Archive for the ‘Paper of the Month’ Category

Paper of the month: Vesicles of double hydrophilic pullulan and poly(acrylamide) block copolymers: a combination of synthetic- and bio-derived blocks

Willersinn et al. report the synthesis of double hydrophilic pullulan and poly(acrylamide) block copolymers.

Double hydrophilic block copolymers (DHBC) self-assemble to various structures in aqueous solutions due to a strong difference in hydrophilicity. This is in contrast to amphiphilic block copolymers that self-assemble due to the insolubility of the hydrophobic block in water. In their recent contribution, Schmidt and co-workers were able to extend the principle of double hydrophilic self-assembly to novel polysaccharide-polyacrylamide block copolymers namely pullulan-b-poly(N,N-dimethylacrylamide) (Pull-b-PDMA) and pullulan-b-poly(N-ethylacrylamide) (Pull-b-PEA). The bio-derived pullulan block was obtained via acid catalyzed depolymerisation, while the polyacrylamide homopolymer blocks were synthesized via reversible addition-fragmentation chain-transfer (RAFT) polymerization. Subsequently the blocks were conjugated via copper catalyzed azide alkyne cycloaddition (CuAAC). The presence of formed vesicular structures was investigated via cryogenic electron microscopy (cryo SEM), static light scattering (SLS) and dynamic light scattering (DLS) measurements showing diameters in the range of 200 to 500 nm. Finally, laser confocal scanning microscopy (LSCM) was employed using Rhodamine B staining or Rhodamine B labelled block copolymers to image the formed particle structures. These novel vesicular structures may find use in a wide range of applications including drug delivery.


Tips/comments directly from the authors:

1. The synthesis of block copolymers via CuAAc can be significantly simplified via the utilization of azide functionalized resins. As described in the paper the alkyne functionalized building block is utilized in excess. The resin is added after sufficient reaction time directly to the reaction mixture to react with residual alkyne end functionalized polymer and finally the resin is easily removed via filtration after the reaction. Therefore, no optimization of equivalents or absolute molecular weight data is needed to obtain a pure product without homopolymer contamination.
2. The choice of blocks is crucial for DHBC self-assembly in water. The blocks have to feature a significant difference in hydrophilicity. Moreover, high polymer concentrations are needed to obtain self-assembled structures.
3. For a pure DHBC self-assembly the utilized blocks should not feature LCST or other stimulus responsive behaviour that alters the solubility of the individual blocks. Otherwise the self-assembly might be driven via hydrophobic interactions as with an amphiphilic block copolymer.

Read this exciting research for free until 16/04/2017 through a registered RSC account:

Vesicles of double hydrophilic pullulan and poly(acrylamide) block copolymers: a combination of synthetic- and bio-derived blocks
Polym. Chem., 2017,8, 1244-1254, DOI: 10.1039/c6py02212j

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About the webwriterAthina Anastasaki

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 website for more information.

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Paper of the month: Probing the causes of thermal hysteresis using tunable Nagg micelles with linear and brush-like thermoresponsive coronas

Blackman et al. report the synthesis of thermoresponsive polymers with tunable aggregation numbers in order to study the causes of thermal hysteresis.

 

Thermoresponsive polymers are materials that exhibit a change in their solubility over a temperature range. Thanks to this unique characteristic, these polymers can be used as smart and switchable materials for a wide range of biomedical applications. However, the cloud points upon cooling and heating a thermoresponsive polymer do not coincide because the process of equilibration takes time. The temperature interval between the cloud points upon cooling and heating is called hysteresis and the reversibility of these polymer’s thermal transitions can be influenced by many factors. In order to shine a light to these factors, O’Reilly, Gibson and Blackman synthesized well-defined, responsive amphiphilic block copolymers containing four different thermoresponsive corona blocks and assembled them in micellar structures in aqueous media. All micelles were designed to have tunable aggregation number enabling the study of the effects of altering the corona chemistry, chain confinement and core hydrophobicity on the thermoresponsive behavior, specifically the degree of hysteresis. It was found that higher core hydrophobicities were associated with a higher degree of hysteresis due to differences in core hydration. Linear corona chains capable of forming polymer-polymer hydrogen bonding interactions (e.g. pNIPAM) showed a greater hysteresis than those that could not (pDEAm). Importantly, the authors demonstrates that polymers with a brush-like architecture (pDEGMA and pOEGMA) exhibit irreversible phase transitions at a critical chain density, owing to irreversible nanoscale rearrangement in the precipitated bulk. These findings offer a deeper and more comprehensive understanding of stimuli-responsive self-assemblies and further highlight the complexity of hysteresis in thermoresponsive polymer systems.

