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Phosphonic Acid-Functionalized Diblock Copolymer Nano-Objects via Polymerization-Induced Self-Assembly: Synthesis, Characterization, and Occlusion into Calcite Crystals
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† Dainton Building, Department of Chemistry, The University of Sheffield, Brook Hill, Sheffield, South Yorkshire S3 7HF, U.K.
‡ School of Chemistry, University of Leeds, Woodhouse Lane, Leeds LS2 9JT, U.K.
*E-mail [email protected] (S.P.A.).
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Macromolecules

Cite this: Macromolecules 2016, 49, 1, 192–204
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https://doi.org/10.1021/acs.macromol.5b02212
Published December 23, 2015

Copyright © 2015 American Chemical Society. This publication is licensed under CC-BY.

Abstract

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Dialkylphosphonate-functionalized and phosphonic acid-functionalized macromolecular chain transfer agents (macro-CTAs) are utilized for the reversible addition–fragmentation chain transfer (RAFT) dispersion polymerization of benzyl methacrylate (BzMA) at 20% w/w solids in methanol at 64 °C. Spherical, worm-like, and vesicular nano-objects could each be generated through systematic variation of the mean degree of polymerization of the core-forming PBzMA block when using relatively short macro-CTAs. Construction of detailed phase diagrams is essential for the reproducible targeting of pure copolymer morphologies, which were characterized using transmission electron microscopy (TEM) and dynamic light scattering (DLS). For nano-objects prepared using the phosphonic acid-based macro-CTA, transfer from methanol to water leads to the development of anionic surface charge as a result of ionization of the stabilizer chains, but this does not adversely affect the copolymer morphology. Given the well-known strong affinity of phosphonic acid for calcium ions, selected nano-objects were evaluated for their in situ occlusion within growing CaCO3 crystals. Scanning electron microscopy (SEM) studies provide compelling evidence for the occlusion of both worm-like and vesicular phosphonic acid-based nano-objects and hence the production of a series of interesting new organic–inorganic nanocomposites.

