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Paper of the month: pH-Sensitive nanogates based on poly(L-histidine) for controlled drug release from mesoporous silica nanoparticles.

18 Feb 2016

Bilalis et al. report the design and synthesis of novel pH-sensitive nanogates based on poly(L-histidine) from mesoporous silica nanoparticles.

The development of novel drug delivery materials necessitates the combination of the knowledge from different scientific fields, including organic and inorganic chemistry. Among the wide range of hybrid organic/inorganic materials, mesoporous silica nanoparticles (MSNs) have attracted considerable attention thanks to their unique characteristics such as high surface area, large specific volume, controllable pore diameter, facile surface functionalization and nontoxicity. On the other hand, polypeptide-coated silica-based systems, including poly(L-histidine) (PHis), have shown great promise in preventing untimely release of drugs and as such the combination of PHis and MSNs can provide an excellent template for the design of advanced drug delivery systems for controlled release applications. To this end, Iatrou, Bilalis and co-workers have exploited surface-initiated ring-opening polymerization (ROP) to synthesize novel pH-sensitive poly(L-histidine)-grafted mesoporous silica nanoparticles through an amino-functionalized MSN intermediate. The successful grafting of the homopolypeptide chains from the surface of MSNs was demonstrated by Fourier Transform-infrared spectroscopy (FT-IR), transmission electron microscopy (TEM) and thermogravimetric analysis (TGA) while size exclusion chromatography (SEC) confirmed the controlled character of the polymerization. Dynamic light scattering (DLS) and zeta potential analysis were also employed to ascertain the pH-responsive nature of the polypeptide-gated MSNs. In addition, drug loading and release studies were performed in order to verify the role of the grafted PHis chains as pH-sensitive nanogates for the MSN pores utilizing the model anticancer drug doxorubicin (DOX). DOX was efficiently loaded within the nanochannels of the hybrid MSN@PHis (~90%) and the drug entrapment and release pattern were proved to be pH-dependent with exert stability at physiological pH. The combination of the positive characteristics of MSNs and poly(L-histidine) enables the described materials as promising drug nanocarriers with potential in vitro and in vivo applications.

Tips/comments directly from the authors:

It is really important to strictly maintain the reported time of reaction during the synthesis of MSNs using TEOS. Longer or less time of reaction will lead to larger or smaller nanoparticles, respectively.
It should be noted that the functionalization of the surface of MSNs with APTES was conducted before the removal of CTAB so as to avoid the grafting of PHis chains from the MSN nanopores.
When following the reported functionalization procedure, a LiOH solution must be used in order to remove HCl traces from the amino groups of MSNs after the removal of CTAB.
The loading procedure of DOX into the MSN nanopores should take place at acidic pH (3.0). In that way the maximum drug encapsulation is ensured, because the PHis nanogates are in an opened state (fully protonated and thus hydrophilic).

pH-Sensitive nanogates based on poly(L-histidine) for controlled drug release from mesoporous silica nanoparticles by P. Bilalis, L.-A. Tziveleka, S. Varlas and H. Iatrou, Polym. Chem., 2016, 7, 1475-1485, DOI: 10.1039/C5PY01841B

Dr. Athina Anastasaki is a Web Writer for Polymer Chemistry. She is currently an Elings fellow working alongside Professor Craig Hawker at the University of California, Santa Barbara (UCSB). Please, visit her webpage for more information.

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Paper of the month: Lights on! A significant photoenhancement effect on ATRP by ambient laboratory light

25 Jan 2016

Zhang et. al. report a significant photoenhancement effect on classical ATRP and ATRP derivatives by ambient laboratory light.