 

Tips/comments directly from the authors:

1. It is important to use a Peltier system fitted with a reference cell with an internal temperature probe in order to accurately measure the hysteresis in the cooling curve. We have found that those without this feature typically over-estimate the hysteresis by a few degrees.
2. When synthesizing the pOEGMA-containing diblock copolymers, careful consideration of the RAFT CTA was necessary; the use of 2-Cyano-2-propyl dodecyl trithiocarbonate, as was employed for pDEGMA-containing diblock copolymers, resulted in a significant amount of high molecular weight polymers in the molecular weight distribution. Additionally, lower conversions had to be employed in order to reduce the presence of such species.
3. It was also important to employ multiple angle light scattering coupled with an algorithm such as REPES, which could enable us to investigate the molecular weights of the major fast mode and filter out scattering from spurious slow modes typically found in thermoresponsive polymer systems attributed to non-Brownian interactive behavior.

 

Read this paper for free until March 14th

Probing the causes of thermal hysteresis using tunable Nagg micelles with linear and brush-like thermoresponsive coronas
Polym. Chem., 2017, 8, 233-244.
DOI: 10.1039/C6PY01191H


About the Webwriter:Athina Anastasaki

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 her website for more information.

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Paper of the month: Synthesis of microcapsules using inverse emulsion periphery RAFT polymerization via SPG membrane emulsification

Ishizuka et al. report the synthesis of polymeric microcapsules with a hydrophilic core using SPG membrane emulsification.

Polymeric nano/micro capsules can be used in many applications ranging from biomedicine to cosmetics, food industry and water treatment. However, the current approaches to prepare hollow particles possess a number of drawbacks including tedious and multistep synthesis and low encapsulation efficiency. In order to address these limitations, Zetterlund, Stenzel and co-workers have elegantly combined inverse emulsion periphery reversible addition-fragmentation chain transfer polymerization (RAFT) polymerization (IEPP) with shirasu porous glass membrane (SPG) emulsification to synthesize hollow particles with a hydrophilic core. An amphiphilic macroRAFT agent, namely poly(di (ethylene glycol)methyl ether methacrylate)17-block-poly (methyl methacrylate), was employed to prepare inverse emulsions with a relatively narrow size distribution. Adjusting the SPG membrane pore size from 0.2 to 3 μm allowed access to good control over the droplet size and subsequently the size of the final capsules. This is a significant improvement over conventional emulsions based techniques (e.g. (mini)emulsion or suspension systems) which typically generate either smaller or larger particles. Importantly, the synthesized emulsions were stable at room temperature for over a month maintaining their size and size distribution post polymerization. Finally, the authors demonstrated the successful encapsulation of a water soluble fluorescent dye without any significant effect on the droplet/microcapsule diameter. Therefore, this process showed great potential and can be applied to a wide range of droplets, enabling the encapsulation of various molecules in nano/micro capsules.


Tips/comments directly from the authors:

1. It is important to use a “hydrophobic membrane” to prepare inverse emulsions. Membranes are reusable after cleaning. Typically after each experiment, the membrane is first washed with THF to remove polymer, then with water to remove salt/water soluble guest molecules, subsequently with THF and finally with toluene at least three times to make sure the membrane is wetted with toluene.
2. In order to prepare monodisperse droplets, the continuous phase has to be stirred gently (in this study 160-170 rpm) during emulsification to create a flow which induces the detachment of droplets from the membrane surface and to prevent creaming (generated droplets would settle down without stirring). The stirring speed may differ depending on the specific emulsification condition.
3. During the crosslinking polymerization, a higher stirring rate (500-600 rpm) is typically required to prevent aggregation of particles. The optimum stirring rate may differ depending on the size of the reaction vessel and/or volume of the reaction mixture.