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Introduction

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It is well-known that phosphorus-based polymers have a wide range of potential applications. They bind metals ions, (1, 2) adsorb strongly onto metal oxide surfaces (3) and biominerals, (4, 5) can serve as proton exchange membrane material, (6) allow the design of halogen-free flame-retardant materials, (7-9) and can exhibit excellent biocompatibility. (10, 11) Literature examples include organophosphorus polymers based on methacrylate-type (12, 13) or acrylamide-type (14) dialkylphosphonates. Anionic polyelectrolytes are also accessible directly either by polymerization of the free acid form of phosphate, (15) phosphonate, (6, 13) or phosphinate (16) monomers or via hydrolysis of dialkylphosphonate polymers. (13, 14) DNA is a particularly important example of a naturally occurring anionic polyphosphate. (17) However, phosphorus-based polymers can also exhibit cationic (e.g., phosphonium-based polymers (18)) or zwitterionic character (e.g., poly(2-(methacryloyloxy)ethyl phosphorylcholine), or PMPC). (19, 20) Both materials are of considerable interest in the field of biomaterials. (21, 22)
Recent progress in the development of controlled/living polymerization methods such as atom transfer radical polymerization (ATRP) (23) and reversible addition–fragmentation transfer (RAFT) polymerization (24) has facilitated the design of a wide range of well-defined controlled-structure polymers. Many examples of well-defined PMPC-based diblock copolymers have been prepared by ATRP (25, 26) and RAFT. (27, 28) Of particular relevance to the present study, (1-ethoxycarbonyl)vinyl dimethylphosphate was homopolymerized via ATRP and also copolymerized using either polystyrene or poly(methyl methacrylate) macroinitiators. (29) While control over the polymerization of this dialkylphosphate monomer was rather poor—incomplete conversions or broad molecular weight distributions (Mw/Mn > 1.60) were obtained depending on the choice of catalyst—subsequent hydrolysis produced the corresponding phosphoric acid-based diblock copolymers. In contrast, RAFT polymerization of either 2-(acryloyloxy)ethyl phosphate or 2-(methacryloyloxy)ethyl phosphate yielded well-defined diblock copolymers when employing 2-(acetoacetoxy)ethyl methacrylate as the second block. (30) Divinyl impurities present in both phosphate monomers limited the molecular weight of the first block to 20 000 g mol–1, otherwise extensive cross-linking was observed.
As far as we are aware, vinylphosphonic acid (VPA) was the first example of a monomer bearing an unprotected phosphonic acid to be polymerized directly via RAFT polymerization. (31) As yet, however, only diblock copolymers with relatively low molecular weights have been obtained using this approach. Considerably higher molecular weights have been reported for the ATRP of 4-vinylbenzyl diethylphosphonate and relatively narrow molecular weight distributions were achieved in this case. (6) Both deprotection (to generate the free phosphonic acid groups) and chain extension (from a polystyrene macroinitiator) were demonstrated for this styrene-type monomer. (32) Similarly, well-defined homopolymers and diblock copolymers were obtained when using 2-(acrylamido)ethyl diethylphosphonate, with subsequent hydrolysis producing the corresponding free phosphonic acid-based polymers. (14)
Another phosphorus-based monomer, methacryloyloxymethyl dimethylphosphonate (MP) has recently become commercially available, but attempts to polymerize this monomer via ATRP have only resulted in rather low conversions and relatively low molecular weights, presumably due to strong copper complexation. (33) Optimizing the ATRP formulation by appropriate selection of the initiator/catalyst system and polymerization temperature was required in order to improve control over the MP polymerization. (34) In this context, RAFT polymerization offers a potentially decisive advantage, since it involves an organosulfur-based chain transfer agent rather than a transition metal catalyst. Moreover, phosphonic acid-based homopolymers were recently prepared directly via RAFT polymerization. (13) In principle, a wide range of functional diblock copolymers incorporating either dialkylphosphonate or phosphonic acid repeat units should be accessible using this approach.
Recently, RAFT polymerization has also been exploited in the context of polymerization-induced self-assembly (PISA), which is a highly efficient and versatile method for directly generating diblock copolymer nano-objects at relatively high solids (up to 50% w/w). (35-38) In such formulations, a soluble macromolecular chain transfer agent is extended with a monomer to form a new block, which becomes increasingly insoluble in the reaction medium. The first soluble block confers steric stabilization, while the growing insoluble block drives in situ self-assembly. Varying the relative block volume fractions while conducting such syntheses at various copolymer concentrations allows predictive phase diagrams to be constructed, which are essential for the reproducible generation of pure phases comprising spherical, worm-like, or vesicular diblock copolymer nanoparticles. (39) This PISA approach has been shown to be generic. Various formulations have been reported in aqueous solution, (40-47) lower alcohols, (48-56)n-alkanes, (57-59) toluene, (60) and solvent mixtures (61, 62) that provide access to not only non-ionic (or zwitterionic) nanoparticles (43, 63-66) but also anionic (67, 68) or cationic nano-objects. (69) All of these aforementioned PISA formulations are based on dispersion polymerization, whereby the core-forming monomer is soluble in the reaction medium. There are also numerous examples of heterogeneous formulations based on aqueous emulsion polymerization. (70-77)
There are very few literature examples of PISA syntheses involving phosphorus-based monomers. Such formulations combine the growing interest in phosphorus-based polymers with straightforward access to a range of diblock copolymer nano-objects without requiring post-polymerization processing. Potential applications include flame-retardant coatings, drug delivery formulations, and biocompatible worm gels for 3D cell growth. In 2011, our group reported aqueous formulations based on a zwitterionic PMPC macromolecular chain transfer agent (macro-CTA) for the preparation of diblock copolymer nano-objects. (43, 66) Very recently, Zhang et al. employed 4-diphenylphosphinostyrene as the core-forming monomer to prepare catalytically-active micellar nanoreactors. (78) However, we are not aware of any examples involving anionic phosphate-based monomers. As mentioned earlier, Monge and co-workers recently reported the RAFT solution polymerization of methacryloyloxymethyl dimethylphosphonate (MP) and methacryloyloxymethylphosphonic acid (MPA), (13) which are of potential interest for biomedical applications. In the present work, we describe the PISA synthesis of dialkylphosphonate- and phosphonic acid-functionalized diblock copolymer nano-objects using these two monomers. A range of nano-objects are prepared using a RAFT alcoholic dispersion polymerization formulation and characterized in terms of their size, morphology, and surface chemistry. Occlusion of selected worm-like and vesicular nanoparticles within growing CaCO3 crystals is also briefly studied as a potential route to novel organic–inorganic nanocomposites.

