Polymer Chemistry Blog

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

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 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.

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.

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 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.

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 M_n 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

Dr. 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.

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

Dr. 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.

The development of “element-block polymers” (defined as a minimum functional unit composed of heteroatoms) and the exploration of controlled methods for their electronic properties is crucial to design new tactics for advanced optical materials. Chujo, Tanaka and co-workers significantly contributed to this direction by developing a new concept for controlling the solid-state luminescence properties of polymers without changing the chemical components. This was achieved by synthesizing a series of alternative copolymers composed of boron diiminate with variable connection points to the comonomer units. The optical measurements revealed that the polymers possessed aggregation-induced emission (AIE) properties originating from boron diiminate. Importantly, the emission colour was varied from green to orange by altering the connection points in the film samples. Careful mechanistic studies suggested that the electron-donating and accepting abilities of the boron diiminate unit can be switched by selecting the connection points. As a result, the chain transfer character in the emission properties of the polymers was changed. Further theoretical investigations proposed that boron diiminate acts as a strong electron-acceptor in the excited state when the comonomers were connected to either one or both of the phenyl groups on the nitrogen atoms. On the contrary, when the comonomers were linked at the phenyl groups on the carbon atoms, a much weaker electron-donating property was induced. These findings pave the way for the design of advanced polymeric materials with precision function tunability without changing the chemical components.

Tips/comments directly from the authors:  

1. Conventional conjugated polymers can show emission only in solution, meanwhile these polymers can present intense emission even in the film. Solid-state luminescent properties were originated from AIE ability of the boron complex.
2. Usually, drastic changes in chemical structures are essential for colour regulation of conjugated polymers. In this boron complex, originating from significant localization of highest occupied molecular orbitals in the boron complex, optical properties can be readily modulated by altering connecting points. Therefore, various types of luminescent materials can be obtained with the same chemical components.
3. The monomers and polymers can be obtained through the several synthetic steps without special techniques. The intermediates and products showed high stability under ambient conditions. The purification for the polymers was simply performed with re-precipitation, and pure materials having good film-formability were successfully obtained.

Luminescent color tuning with polymer films composed of boron diiminate conjugated copolymers by changing the connection points to comonomers, Polym. Chem., 2018, 9, 1942-1946, DOI: 10.1039/C8PY00283E

This paper is free to read until 30 May

About the web writer

Dr. 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.