Since the introduction of atom transfer radical polymerisation (ATRP) by Matyjaszewski and Sawamoto, a large diversity of other copper adjuncts have attracted considerable interest including activator regenerated by electron transfer (ARGET)-ATRP, activator generated by electron transfer (AGET)-ATRP, initiators for continuous activator regeneration (ICAR)-ATRP and single electron transfer living radical polymerisation (SET-LRP). Recently photoinduced copper radical polymerization has also drawn significant attention as CuBr₂, typically referred to as deactivating species, can be reduced in situ generating the active CuBr species in the presence of UV or visible light. In this contribution, Jordan and co-workers investigated on whether the typical laboratory light would have any considerable impact on standard ATRP reactions. Interestingly, the vast majority of the ATRP techniques (with the exception of ARGET-ATRP) demonstrated a remarkable photoenhancement effect by ambient light, originated from common fluorescent lamps. It was observed that when less copper complex is utilized for the polymerizations, a stronger influence of the ambient light in the monomer conversion is evident and this effect was significant even in the presence of additional reducing agents. As a general rule, it was concluded that the slower the polymerization is, the more pronounced the effect of the ambient laboratory light can be. As it was proved that it makes a difference if one is performing an ATRP reaction with the hoods on and off, the effect of the laboratory light on the polymerization can no longer be neglected and the authors encourage the report of the light conditions in typical ATRP experiments in order to ensure the reproducibility.

Tips/comments directly from the authors:

Comments:
In this study, we provide conclusive evidence that common laboratory light especially originating from fluorescence lamps has a significant impact on ATRP. This is most probably one main reason why reproducibility of ATRP reactions are not as it should be which impairs further development of controlled radical polymerization techniques. As shown, the impact of light is different for the different ATRP recipes and will also strongly vary with the:

1. Type of complex formed regarding the metal and the ligands;
2. Quantity/concentration of metal complex formed and surely;
3. Type of illumination (natural light, type of fluorescent lamps installed in the laboratory and light intensity in the reaction vial). Strong influence were found for the “hood light illumination” (see Fig. 1 for description and Fig. 2 for experimental data) which was also very surprising for us. We therefore suggest to check the type of light bulbs/source of light, consider their emission spectra and resulting intensity at the location of the reaction and possibly provide these details in the experimental section to ensure reproducibility.

Tips:
1. The various metal complexes used in ATRP are most probably photosensitive but to a different extent. Thus, the influence of laboratory light will vary.
2. Perform the ATRP reactions always under the same light settings with the same light source and provide details (type of lamp, light intensity at each location of the reaction) in the experimental section.
3. Especially modern fluorescent lamps have a higher emission in the blue range and thus may have a stronger/other influence upon conversion/kinetics of the ATRP. Check the emission spectra of the lamps installed in the laboratory and especially in the chemical fume hoods.

Lights on! A significant photoenhancement effect on ATRP by ambient laboratory light by Tao Zhang Dan Gieseler and Rainer Jordan, Polym. Chem., 2016, 7, 775-779

Dr. Athina Anastasaki is a Web Writer for Polymer Chemistry. She is currently an Elings Fellow working alongside Professor Craig Hawker at the University of California, Santa Barbara (UCSB).Visit her webpage for more information.

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Paper of the month: Solvent-free mechanochemical synthesis of a bicyclononyne tosylate: a fast route towards bioorthogonal clickable poly(2-oxazoline)s

23 Dec 2015

Glassner et. al. report a new solvent-free mechanochemical synthesis of a bicyclononyne tosylate.

The strain promoted azide-alkyne cycloaddition of cyclooctynes (SPAAC) is a popular bioorthogonal reaction that enables the tracking of biomacromolecules in living systems. Among the various cylooctynes that have been developed for SPAAC reactions, derivatives of bicyclo[6.1.0]non-4-yne (BCN) have attracted considerable interest for a number of reasons including their low hydrophobicity and ability to undergo fast cycloadditions not only with azides but also with nitrones. At the same time, poly(2-oxazoline)s (PAOx) represent a promising class of polymers for biomedical applications that can easily incorporate clickable end-groups by employing functional initiators or terminators. In this paper, Hoogenboom and co-workers present the first mechanochemical synthesis of a BCN tosylate (BCN-OTs) initiator via solvent-free reaction conditions utilizing high speed vibration milling instead of manual grinding. The BCN-OTs was subsequently used for the cationic ring opening polymerization (CROP) of 2-ethyl-2-oxazoline (EtOx) yielding a well-defined polymer and demonstrating controlled/living polymerization features such as narrow molecular weight distributions and high chain end fidelity. This represents a rapid route to prepare defined BCN functionalized PEtOx that can be used for bioorthogonal strain-promoted conjugation reactions. This was successfully demonstrated for the synthesis of a PS-b-PEtOx copolymer and a PEtOx-protein conjugate. As such, BCN-OTs may find potential applications as intermediate in the synthesis of functional BCN derivatives and BCN-PAOx. In addition, the ball-milling methodology utilized for this study is a promising tool for the synthesis of other unstable/highly reactive tosylates.