Experimental Section

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Materials

Methacryloyloxymethyl dimethylphosphonate (MP; Specific Polymers, France), glycerol monomethacrylate (GMA; kindly donated by GEO Specialty Chemicals, Hythe, UK), 2-hydroxypropyl methacrylate (HPMA, Alfa Aesar, 98%), 2,2′-azobis(isobutyronitrile) (AIBN; Molekula, Germany), 4,4′-azobis(4-cyanopentanoic acid) (ACVA; Sigma-Aldrich, ≥98%), 1,3,5-trioxane (Sigma-Aldrich, UK), bromotrimethylsilane (TMSBr; Acros, 98%), CaCl2·2H2O (Sigma-Aldrich), and (NH4)2CO3 (Sigma-Aldrich) were used as received. Benzyl methacrylate (BzMA, 96% purity, Sigma-Aldrich) was passed through a column to remove inhibitor prior to its polymerization. 2-Cyano-2-propyl dithiobenzoate (CPDB; Strem Chemicals, UK) had a CTA efficiency of approximately 80%. This efficiency was taken into account for the RAFT syntheses of the various macro-CTAs. Methanol, ethanol, and DMF were all of solvent grade, while anhydrous dichloromethane (DCM) was used for the hydrolysis experiments. Deuterated NMR solvents (CD3OD and d6-DMSO) were purchased from Cambridge Isotopes Laboratories, Inc.

Synthesis of Poly(methacryloyloxymethyl dimethylphosphonate) [PMP] Macro-CTA

The RAFT polymerization of MP was conducted as reported by Canniccioni et al. (13) using a 25% w/w MP solution in DMF and a CPDB/AIBN molar ratio of 3.0. In a typical protocol, the monomer (22.50 g, 108.1 mmol), CPDB (1.813 g, 4.9 mmol), and AIBN (266.3 mg, 1.6 mmol) were dissolved in 73 mL of DMF in a 250 mL round-bottomed flask. After purging with nitrogen for 20 min, the flask was sealed and placed in a preheated oil bath at 70 °C for 3.5 h, before quenching the reaction by cooling with an ice bath. Samples were taken during the course of polymerization for 1H NMR measurements, and the final conversion was determined to be 65%. The crude PMP was purified via precipitation into excess cold diethyl ether (twice) and finally freeze-dried from aqueous solution overnight. A mean degree of polymerization of 24 (or Mn = 5.2 kg mol−1) was calculated via end-group analysis using 1H NMR spectroscopy by comparing the integrated intensity of the two aromatic protons of the CTA at 7.42–7.56 ppm with the five protons assigned to the methacrylic backbone at 0.2–2.4 ppm. DMF GPC analysis indicated apparent Mn and Mw/Mn values of 5.4 kg mol−1 and 1.20, respectively. Two other PMP macro-CTAs were also prepared with higher target mean degrees of polymerization: DMF GPC indicated a Mn of 7.0 kg mol−1 and Mw/Mn = 1.23 for PMP32 and a Mn of 9.5 kg mol−1 and Mw/Mn = 1.22 for PMP42.

