaddition. Finally, catalase was coupled to the nanotubes surface via EDC coupling. Importantly, the catalytic conversion of H2O2 by the enzyme provided adequate propulsion to move the nanotubes forward. In addition, both hydrophobic and hydrophilic drugs could be simultaneously loaded in the tubes. Given the advantageous characteristics of tubular-shaped polymersomes, such as high-aspect-ratio and higher loading capacity, such materials can be potentially used as excellent nanocarriers for drug delivery.

Tips/comments directly from the authors:

  1. For the synthesis of PEG-PDLLA, keep the ratio of catalyst to initiator bellow 0.5 equivalents to obtain polymers with low PDI. Furthermore, the reaction is oxygen and water sensitive and should thus be carried out in inert atmosphere.
  2. The formation of polymersomes requires stable conditions, as small changes can affect the morphology and the polydispersity of the vesicles. Pre-cooled water is used for the shape transformation by dialysis while the dialysis is carried out in in the fridge at 4°C.
  3. Centrifugation of the nanotubes for functionalization reaction should not exceed 5000 rpm and should not be longer than 10 min to prevent aggregation and clogging of the spin filter and breakage of the tubes.
  4. It is recommended to use low concentration of nanomotors (< 109 particles/ml) when measuring their movement with the Nanosight LM10, as high oxygen production can lead to drift of the sample (dilute such that single motors are visualized).

 

Enzyme-driven biodegradable nanomotor based on tubular-shaped polymeric vesicles, Polym. Chem., 2018, 9, 3190-3194, DOI: 10.1039/C8PY00559A

 

This paper is free to read and download until 6 August!

 

About the webwriter

AthinaDr. Athina Anastasaki is a Web Writer for Polymer Chemistry. She is currently a Global Marie Curie Fellow working alongside Professor Craig Hawker at the University of California, Santa Barbara (UCSB). Please, visit http://hawkergroup.mrl.ucsb.edu/members/athina-anastasaki for more information.

 

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Paper of the month: Discrete oligodimethylsiloxane–oligomethylene di- and triblock co-oligomers: synthesis, self-assembly and molecular organisation

A long lasting challenge in polymer chemistry is to design new block copolymer combinations that allow a decrease of feature sizes and to propose models that describe the molecular organization within the microphase-segregated structures. A recently developed way to achieve this is through a new class of low molecular weight, discrete block copolymers (dispersity of 1). In this contribution, Meijer, Palmans and co-workers designed and synthesized a new class of discrete-length block co-oligomers comprising of oligodimethylsiloxane (oDMS) and oligomethylene (oM). By employing differential scanning calorimetry and small-angle X-ray scattering it was shown that all block co-oligomers exhibit microphase separation into well-ordered lamellar morphologies, driven by the crystallization of the oM blocks. Pre-melting order-order transitions were found for a number of block co-oligomers, resulting in an alternation of the oM crystal packing and in changes of the overall microphase-segregated structure. Importantly, uniform microphase-segregated domains were discovered and among them, one of the smallest domain spacing ever reported (dLAM=5.8 nm), highlighting that the combination of small feature sizes and structural perfection is unique for this type of materials. The authors also elegantly proposed models to describe the molecular organisation within the microphase-segregated structures. This was achieved by evaluating the changes in the lamellar thickness upon alternation of the block co-oligomer architecture. Such type of materials are of critical importance to fundamentally understand the molecular structure and the self-assembly of polymeric materials.

DOI: 10.1039/c8py00355f

 

Tips/comments directly from the authors: 

  1. The large difference between the affinities of oDMS hydride and oDMS with a silanol endgroup toward silica remains a useful tool to separate traces of starting material from the product during the oDMS synthesis. Secondly, the large difference in solubility of short (< 11 repeat units) and long (> 11 repeat units) siloxane oligomers was used frequently to purify the materials.
  1. During the synthesis of the oM blocks, we routinely used a two-stage protection and deprotection protocol of the cyclic ethylene acetal via a dialkyl acetal intermediate.
    Thus, stages involving the free aldehyde could be conducted at room temperature, minimizing the risk of degradation of the aldehyde functionality, which otherwise might led to inseparable side-products (e.g., the result of unwanted condensation reactions).
  1. To ensure good solubility of the oM blocks, a molecular design containing at least one double bond per 30 carbon atoms is advised.
  1. Crystallisation kinetics in oDMS–oM and related systems generally are very fast. In a select number of cases we clearly noticed the benefits of very slow (< 0.1 °C min-1) cooling from the melt in order to decrease the number of defects/increase the size of the crystalline domains in the phase-segregated systems.

 

This article is free to read and download until 26 June

 

Discrete oligodimethylsiloxane–oligomethylene di- and triblock co-oligomers: synthesis, self-assembly and molecular organisation, Polym. Chem., 2018, 9, 2746-2758, DOI: 10.1039/c8py00355f

 

 

About the webwriter 

Dr. Athina Anastasaki is a Web Writer for Polymer Chemistry. She is Athinacurrently a Global Marie Curie Fellow working alongside Professor Craig Hawker at the University of California, Santa Barbara (UCSB). Please, visit this link for more information.

 

 

 

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