Read this exciting research for free until 25/02/2017 through a registered RSC account:

Synthesis of microcapsules using inverse emulsion periphery RAFT polymerization via SPG membrane emulsification
Polym. Chem., 2016, 7, 7047-7051, DOI: 10.1039/c6py01584k

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About the webwriterAthina Anastasaki

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 this website for more information.

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Paper of the month: SET-LRP of acrylates catalyzed by a 1 penny copper coin

Aksakal et al. report the Cu(0)-mediated living radical polymerization of acrylates using a British penny coin.

Cu(0)-mediated living radical polymerization, typically referred to as single electron transfer living radical polymerization (SET-LRP), is a versatile tool for the synthesis of advanced materials. The groups of Becer and Resmini have further highlighted this versatility by reporting the SET-LRP of acrylates catalyzed by a penny copper coin. Impressively, a wide range of hydrophobic and hydrophilic monomers were successfully polymerized yielding well-defined polymers with low dispersities, near-quantitative conversions and high end group functionality. The scope of the system was subsequently expanded to include the synthesis of star polymers through the core first approach. Interestingly, the authors used two type of coins, the first one (issued in 1971-1992) consisting of 97% of copper and the second one (issued after 1992) consisting of 6% of copper. Both coins exhibited near identical polymerization results. A series of polymerizations targeting different degrees of polymerization were also conducted, all proceeding with very good control over the molecular weight distributions. In comparison to traditional Cu(0)-wire systems, British penny coins have the additional advantage of prohibiting the induction period, which is typically observed for many SET-LRP reactions. Finally, the scalability of these polymerizations up to 50 gram scale was also successful and thus demonstrating an economic, efficient and easily accessible catalyst for SET-LRP of various acrylic monomers.


Tips/comments directly from the authors:

Comments:

In this study, we provide direct evidence that the traditionally used Cu(0)-wire can be replaced with a copper coin, regardless to its year of issue. To avoid any induction period, this method can be employed for the synthesis of well-defined polymers of acrylates with both linear and star shaped initiators.


Tips:

  1. Since the coins are usually contaminated due to prior circulation, we suggest for reproducibility purposes a quick rinse with a freshly prepared HCl before the polymerization.
  2. As mentioned in the manuscript, both penny coins issued before and after 1992 exhibit near identical polymerization results. However, the coins issued after 1992 consist of 94% steel and are magnetic. Due to this, it should be taken into account that the magnetic stirrer can occasionally spin out of its axis. Therefore, we suggest the use of narrow and long Schlenk tube to avoid any splashing of the polymerization mixture.
  3. Due to the reactivity of the polymerization mixture, samples for both NMR and GPC should be diluted immediately in order to avoid errors during kinetic sampling.
  4. The inhibitor of monomers can be easily removed by passing over a plug of basic aluminium oxide. Due to the high viscosity of the OEGA480 monomer, larger volumes can be diluted in a volatile solvent to decrease viscosity. Evaporation of the solvent will provide the inhibitor-free monomer.

Read this exciting research for free until 25/11/2016 through a registered RSC account:

SET-LRP of acrylates catalyzed by a 1 penny copper coin
R. Aksakal, M. Resmini and C.R. Becer
Polym. Chem.
, 2016, 7, 6564-6569
DOI: 10.1039/C6PY01295G

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About the webwriterAthina Anastasaki

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 this website for more information.

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Paper of the month: Regioselective cross metathesis for block and heterotelechelic polymer synthesis

Radlauer et al. report the regioregularity of ring-opening metathesis polymerization and cross metathesis reactions for the synthesis of block and heterotelechelic materials.