Hydrolysis of Poly(methacryloyloxymethyl dimethylphosphonate) [PMP] Yielding Poly(methacryloxymethylphosphonic acid) [PMPA]

To generate the free phosphonic acid form, PMP was hydrolyzed with trimethylsilyl bromide (TMSBr). (13) In a typical reaction, PMP24 (10.0 g) was dissolved in anhydrous DCM (100 mL), followed by addition of TMSBr (4 equiv per phosphonate group). The reaction mixture was stirred for 3 h at 20 °C, and then the solvent was removed under reduced pressure. The free phosphonic acid homopolymer, PMPA, was generated by addition of methanol (500 mL) followed by stirring for 1 h at 20 °C. After concentration under reduced pressure, the crude polymer was purified via dialysis against water, followed by freeze-drying overnight. Aqueous GPC analysis of the three PMPA homopolymers using 30% methanol cosolvent at pH 9 gave the following data: PMPA24Mn = 10.1 kg mol−1, Mw/Mn = 1.03; PMPA32Mn = 11.1 kg mol−1, Mw/Mn = 1.03; PMPA42Mn = 12.6 kg mol−1, Mw/Mn = 1.05.

Chain Extension Experiments with PMPA Macro-CTAs

Macro-CTAs were chain-extended with glycerol monomethacrylate (GMA) to examine their blocking efficiencies. RAFT solution polymerizations were conducted at 20% w/w solids at 64 °C for 16 h. In a typical protocol, the PMPA24 macro-CTA (140.5 mg, 31.22 μmol), AIBN (1.0 mg, 6.24 μmol, CTA/AIBN molar ratio = 5.0), GMA (1.00 g, 6.24 mmol), and 1,3,5-trioxane (11.3 mg, 124.87 μmol, GMA/1,3,5-trioxane molar ratio = 50) were dissolved in methanol (4.0 mL). The solution was purged with nitrogen for 20 min, and then the sealed vial was placed in a preheated oil bath at 64 °C for 16 h, before quenching the reaction by cooling with an ice bath. Samples were taken during the course of the polymerization for 1H NMR measurements. For the three different blocking experiments, final GMA monomer conversions were determined to lie between 46 and 73%. The resulting PMPA–PGMA diblock copolymers were precipitated twice from cold diethyl ether and freeze-dried from aqueous solution overnight prior to analysis by aqueous GPC (with 30% methanol cosolvent at pH 9): PMPA24–PGMA140Mn= 18.3 kg mol−1, Mw/Mn = 1.05; PMPA32–PGMA146Mn = 20.0 kg mol−1, Mw/Mn = 1.07; PMPA42–PGMA92Mn = 19.5 kg mol−1, Mw/Mn = 1.07.

Diblock Copolymer Synthesis via Alcoholic Dispersion Polymerization

Alcoholic RAFT dispersion polymerizations were conducted with both the dialkylphosphonate (PMP) macro-CTAs as well as the phosphonic acid (PMPA) macro-CTAs at 20% w/w solids under identical conditions. In a typical synthesis targeting a diblock composition of PMPA24–PBzMA300, the protocol was as follows: PMPA macro-CTA (37.5 mg, 8.32 μmol), AIBN (0.30 mg, 1.67 μmol, CTA/AIBN molar ratio = 5.0), and BzMA (440 mg, 2.50 mmol) were weighed into a sample vial and dissolved in pre-degassed methanol (1.91 g). This vial was then sealed with a septum and purged using a gentle nitrogen flow for 10 min while cooling in ice to minimize evaporation. Polymerization was initiated by placing the vial in a preheated oil bath at 64 °C. In order to guarantee a high BzMA conversion, the polymerization was conducted at this temperature for 24 h before quenching by exposure to air. In the case of the alcoholic dispersion polymerization syntheses conducted using the PMP macro-CTAs, the final PMP–PBzMA diblock copolymers were also analyzed using DMF GPC (see Tables 1 and 2 for details).

