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.

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.

p-type organic semiconductor polymers can find a use in organic electronics, including organic light-emitting diodes (OLEDs), solar cells, and organic thin-film transistors. These materials offer unique characteristics over inorganic semiconductors such as flexibility and light weight. To maximize their potential, reversible deactivation radical polymerization (RDRP) methodologies are often used with traditional atom transfer radical polymerization and reversible addition/fragmentation chain transfer polymerization dominating in this area. To this end, Hudson and co-workers exploited Cu(0)-RDRP as an effective method for preparing functional acrylate-based polymers with p-type organic semiconductors as side chains. Impressively, all polymers were obtained in high yields (~ 90 %) with low dispersity and high end group functionality while the polymerizations displayed first order kinetics. Both low and high molecular weight polymers could be prepared in a facile manner and the choice of solvent seemed to be crucial to maintain good control over the molecular weight distributions. It should be highlighted that the described technique represents the most simple, low-cost and efficient way to synthesize these materials with improved end group functionality and yields over previous methods. The optical, electrochemical and thermal properties of each of these p-type materials were also carefully investigated with cyclic voltammetry and thermogravimetric analysis revealing the potential for further studies in optoelectronic applications. The Hudson group will now focus on the synthesis of more complex materials, including multiblock copolymers, and subsequently utilize them for optoelectronics.

Tips/comments directly from the authors:  

1. The Cu(0) wire should be prepared immediately before use for best activity, as substantial reductions in polymerization rate are observed when the wire is cleaned and stored.
2. Reducing the relative amount of Cu(0) wire when attempting the synthesis of high molecular weight polymers reduces the polymerization rate, but provides improved control over the polydispersity of the products.
3. For long-term storage all monomers should be stored in the freezer (–10 ºC), but are stable on the bench top under air for 1-2 days.
4. Yields of pure monomers 5a-c are substantially improved when purification is conducted quickly (<5 min) on a short silica column to minimize decomposition; the same urgency is not required for 5d.

Cu(0)-RDRP of acrylates based on p-type organic semiconductors, Polym. Chem., 2018, 9, 1397-1403, DOI: 10.1039/C8PY00295A

This article is free to read until 30 April

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 site for more information.