Tips/comments directly from the authors:

1) KOH and K₂CO₃ should be finely grounded and dried in a vacuum oven prior to use.

2) During the synthesis of BCN-OTs, removal of solvents has to be performed at ambient temperature (turn of the heating bath of the rotary evaporator).

3) BCN-OTs has to be used immediately for the next step because of its limited stability at ambient conditions.

Solvent-free mechanochemical synthesis of a bicyclononyne tosylate: a fast route towards bioorthogonal clickable poly(2-oxazoline)s by M. Glassner, S. Maji, V. de la Rosa, N. Vanparijs, K. Ryskulova, B. De Geest and R. Hoogenboom, Polym. Chem., 2015, 6, 8354-8359

Dr. Athina Anastasaki is a Web Writer for Polymer Chemistry. She is currently a Warwick (UK)/ Monash (Australia) research fellow working under the Monash Alliance. Visit http://haddleton.org/group-members for more information.

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Paper of the month: Synthesis and characterization of TEMPO- and viologen-polymers for water-based redox-flow batteries

25 Nov 2015

Janoschka et al. report TEMPO and viologen containing polymers for water-based redox-flow batteries.

Redox-flow batteries (RFBs) provide an inexpensive, safe and long lasting approach towards the development of stationary large-scale energy storage. Typically, RFBs employ redox- active materials that dissolve in either aqueous or organic solutions, although many of the existing systems challenge the chemical stability of installed battery components and impose a safety risk. Vanadium salts in sulphuric acid consist the most thoroughly studied system and when aqueous solutions are additionally employed further advantages can be achieved including low corrosivity, high safety and affordable size-exclusion membranes. In this current contribution, Schubert and co-workers present the screening of two series of redox-active polymers that can potentially find application in water-based polymer redox-flow batteries (pRFBs). The rheological and electrochemical properties of both viologen and TEMPO containing polymers have been investigated in details. The viologen-homopolymer poly(1-methyl-1’-(4-vinylbenzyl)-[4,4’-bipyridine]-1,1’-diium dichloride) was found to exhibit good solubility in water (even in the reduced state) and a redox potential of -0.4 V (vs Ag/AgCl), demonstrating its suitability as an anode material. On the other hand, the TEMPO-polymer showed reduced solubility in aqueous solution and thus requiring a solubilizing comonomer. Although poly(ethyleneglycol)-based comonomers and methacrylamide were subsequently tested, the unfavourable LCST and the requirement of high mole fractions respectively led to the investigating of a third alternative. Pleasingly, poly(4-methacryloyloxy-2,2,6,6-tetramethylpiperidine-1-oxyl-co-2-(methacryloyloxy)-N,N,N-trimethylethane ammonium chloride) proved to be a suitable cathode material for pRFBs when a choline moiety was used as the solubilizing unit. In summary, the presented work paves the way for the production of economical energy storage devices that are safe, metal free and utilise all-organic raw materials.

Tips/comments directly from the authors:

1. It should be noted that the redox-active polymer solutions can be used straight from the reaction vessel and without further purification for the designated application in pRFBs.

2. The TEMPO-polymers are excellent surfactants and prone to foam formation. The authors highly encourage the reader to come up with ideas for potential applications of this redox-active, radical-bearing surfactant.

3. When preparing polymers for pRFBs the molar mass of the polymer should be tailored in order to fit the size-exclusion membrane (dialysis membrane) used in the battery. The dispersity Đ should be low to ensure good rheological properties.

Synthesis and characterization of TEMPO- and viologen-polymers for water-based redox-flow batteries, by T. Janoschka, S. Morgenstern, H. Hiller, C. Friebe, K. Wolkersdörfer, B. Häupler, M.D. Hager and U.S. Schubert, Polym.Chem., 2015, 6, 7801-7811

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Paper of the month: Micro-patterned polymer brushes by a combination of photolithography and interface-mediated RAFT polymerization for DNA hybridization

27 Oct 2015

Cimen et al. report the fabrication of micro-patterned polymer brushes for DNA attachment and subsequent hybridization.