CaCO3 Precipitation via the Ammonia Diffusion Method (79)

For the crystallization experiments, 50 μL of a 1.0% w/w aqueous copolymer dispersion was diluted with water (2.50 mL) and a 3 mM CaCl2 solution (2.50 mL) in a 30 mL sample vial, giving a final 0.01% w/w copolymer dispersion in a 1.5 mM CaCl2 aqueous solution. A glass slide that had been pre-cleaned with piranha solution was placed on the base of the vial, and then the samples were transferred to a desiccator previously charged with (NH4)2CO3 powder (5.0 g). Crystallization was allowed to proceed for 24 h at 20 °C, and then the glass slides were removed from solution, washed with deionized water and ethanol, and finally air-dried. A second set of crystallization experiments was also conducted using a 0.005% w/w aqueous copolymer dispersion. Control experiments were conducted in the absence of added copolymer.

Characterization

NMR Spectroscopy

1H NMR spectra were recorded in d6-DMSO for PMP, PMPA and their related diblock copolymers using either a 400 or 250 MHz Bruker spectrometer. A spectrum of the PGMA homopolymer was also recorded in CD3OD. 31P NMR spectra for the PMP and PMPA macro-CTAs were recorded in d6-DMSO using a 250 MHz Bruker spectrometer (400 scans).

Gel Permeation Chromatography (GPC)

GPC analyses of PMP and PGMA homopolymers and PMP–PBzMA diblock copolymers were conducted using two Polymer Laboratories PL gel 5 μm mixed C columns and one PL polar gel 5 μm guard column arranged in series and maintained at 60 °C, followed by a Varian 390 LC refractive index detector. The DMF eluent contained 10 mM LiBr, and the flow rate was 1.0 mL min–1. Calibration was achieved using a series of near-mondisperse poly(methyl methacrylate) standards (ranging from 645 to 2.48 × 106 g mol–1). Molecular weight distributions for the PMPA and PMPA–PGMA diblock copolymers were assessed using aqueous GPC, which comprised an Agilent 1260 Infinity setup fitted with two Agilent PS Aquagel-OH 8 μm columns at 35 °C and a refractive index detector. The eluent was a 10 mM NaH2PO4 pH 9 buffer containing 200 mM NaNO3 and 30% methanol at a flow rate of 1.0 mL min–1. Calibration was achieved using a series of near-monodisperse poly(methacrylic acid) standards ranging from 1.27 to 4.83 × 105 g mol–1.

Dynamic Light Scattering (DLS) and Zeta Potential Measurements

Intensity-average hydrodynamic diameters of the dispersions were obtained by DLS using a Malvern Zetasizer NanoZS instrument. This instrument detects scattered light at an angle of 173° and was equipped with a 4 mW He–Ne laser operating at 633 nm, an avalanche photodiode detector with high quantum efficiency, and an ALV/LSE-5003 multiple τ digital correlator electronics system. Copolymer dispersions of ∼0.20% w/w solids were analyzed using the cumulants method to obtain the hydrodynamic (z-average) diameter and polydispersity index (PDI). Aqueous electrophoresis studies were performed on 0.01% w/w copolymer dispersions using the same instrument equipped with DTS1070 cells. The solution pH was adjusted by the manual addition of 0.01–1.0 M HCl or NaOH, and the background electrolyte was 1 mM NaCl.

Transmission Electron Microscopy (TEM)

Copolymer particle morphologies were assessed using a Philips CM 100 instrument operating at 100 kV and equipped with a Gatan 1k CCD camera. For sample preparation, 8 μL of a diluted copolymer dispersion (∼0.20% w/w) was dropped onto a freshly glow-discharged grid, left for 30 s, and then blotted with filter paper to remove excess solution. Subsequently, staining was performed for 30 s using a 8 μL droplet of 0.75% w/v uranyl formate solution, followed by blotting the excess stain and drying with a vacuum hose. TEM grids were prepared by coating copper/palladium TEM grids (Agar Scientific, UK) with a thin film of amorphous carbon.