Among the two distinct olefin metathesis polymerization methods, namely acyclic diene metathesis (ADMET) polymerization and ring opening metathesis polymerization (ROMP), ROMP possesses the advantage of proceeding by a chain growth mechanism and has been demonstrated to deliver polymers with an R substituent every 8 carbons from 3-substituted cyclooctene (3RCOE) monomers. Although secondary metathesis such as cross metathesis (CM) can occur between chains during ROMP, when the selectivity of 3RCOE monomers was examined, no studies on the CM were conducted. Towards this direction, Hillmyer and co-workers determined the CM of 3RCOE derivatives to be both regio- and stereoselective, making only a small fraction of t-TT and t-HH errors. As only a marginal increase in errors over time was observed, it was concluded that the system reached an equilibrium between the formation and fixing of errors. This stereo- and regioselectivity can be extended by combining polymers with different degrees of polymerization, or by combining polymers with different R substituents leading to the formation of multiblock or statistical copolymers depending on how long the CM is allowed to proceed. In the latter case, differential scanning calorimetry (DSC) confirms the transition from two Tgs to one Tg corresponding to a multiblock and a statistical copolymer, respectively. Additionally, the location of end groups from an asymmetric chain transfer agent can be controlled, thus allowing access to predominantly heterotelechelic oligomers or polymers despite the CM reactions that can occur between the chains.

Tips/comments directly from the authors:

  1. In our experience if a polymer turned brown, it was generally caused by the presence of deactivated Grubbs catalyst. To remove this contaminant at the end of the polymerization, the polymer was dissolved in chloroform and stirred with carbon black. Subsequent filtration and reprecipitation of the polymer generally yielded clear and colorless materials.
  2. To obtain SEC data that was more easily interpreted, we used adjusted ratios of the polymers of different degrees of polymerization in the polymer-polymer cross metathesis experiments. Though the 1:1 scenario still showed immediate CM to an average N, the resolution between the two peaks was quite poor.

Any experiment involving the acetal-containing chain transfer agent required that each reaction component be filtered over basic alumina to avoid deprotection of the acetal moiety.

Read this exciting research for free until 25/11/2016 through a registered RSC account:

Regioselective cross metathesis for block and heterotelechelic polymer synthesis
M.R. Radlauer, M.E. Matta and M.A. Hillmyer,
Polym. Chem., 2016, 7, 6269-6278,
DOI: 10.1039/C6PY01231K

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About the webwriterAthina Anastasaki

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 this website for more information.

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Paper of the month: Well-defined 4-arm hydroxy-terminated PE stars

Zhang et al. report in Polymer Chemistry the synthesis of well-defined polyethylene-b-polycaprolactone and polyethylene-b-polymethylmethacrylate star copolymers.

Star homo- and copolymers have always been the centre of attention thanks to their interesting solution and self-assembly properties in comparison to their linear counterparts. Among the available strategies of synthesizing star polymers, the “core first” approach is of significant importance although polyethylene-based (PE) stars via this method have been rarely reported. Towards this end, Hadjichristidis and co-workers have prepared a 4-arm OH-terminated polyethylene macroinitiator in three steps via cyclic hydroboration/polyhomologation/oxidation reactions.

The successful synthesis has been confirmed by both nuclear magnetic resonance (NMR) and gel permeation chromatography (GPC) analysis. The hydroxyl groups of the star macroinitiator were subsequently utilized to initiate the ring-opening polymerization (ROP) of ε-caprolactone with the GPC confirming a clear shift to higher molecular weights while retaining narrow molecular weight distributions.

In order to demonstrate the potential of this approach, the hydroxyl groups have also been esterified to yield eight initiating sites capable of enabling the atom transfer radical polymerization (ATRP) of methyl methacrylate (MMA). Pleasingly, a monomodal distribution and a quantitative shift to higher molecular weights were observed by GPC, suggesting the efficiency of the reaction.

As such, this strategy revealed the compatibility of polyhomologation with other controlled/”living” polymerization techniques and thus allowing access to the synthesis of novel and well-defined materials.

Tips/comments directly from the authors:

  1. An excess of thexylborane should be used to ensure all divinyl groups are reacted (cyclic hydroboration), otherwise the tetrafunctional initiator will be contaminated with difunctional.
  2. Unreacted thexylborane should be quenched with methanol before the addition of ylide since it will initiate polyhomologation too towards linear PE.
  3. Temperature higher than 80 oC should be used for ROP and ATRP to ensure good solubility of polyethylene.



Read this exciting research for free until 30/10/2016 through a registered RSC account:

Well-defined 4-arm stars with hydroxy-terminated polyethylene, polyethylene-b-polycaprolactone and polyethylene-b-(polymethyl methacrylate)2 arms
Z. Zhang, Y. Gnanou and N. Hadjichristidis
Polym. Chem., 2016, 7, 5507-5511
DOI: 10.1039/C6PY01090C

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About the webwriterAthina Anastasaki

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 this website for more information.