The covalent immobilization of DNA probes has been extensively studied over the past ten years. Among others approaches, the binding of protected organic molecules on the silicon surfaces followed by the deprotection of the attached groups is perhaps one of the most popular routes. However, the harsh conditions typically utilized for the deprotection have a detrimental effect on the quality of the surface and as such alternative approaches have also been exploited including growth of brushes from a range of substrates.

In this paper, Caykara and Cimen investigated the micro-patterning of poly(6-azidohexylmethacrylate) [poly(AHMA)] brushes via a combination of photolithography and interface-mediated reversible addition-fragmentation chain transfer (RAFT) polymerization for DNA hybridization. Upon immobilization of the RAFT agent on the Si surface followed by the attachment of 2-(2-carboxyethylsulfanylthiocarbonylsulfanyl)propionic acid (TTC5) (photoresist-coated substrate), poly(AHMA) brushes were grown via interface-mediated RAFT polymerization. AFM was used to determine the thickness of the polymer brush which was found to approach the average thickness of a uniform (AHMA) brush film. Propargylamine was subsequently used to functionalize the azide groups of poly(AHMA) via a click reaction as confirmed by X-ray photoelectron spectroscopy (XPS). The micro-patterned poly(AHMA) brushes with amine pendant groups were then chemically modified in order to function as selective adsorption sites for DNA hybridization. In order to form a non-active background, the probe DNA molecules were successfully coupled onto the micro-patterned poly(AHMA) brushes, as shown by AFM and ellipsometry. The probe DNA modified surfaces were subsequently incubated in the presence of complementary DNA and the successful hybridization of complementary DNA molecules on the micro-patterned poly(AHMA) was confirmed by confocal microscopy and AFM. This simplified and straightforward approach provides a platform for the immobilization of complicated structures on silicon surfaces with potential applications in biosensing and the production of novel materials.

Tips/comments directly from the authors:

1. 11-amino-1-undecene (AUD) cannot be attached directly to the hydrogen terminated silicon surface because of the free amino functional groups. So, in this study, the amino group was protected by the t-butyloxycarbonyl group (t-BOC). The protection reaction is easily performed but during the vacuum purification of the product, control of the temperature and pressure is of vital importance. Gentle heating and constant vacuum pressure must be carried out to prevent the decomposition of t-BOC-AUD.

2. Si-H surface can be easily oxidized at atmosphere. So, rinsing of Si-H surface and coating of t-BOC-AUD must be performed immadiately in a glove box.

3. In the photolithography process, coating of the photoresist was carried out under yellow light. Until removing of the unexposed areas, the surfaces must be kept under safe light.

4. To avoid loss of resolution during the UV exposure, obtaining a thin and homogenous coating on the silicon surface is extremely important. Therefore, the silicon wafers should be cleaved into uniform square or rectangle pieces before the lithography process and the surfaces must be modified homogeneously.

Micro-patterned polymer brushes by a combination of photolithography and interface-mediated RAFT polymerization for DNA hybridization, by D. Cimen and T. Caykara, Polym. Chem., 2015, 6, 6812-6818

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Paper of the month: Fe(III)-mediated ICAR ATRP in a p-xylene/PEG-200 biphasic system: facile and highly efficient separation and recycling of an iron catalyst

28 Sep 2015

Zhang et al. report a highly efficient separation and recycling of an iron catalyst in a p-xylene/PEG-200 biphasic system exploiting Fe(III)-mediated ICAR ATRP.