Optical Microscopy, Scanning Electron Microscopy (SEM), and Raman Microscopy

The dimensions and morphologies of the CaCO3 (calcite) crystals were assessed using optical microscopy and scanning electron microscopy. Optical microscopy images of crystals grown on glass slides were recorded using a Motic DMBA 300 digital biological microscope with a built-in camera and analyzed using Motic Images 2.0 ML software. For SEM studies, the glass slides supporting the CaCO3 crystals were mounted on SEM stubs using adhesive pads. Imaging of the uncoated samples was performed using a FEI Nova NanoSEM 450 operating at 2 kV. Cross sections of the CaCO3 crystals were also imaged using SEM to investigate the extent of particle occlusion. Such samples were prepared by placing a clean glass slide on top of the glass slide coated with CaCO3 crystals and pressing down while slightly twisting the upper glass slide. This protocol resulted in fracture of the crystals, hence revealing their interior structure. Characterization of the crystal polymorph was conducted via Raman microscopy studies of individual particles using a Renishaw Raman 200 System microscope operating at a laser wavelength of 785 nm.

Results and Discussion

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Scheme 1

Scheme 1. RAFT Solution Polymerization of Methacryloyloxymethyl Dimethylphosphonate (MP) and Subsequent Hydrolysis of the PMP Precursor To Obtain Poly(methacryloyloxymethylphosphonic acid) (PMPA); RAFT Dispersion Polymerization of BzMA in Methanol Using Either the PMP or PMPA Macro-CTA as Steric Stabilizer Produced Spherical, Worm-like, or Vesicular Nano-Objects, Depending on the Precise PISA Formulation
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Synthesis of Poly(methacryloyloxymethyl dimethylphosphonate) and Poly(methacryloyloxymethyl phosphonic acid) Macro-CTAs

Methacryloyloxymethyl dimethylphosphonate (MP) was used for the synthesis of the macro-CTA in order to prepare phosphonic acid-stabilized nano-objects via RAFT dispersion polymerization. The RAFT solution polymerization of this commercially available monomer was recently studied using 2-cyano-2-propyl dithiobenzoate as chain transfer agent. (13) The resulting PMP macro-CTA can subsequently be converted into PMPA via ester hydrolysis (see Scheme 1). Three near-monodisperse PMP macro-CTAs with DPs of 24, 32, or 42 were synthesized in DMF at 70 °C. The corresponding DMF GPC traces are shown in Figure 1a; Mw/Mn values were below 1.25 in each case. Subsequent hydrolysis in dichloromethane with trimethylsilyl bromide and methanol yields the corresponding PMPA in its free phosphonic acid form, as indicated by a shift from 21.6 to 14.0 ppm in the 31P spectrum (Figure 1b and Figure S1). Because of unfavorable column interactions during attempted DMF GPC analysis, the PMPA macro CTAs had to be characterized using aqueous GPC (see Figure S2). These analyses indicated relatively low Mw/Mn values (less than 1.05).

Figure 1

Figure 1. (a) DMF gel permeation chromatography traces obtained for the three PMP macro-CTAs used in this study. (b) 31P NMR spectra recorded for the PMP24 macro-CTA before (red spectrum, top) and after hydrolysis (black spectrum, bottom). This indicates complete hydrolysis of the dimethyl ester groups on the PMP precursor to produce a well-defined PMPA macro-CTA.

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Polymerization Induced Self-Assembly (PISA) in Aqueous and Alcoholic Media