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Paper of the month: Quantitative end-group functionalization of PNIPAM from aqueous SET-LRP via in situ reduction of Cu(II) with NaBH4

Gavrilov et al. report the quantitative and in situ functionalization of PNIPAM in aqueous media.

One major issue in aqueous copper mediated approaches is the hydrolysis of the halide end group which significantly compromises the end group fidelity of the resulting materials and therefore limits post polymerization modifications. In this current contribution, Monteiro and co-workers have carefully investigated the kinetics of hydrolysis of poly(N-isoproplyacrylamide) (PNIPAM) obtained via a new single electron transfer (SET) polymerization method to reduce Cu(II) directly and quantitatively to Cu(0). It was shown that the rate of hydrolysis is independent of the molecular weight of the polymer and the copper content in solution and reaches completion in approximately 15 hours.

In order to circumvent the hydrolysis issue, the authors elegantly conducted an in situ azidation of the bromine end group resulting in the quantitative transformation of the bromine end groups in a matter of 30 seconds, as indicated by matrix assisted laser desorption ionization time of flight mass spectrometry (MALDI-ToF-MS). This is quite a remarkable achievement given that organic solvents facilitate the same process in much longer reactions times, ranging from 10 to 24 hours. The fast reaction rate can be attributed to the enhanced solubility of sodium azide in water and the greater ion pair separation that facilitates nucleophilic attack.

In an attempt to further confirm the data obtained by MALDI-ToF-MS, the azide terminated PNIPAM was subsequently coupled to alkyne functional polymers (after purification) with coupling efficiencies greater than 97%. Thus, it was demonstrated that this approach can not only overcome the hydrolysis issues but also allow rapid synthesis of functional materials.

Tips/comments directly from the authors:

  1. The method of addition of the reactants is important, and by varying the order of addition the polymerizations rates can vary.
  2. It is important to note that the NaBH4 is hygroscopic, and care should be taken to store it in a water-free environment as errors can arise from weighting wet reagents.
  3. We believe that quantitative information can be obtained from the molecular weight distribution using the SEC/LND-model. This is especially important when determining quantitative coupling information of end-groups at high polymer molecular weights, as conventional techniques such as NMR and MALDI-ToF lose sensitivity. We use the weight distribution (i.e. w(M)) as the total weight of the reactants and products remains constant over the reaction; thus, the area under the w(M) vs M curve is the same before and after the reaction.

Read this exciting research for free until 30/09/2016 through a registered RSC account:

Quantitative end-group functionalization of PNIPAM from aqueous SET-LRP via in situ reduction of Cu(II) with NaBH4
M. Gavrilov, Z. Jia,, V. Percec and M.J. Monteiro
Polym. Chem., 2016, 7, 4802-4809
DOI: 10.1039/C6PY00968A

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About the webwriterAthina Anastasaki

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).

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Paper of the month: Investigation into the photolytic stability of RAFT agents and the implications for photopolymerization reactions

McKenzie et al. report the stability of a wide range of RAFT agents during photopolymerization.

The use of external stimuli to mediate the polymerization process has recently received significant attention with light being one of the most popular stimuli mainly due to its natural abundance and the possibility for spatiotemporal control. Photopolymerizations involving reversible addition fragmentation chain transfer (RAFT) have been widely investigated and studied exhibiting impressive characteristics such as fast reaction rates, good spatiotemporal control, and high-end group fidelity.

However, a report on the stability of these RAFT agents has been clearly missing from the literature. Qiao and co-workers recently discovered an initiator-free photopolymerization where the RAFT agent is activated by a blue LED. Following this work, they have investigated the photolytic stability of a range of RAFT agents under blue light irradiation. Careful NMR studies regarding the initiation process and the induction period revealed that the photopolymerization reaction is strongly dependent on the structure of the employed trithiocarbonates (TTCs).