Atom transfer radical polymerization (ATRP) is a well-established polymerization protocol which allows access to the facile preparation of well-defined materials. As copper is considered an unwanted contamination in some applications, a significant attention has been directly towards the investigation of ATRP catalyst separation and recycling. However, most of the recycling studies are conducted with copper catalysts neglecting other catalytic species such as iron which are less toxic, abundant and biocompatible. Inspired by the successful application of biphasic systems in organic synthesis, Cheng, Zhang and co-workers utilized a PEG-200/p-xylene biphasic system to afford a thermo-regulated phase-separable catalysis (TPSC) via Fe(III)-mediated initiators for continued activator regeneration ATRP (ICAR ATRP). Although PEG-200 and p-xylene are immiscible at ambient temperature, they become homogeneous when heated to 70 °C. Upon commencement of the polymerization, followed by a subsequent cooling period, the reaction mixture separates in two phases. The PEG-200 phase includes the catalyst complex and could be re-used 10 times while still maintaining high catalyst activity while the p-xylene layer contains well-defined polymers with less than 4 ppm of catalyst. Importantly, the versatility and robustness of this protocol was demonstrated by the polymerization of a large diversity of monomers, including methacrylates, acrylates and styrene. In all cases, narrow molecular weight distributions (Ð <1.27) were obtained while high end-group fidelity was verified through successful chain extension experiments that confirmed the “living”/controlled nature of the system. This novel strategy complements previous studies in the field and clearly shows a trend of using alternative metals for controlled polymerizations while at the same time recycling the catalyst to minimize cost and purification steps.

Tips/comments directly from the authors:

1. Iron catalysts have unique advantages over copper catalysts from the view point of catalyst abundancy, biocompatibility and toxicity. Therefore, iron catalysts are better candidates than others for the synthesis of polymeric materials, especially those used for biomedical applications, by the ATRP method.

2. For this Fe(III)-mediated ICAR ATRP, it should be noted that choosing a facile and highly efficient separation biphasic TPSC system for the features of homogeneous catalysis at high temperatures (polymerization temperature) and phase separation at room temperature is important.

3. In this system iron catalyst complexes can be separated and recycled in situ more than 10 times. However, a small amount of PEG-200 may dissolve in p-xylene, as a consequence, we can add some fresh PEG-200 to keep a more efficient TPSC strategy.

4. The organic phase (p-xylene layer with the resultant polymers) can be transferred at room temperature by simple decantation and washed with p-xylene in recycling procedure.

Fe(III)-mediated ICAR ATRP in a p-xylene/PEG-200 biphasic system: facile and highly efficient separation and recycling of an iron catalyst, by B. Zhang, X. Jiang, L. Zhang, Z. Cheng and X. Zhu, Polym. Chem., 2015, 6, 6616-6622

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Paper of the month: Efficiency assessment of single unit monomer insertion reactions for monomer sequence control: kinetic simulations and experimental observations

25 Aug 2015

Haven et al. describe the efficiency of single monomer insertion via both kinetic simulations and experimental observations.

So-called sequence controlled materials have recently received considerable interest due to the precise and freely selectable order of monomers in a monodisperse chain. Such materials exhibit the precision of the peptides in all aspects and differentiate this approach from the synthesis of multiblock copolymers, where a significant dispersity (albeit <1.10 in many occasions) is displayed. Herein, Junkers and co-workers provide an in depth elucidation of the crucial factors that should be taken into account when performing single unit monomer insertion (SUMI) reactions. Both modelling and experimental data confirm that isolated yields of each insertion are comparatively low when going beyond the third monomer addition and as such, even lower yields must be expected for further monomer insertions. Kinetic simulations have shown that most reaction conditions play only a minor role for the success of the insertions and thus, a wide range of conditions can be applied for the synthesis of such materials. Moreover, the effect of the chain-length dependency on the SUMI reactions has also been critically evaluated. Importantly, the carefully optimized conditions obtained from microreactor experiments and kinetic modelling has been subsequently applied to upscale the SUMI reactions in a mesoflow reactor. Although the facile access to such materials demonstrates the pathway towards future developments in the synthesis of longer sequence controlled oligomers, the challenge remains whether oligomers with chain length above 5 will also be available

Tips/comments directly from the authors:

For Single Unit Monomer Insertion reactions (SUMIs), product yield optimization is by stopping the reaction after exactly one monomer equivalent consumption. The reaction rate, thus radical initiator concentration, temperature and overall monomer conversions play a minor role; SUMIs can thus be performed within few minutes.
To study the yield of a SUMI reaction, one needs to distinguish isolated yield from the yield in the crude product mixture. Practically, isolated yields are very dependent on the efficiency of the product isolation method. Yields from the crude can be obtained by careful calibration of mass spectra intensities.
As long as monomers with more or less equal reactivities are chosen, a yield of ~50% is the theoretical maximum.
Evaluation of experimental yields under optimized conditions show that the yield decreases with increasing length of the sequence-defined oligomers. This effect is attributed to a strong chain-length dependency of the monomer propagation rate coefficients.
For upscaling of SUMI reactions, micro- and mesoflow reactors offer the perfect solution.

