Polymer Chemistry Blog

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.

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.

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.

2. 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.

3. 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 31^st August!

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

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.

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.

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 (d_LAM=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.

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.

2. 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).

3. To ensure good solubility of the oM blocks, a molecular design containing at least one double bond per 30 carbon atoms is advised.

4. 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 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.

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.