Degradation studies under polymerization relevant conditions showed that photolytic degradation of TTCs with more labile R-groups is observable within the reaction time scale up to 12% for a cyanosubstituted tertiary fragmenting group. On the contrary, when less stable (i.e. primary and secondary) R-group-derived radicals are employed, no degradation is detected.

Two main conclusions can be derived from these studies. Firstly, under identical photochemical conditions, the polymerization of acrylates will lead to higher end group fidelity polymers when compared to the polymerization of methacrylates. In addition, the induction period is dependent on the ability of the RAFT agent to fragment photolytically. As such, this work significantly contributes towards the understanding of the RAFT mechanism and side reactions during photopolymerization processes.

Tips/comments directly from the authors:

  1. The rate of photolysis, although demonstrated here under blue light irradiation (λmax ~ 460 nm) of constant intensity (ca. 1.5 mW/cm2), is likely strongly dependent on both the wavelength and intensity of the employed light source.
  2. Less stable (i.e. faster fragmenting) RAFT agents can be used with acrylate type monomers with minimal degradation due to conversion of the fragmenting species from tertiary to secondary during initiation.
  3. The rate of photopolymerization is also dependent on the initial RAFT agent concentration, and hence the targeted degree of polymerization.
  4. Trithiocarbonates are also more hydrolytically stable than many dithiobenzoates, so these photopolymerization reactions are also amenable to aqueous reaction conditions.



Read this exciting research for free until 31/08/2016 through a registered RSC account:

Investigation into the photolytic stability of RAFT agents and the implications for photopolymerization reactions
T. G. McKenzie, L. P. da M. Costa, Q. Fu, D. E. Dunstan and G. G. Qiao
Polym. Chem., 2016, 7, 4246-4253
DOI: 10.1039/C6PY00808A

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About the webwriterAthina Anastasaki

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).

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Paper of the month: Bespoke cationic nano-objects via RAFT aqueous dispersion polymerisation

Williams et al. utilise addition-fragmentation chain transfer aqueous dispersion polymerisation for the synthesis of bespoke cationic nano-objects directly in water.

Polymerisation-induced self-assembly (PISA) via reversible addition-fragmentation chain transfer (RAFT) polymerisation enables the direct and efficient formation of various diblock copolymer morphologies (e.g. spherical micelles, work-like micelles, vesicles etc.) in aqueous solution. Here the first block is selected to be water-soluble, while the growing second block is water-insoluble and hence drives in situ self-assembly. This versatile approach can be conducted at much higher copolymer concentrations than traditional block copolymer self-assembly based on post-polymerisation processing.

Now Williams and co-workers report the synthesis of a range of cationic diblock copolymer nano-objects utilising a judicious binary mixture of chain transfer agents, namely non-ionic poly(glycerol monomethacrylate) (PGMA) and cationic poly[2-(methacryloyloxy)ethyl trimethylammonium chloride] (PQDMA) and using poly(2-hydroxypropyl methacrylate) (PHPMA) as the hydrophobic core-forming block. Systematic variation of the PQDMA mol fraction and the mean degree of polymerisation of the core-forming PHPMA block enabled the formation of well-defined spheres, worms or vesicles that remain cationic over a wide pH range.

Interestingly, higher cationic character led to the formation of kinetically-trapped spheres; this is because more effective electrosteric stabilisation prevents sphere-sphere fusion. In addition, using 5 mol% PQDMA stabiliser enabled preparation of a 12.5% w/w cationic worm gel that exhibited a zeta potential of +20 mV and a storage modulus of 137 Pa, as demonstrated by variable temperature rheology studies. This worm gel proved to be thermoresponsive: it underwent reversible degelation on cooling from 25 °C to 5 °C. Finally, such cationic gels exhibited weak antimicrobial activity towards the pathogen Staphylococcus aureus.