Efficiency assessment of single unit monomer insertion reactions for monomer sequence control: kinetic simulations and experimental observations, by J.J. Haven, J. Vandenbergh, R. Kurita, J. Gruber and T. Junkers, Polym. Chem., 2015, 6, 5752-5765.

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Paper of the month: From drug to adhesive: a new application of poly(dihydropyrimidin-2(1H)-one)s via the Biginelli polycondensation

29 Jul 2015

Zhao et al. describe the potential of the Biginelli polycondensation to improve metal bonding strength.

Recently, the introduction of efficient reactions (e.g. click reactions, Diels-Alder reactions) to polymer chemistry aiming to synthesize new condensation polymers with improved properties and characteristics has attracted considerable interest. However, on many occasions, the requirement of extensive synthetic steps (e.g. for monomer synthesis) in combination with the usage of unsafe reagents (e.g. toxic, explosive etc.) necessitates the need for the development of alternative strategies that will provide access to large scale functional materials. Towards this goal Tao and co-workers employed the Biginelli polycondensation reaction to polymerize a novel difunctional monomer consisting of benzaldehyde and beta-keto ester groups, to yield poly(dihydropyrimidin-2(1H)-one)s (poly(DHMPs) (M_n ~ 22000 g mol^-1) within 1 h. Interestingly and in contrast with the small molecular DHMPs, the Biginelli polycondensates presented metal bonding capability and adhesive properties (up to ~ 2.8 Mpa). In addition, when monomers containing more functional groups were employed, stronger tensile shear strength was demonstrated indicating that the cross-linked polymer network has a positive effect on the bonding strength (3.9-5.9 Mpa). Finally, the efficiency of the reaction was further demonstrated by performing the reaction using an electric heat gun. The preparation of the monomers on a large scale, the facile nature of the polymerisation and the excellent metal bonding performance paves the way for the synthesis of new functional polymers.

Tips/comments directly from the authors:

1. When finishing the polycondensation of monomer AB, the final polymer should be precipitated into cold water immediately, because the viscosity of they system will increase after cooling down (solid can even be formed), which will make the purification challenging.

2. When precipitating the polymer, adding some base (e.g. NaOH) in water is helpful for the removal of the acetic acid. In addition, strong stirring during the precipitation is necessary to achieve satisfactory purification.

3. During the metal bonding test, evenly heating could improve the efficiency of bonding. An open system for the volatilization of water will also enhance the glue effect.

4. As the monomer A2B2 is viscous, gentle heating prior to use is helpful for measuring.

From drug to adhesive: a new application of poly(dihydropyrimidin-2(1H)-one)s via the Biginelli polycondensation, by Y. Zhao, Y. Yu, Y. Zhang, X. Wang, B. Yang, Y. Zhang, Q. Zhang, C. Fu, Y. Wei and L. Tao, Polym. Chem., 2015, 6, 4940-4945.

Dr. Athina Anastasaki is a Web Writer for Polymer Chemistry. She is currently a Warwick (UK)/ Monash (Australia) research fellow working under the Monash Alliance. Visit http://haddleton.org/group-members for more information.

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Paper of the month: Preparation of complex multiblock copolymers via aqueous RAFT polymerization at room temperature

07 Jul 2015

Martin et al. describe the synthesis of multiblock copolymers via RAFT polymerization at room temperature.

The preparation of high-order multiblock copolymers in a one pot process using reversible addition-fragmentation chain transfer (RAFT) is highly attractive due to the rapid polymerization rates, the achievement of quantitative conversions for each block, the lack of purification steps between the intermediate monomer additions (time effective and resource effective) and the narrow molecular weight distributions that can be attained. The “secret” of this success is the choice of high k_p acrylamide monomers and water as the reaction solvent allowing for full monomer conversion to be obtained whilst employing very low amounts of free radical initiator. However, applying this polymerization methodology to lower k_p monomers, such as methacrylates and acrylates, can be problematic as a higher concentration of initiator will be required to “push” the reaction towards completion and side reactions are also likely to occur at elevated temperatures, typically employed for this polymerization protocol. High temperatures are also disadvantageous for the polymerization of monomers that possess an LSCT upon polymerization (e.g. N-isopropyl acrylamide (NIPAM)).