Tips/comments directly from the authors:

  1. It is really important to map out a detailed phase diagram for the reliable and reproducible identification of pure copolymer phases. This is particularly true for the elusive worm phase, since this occupies relatively narrow phase space.
  2. Using pairs of stabiliser blocks is a powerful and versatile means of tuning the copolymer morphology. If a wholly cationic stabiliser is used, only spheres can be obtained. However, using a binary mixture of a non-ionic and a cationic stabiliser allows access to cationic spheres, worms or vesicles. This is because the non-ionic stabiliser dilutes the charge density within the coronal layer. If maximum cationic character is desired, then the ionic block should have a higher degree of polymerisation than the non-ionic block. This will enable it to protrude from the layer of non-ionic stabiliser chains and influence the electrophoretic footprint of the diblock copolymer nano-objects.
  3. When diluting thermoresponsive worm dispersions to the relatively low concentrations typically used for TEM or DLS analysis, it is important for dispersions to be stored at ambient temperature. This is because the thermoresponsive degelation behaviour becomes irreversible below a certain critical copolymer concentration. Thus storing highly dilute (< 1 %) dispersions in a refrigerator at 4-5 °C simply leads to kinetically-trapped spheres – worms are no longer reformed on returning to ambient temperature under these conditions.



Read this exciting research for free until 31/07/2016 through a registered RSC account:

Bespoke cationic nano-objects via RAFT aqueous dispersion polymerisation
M. Williams, N. J. W. Penfold, J. R. Lovett, N. J. Warren, C. W. I. Douglas, N. Doroshenko, P. Verstraete, J. Smets and S. P. Armes
Polym. Chem., 2016, 7, 3864-3873
DOI: 10.1039/C6PY00696E

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About the webwriterAthina Anastasaki

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).

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Paper of the month: Thermoresponsive hydrogels from triblock copolymers

Despax et al. report the synthesis of triblock copolymers and their application as thermoresponsive hydrogels.

c6py00499g

Temperature responsive gelators can benefit a wide range of biomedical applications and typically comprised of triblock copolymers with a central hydrophilic block and terminal blocks that undergo a hydrophilic to hydrophobic transition at a specific temperature. However, typical ABA triblock copolymers obtained from commercially available monomers require concentrations of at least 50-100 g L-1.

Harrisson, Destarac and co-workers have managed to circumvent this by synthesizing high molecular weight triblock copolymers via low temperature reversible addition-fragmentation chain-transfer (RAFT) gel polymerization. The targeted triblock copolymers were based on polydimethylacrylamide (PDMA) as the long central hydrophilic block and poly(N-isopropylacrylamide) (PNIPAM) as the shorter terminal blocks and the gel formation was initially demonstrated via vial-inversion tests.

Two different molecular weight triblock copolymers were tested with the PDMA block varying from 58 kg mol-1 to 421 kg mol-1 showing self-supporting gels at 30 g L-1 and 6 g L-1 concentration respectively, which is already a significant improvement over previously reported materials. As the vial-inversion test is subject to experimental variations, a more objective measure of the effect of the temperature was obtained from the evolution of the storage and loss moduli of aqueous polymer solutions.

For the lower molecular weight polymer, a two-step transition consisting of an initial thickening of the solution at the lower critical solution temperature (LCST) of PNIPAM occurred followed by gel formation at 38–39 °C requiring a minimum concentration of 20 g L-1. For the longer polymer, only the second transition was observed; gel formation occurred at 40-45 °C with a minimum concentration of 4 g L-1. With a storage modulus of only 0.1 Pa however, this gel is likely too soft for practical use.

In an attempt to improve the mechanical properties of the gels, 2-acrylamido-2-methylpropanesulfonic acid was also incorporated (20 mol% of DMA) resulting on the formation of self-supporting gels at 2 g L-1, an order of magnitude improvement over previously-reported ABA copolymers. These results approach the performance obtained from exotic polymers such as polyisocyanopeptides.

Tips/comments directly from the authors:

  1. High monomer concentrations are helpful to obtain high molecular weights. However, the polymerization of acrylamides is very exothermic so it is important not to exceed 30 wt%.
  2. As very low initiator concentrations are used, it is important to thoroughly degas all solutions prior to polymerization.
  3. Take care to exclude any air bubbles from the solution when carrying out rheology measurements.



Read this exciting research for free until 03/07/2016 through a registered RSC account:

Low concentration thermoresponsive hydrogels from readily accessible triblock copolymers
L. Despax, J. Fitremann, M. Destarac and S. Harrisson
Polym. Chem., 2016, 7, 3375-3377
DOI: 10.1039/C6PY00499G
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About the webwriterAthina Anastasaki

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).

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