In this work, a new approach to prepare multiblock copolymers via room temperature aqueous RAFT is presented. The authors implement the suitable redox couple tert-butyl hydroperoxide/ascorbic acid (TBHP/AsAc) to polymerize both acrylate and acrylamide multiblock copolymers with low dispersity values and high end-group fidelity exemplified by several in situ chain extensions. The challenge of working with slightly lower k_p monomers is also highlighted as both low and high molecular weight tailing is evident for the acrylate multiblocks whilst only gradual broadening and no shoulders are observed for the acrylamide analogues. A multiblock that consists of both acrylamide and acrylate monomers has also been targeted, demonstrating the versatility of the approach to obtain more complex multiblock structures. The main advantage of this work is the possibility of incorporating thermoresponsive blocks (e.g. NIPAM and diethyl acrylamide (DEA)) in the multiblock composition and limiting side reactions, often occurring at higher temperatures. Another interesting feature of this paper is the ability to control the polymerization of more hydrophobic (and not water soluble) monomers (e.g. methyl acrylate and ethylene glycol methyl ether acrylate) which were also successfully included in the multiblock sequence with a high degree of control. In contrast with multiblock copolymers obtained via single electron transfer living radical polymerization (SET-LRP) or atom transfer living radical polymerization (ATRP) methods, RAFT offers the additional advantage of allowing the incorporation of acidic monomers in the multiblock composition. The next challenge to tackle is to polymerize even lower k_p monomers (such as methacrylates) with a similar level of control.

Tips/comments directly from the authors:

1) When working at room temperature the viscosity is high. To avoid a loss of MW control after few block extension, a strong stirring for a good homogenization of the polymerization mixture is necessary.

2) The mixing of acrylate and acrylamide blocks is rather difficult because of the difference in reactivity of each family of monomers. Normally poly(acrylates) are better reinitiating group than poly(acrylamides) and therefore should be polymerized first.

3) In the redox initiator couple tert-butyl hydroperoxide/ascorbic acid (TBHP/AsAc), we found that a lot less AsAc could be used than that reported, and yet still give efficient initiation. In fact we observed that AsAc could act as an inhibitor of the radical polymerization. We are currently investigating the optimal ratio of the reducing and oxidizing agents.

4) The water soluble hydroxyethylacrylate monomer needs to be carefully purified because of diacrylate contamination, which is responsible for the shoulder observed at high MW on SEC analyses.

Preparation of complex multiblock copolymers via aqueous RAFT polymerization at room temperature, by L. Martin, G. Gody and S. Perrier, Polym. Chem., 2015, 6, 4875-4886

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Paper of the Month: Rapid synthesis of ultrahigh molecular weight and low polydispersity polystyrene diblock copolymers by RAFT-mediated emulsion polymerization

26 May 2015

Truong et al. describe the synthesis of ultrahigh molecular weight and low polydispersity polystyrene diblock copolymers by RAFT-mediated emulsion polymerisation.

aper of the month: Rapid synthesis of ultrahigh molecular weight and low polydispersity polystyrene diblock copolymers by RAFT-mediated emulsion polymerization

Ultra high molecular weight (UHMW) polymers have always been an ambitious target for synthetic polymer chemists. However, synthesising these materials using controlled radical polymerisation is challenging due to high levels of bimolecular termination and chain transfer to monomer that impede the growth of polymer chains. Monomers with higher propagation rate coefficient (k_p) such as acrylamides and acrylates (and even methacrylates) have been successfully polymerised up to 10⁶ g mol^-1 but lower k_p monomers (e.g. styrene) typically present broad molecular weight distributions when high molecular weight (e.g. 10⁶ g mol^-1) is targeted.

Truong et al. envisaged that a high polymerisation rate for the polymer chains would be required in order to produce well-defined UHMW polystyrene, whilst at the same time termination and side reactions would need to be minimised. The authors addressed this by employing the use of novel macromolecular chain transfer agents (CTA) in reversible addition fragmentation chain transfer polymerisation (RAFT)-mediated emulsion polymerisation. N-hydroxyethyl acrylamide (HEAA) and poly(ethylene glycol) methyl ether acrylate (PEGA) were copolymerised under judiciously selected reaction conditions to identify the most effective macrostabiliser for the emulsion polymerisation. The choice of these monomers proved crucial for the polymerisation, with PEGA conferring excellent antifouling characteristics while HEAA improves the water solubility of the macromolecular CTA and reduces partitioning of the macro-stabilisers within the styrene droplets and/or at the water/droplet interfaces. Under carefully optimised conditions, UHMW polystyrene of 10⁶ g mol^-1 could be obtained with relatively low dispersity values (<1.4) and unimodal molecular weight distributions even at near-quantitative conversions (>90%). Moreover, UV-Vis analysis confirmed the presence of the CTA, further suggesting that the reversible-deactivation radical polymerisation mechanism remained operative even to this very high conversion and molecular weight. Another interesting feature of this work is the linear relationship between particle size and molecular weight in this system, which seems to depart from the packing parameter theory, typically used to explain morphology transformations during emulsion or aqueous dispersion polymerisations. TEM analysis in all samples revealed uniformly spherical nanoparticles, even when the chain length of the polystyrene core was well above the threshold for the worm and vesicle formation. These experiments suggest that the packing parameter theory cannot be applied for all polymerisation-induced self-assembly systems and that further theoretical models are potentially required to fully understand the equilibrium morphology of soft nanoparticles.

In short, this article has overcome a longstanding challenge in the synthesis of UHMW polymers and created a new nanomaterial which offers great potential in numerous applications.

Summary points from the authors:

4,4′-Azobis(4-cyanopentanoic acid (ACPA) (free radical initiator) completely dissolves in water only after stirring for about 30 min. The stock ACPA solution should be made up fresh and not be stored for later use.
To avoid the loss of styrene monomer (by evaporation) during polymerisation, there is no need to keep the emulsion polymerisation under the continuous flow of nitrogen.
Samples prepared for dynamic light scattering measurements were not filtered, and as such filtration might enable a further reduction in the particle size distribution.
The molar ratio of HEAA to PEGA in the macromolecular CTAs was optimised at 1 to 1. A higher molar ratio of HEAA to PEGA results in aggregation when targeting ultra-high molecular weight polystyrene. A lower molar ratio of HEAA to PEGA results in a higher portion of macromolecular CTAs partitioning within the styrene droplets.

Rapid synthesis of ultrahigh molecular weight and low polydispersity polystyrene diblock copolymers by RAFT-mediated emulsion polymerization by Nghia P. Truong, Marion V. Dussert, Michael R. Whittaker, John F. Quinn and Thomas P. Davis, Polym. Chem., 2015, 6, 3865-3874

Dr. Athina Anastasaki is a is a guest web-writer for Polymer Chemistry. She is currently a Warwick University (UK) and Monash University (Australia) research fellow working under the Monash Alliance. Visit the Haddleton group’s website for more information.

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Archive for the ‘Paper of the Month’ Category

Paper of the month: pH-Sensitive nanogates based on poly(L-histidine) for controlled drug release from mesoporous silica nanoparticles.

Paper of the month: Lights on! A significant photoenhancement effect on ATRP by ambient laboratory light

Paper of the month: Solvent-free mechanochemical synthesis of a bicyclononyne tosylate: a fast route towards bioorthogonal clickable poly(2-oxazoline)s

Paper of the month: Synthesis and characterization of TEMPO- and viologen-polymers for water-based redox-flow batteries

Paper of the month: Micro-patterned polymer brushes by a combination of photolithography and interface-mediated RAFT polymerization for DNA hybridization

Paper of the month: Fe(III)-mediated ICAR ATRP in a p-xylene/PEG-200 biphasic system: facile and highly efficient separation and recycling of an iron catalyst

Paper of the month: Efficiency assessment of single unit monomer insertion reactions for monomer sequence control: kinetic simulations and experimental observations

Paper of the month: From drug to adhesive: a new application of poly(dihydropyrimidin-2(1H)-one)s via the Biginelli polycondensation

Paper of the month: Preparation of complex multiblock copolymers via aqueous RAFT polymerization at room temperature

Paper of the Month: Rapid synthesis of ultrahigh molecular weight and low polydispersity polystyrene diblock copolymers by RAFT-mediated emulsion polymerization

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