First, the performance of the PMP macro-CTA as non-ionic stabilizer block for the aqueous dispersion polymerization of HPMA was investigated at pH 5. This facilitates comparison with later experiments conducted with the anionic PMPA macro-CTA, which is singly ionized at this pH. (13) Unfortunately, stable colloidal dispersions could not be obtained when targeting a PHPMA block with a degree of polymerization of 200 at 70 °C using ACVA initiator. This behavior was rather unexpected, as each of the PMP macro-CTAs exhibited excellent water solubility and no LCST-like behavior was observed. PISA syntheses were also attempted at 50 °C using AIBA as a low temperature radical source, (64) but again precipitation was observed. Since we have recently developed various robust PISA formulations based on the RAFT alcoholic dispersion polymerization of BzMA, (49, 50, 55, 56) we switched to using either ethanol or methanol as the continuous phase. However, coagulation was again obtained for the former solvent when targeting DPs of 200. It is perhaps noteworthy that complete dissolution of the steric stabilizer macro-CTA required stirring for around 1 h regardless of its molecular weight, which suggests that ethanol is perhaps a rather marginal solvent for PMP. Consequently, we selected methanol as a more polar solvent, which led to significantly faster PMP dissolution under the same conditions.
For kinetic studies of PISA syntheses conducted in methanol, an initial BzMA polymerization was conducted at 64 °C for 24 h at 20% w/w solids (see Scheme 1) using a PMP24 macro-CTA and targeting a final PBzMA block DP of 300. No precipitation was observed for this formulation, which produced a turbid dispersion. More than 99% conversion was attained after 24 h as judged by 1H NMR studies, with almost complete monomer consumption (>97%) being achieved within 13 h (Figure 2a). Moreover, an enhanced rate of polymerization is only observed relatively late in the polymerization—well after micellar nucleation has occurred (see inset in Figure 2a). This differs markedly from previous observations for similar PISA formulations based on RAFT alcoholic dispersion polymerization, where the increase in the rate of propagation coincides with the onset of micellar nucleation. (50, 80) A tentative explanation for this observation might be the relatively small size of the spherical nanoparticles initially formed during this PISA formulation (Dh < 25 nm, see Table S1). This suggests the presence of rather loose, hydrated aggregates and thus perhaps relatively inefficient initial solubilization of the BzMA monomer within the nascent nuclei. For PISA syntheses conducted under appropriate conditions, the copolymer morphology is known to evolve from spheres to vesicles via various intermediate morphologies, including worms and “jellyfish”. (81) When the vesicle dimension (i.e., mean diameter and/or mean membrane thickness) reaches a certain critical value, these aggregates are able to solubilize BzMA. Thus, a significant increase in the rate of BzMA polymerization occurs at around 6 h, as seen in the third stage in the first-order kinetics plot.
Despite this unusual kinetic behavior, the PISA formulation proved to be both efficient and reasonably well-controlled in terms of both copolymer morphology and molecular weight distribution. DLS studies of aliquots extracted during polymerization indicate that vesicles of around 160 nm are formed within 6 h, which suggests that the observed rate enhancement is associated with this morphology, rather than the initial spherical nuclei (see inset in Figure 2a and Table S1). These vesicles eventually attain a final hydrodynamic diameter of approximately 180 nm. A post mortem TEM image of the final copolymer morphology obtained after 24 h is shown in Figure S3b; the vesicular morphology is confirmed, and the estimated mean vesicle diameter is in reasonably good agreement with the DLS studies. Given the non-ionic nature of the PMP macro-CTA used in this set of experiments, DMF-GPC analysis of the corresponding PMP24–PBzMAx diblock copolymers was possible. Selected chromatograms are plotted in Figure 2b. The unimodal GPC traces are symmetric and show no significant tailing due to prematurely terminated macro-CTA chains, indicating high blocking efficiencies. This is supported by the linear evolution of the number-average molecular weight, Mn, while maintaining relatively narrow molecular weight distributions, with Mw/Mn values typically below 1.16 (Figure S3a and Table S1). These observations are consistent with the anticipated pseudo-living character of this RAFT dispersion polymerization. In addition, a high molecular weight shoulder gradually becomes more prominent above 30% conversion. Similar observations were also reported for another alcoholic PISA formulation utilizing BzMA as the core-forming block. (50)

Figure 2

Figure 2. (a) Kinetic data derived from 1H NMR studies of the RAFT alcoholic dispersion polymerization of BzMA (target DP = 300) at 64 °C using the PMP24 macro-CTA at 20% w/w solids in methanol. The inset shows the evolution of the hydrodynamic diameter Dh and the first-order kinetic plot with respect to monomer. (b) The corresponding DMF GPC traces and the associated Mn and Mw/Mn data obtained for selected copolymers on sampling this formulation, indicating its reasonably good controlled/living character (see Table S1 for a more complete set of GPC data).

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Using various PMPx macro-CTAs (x = 24, 32, and 42) and performing all BzMA polymerizations at 20% w/w solids in methanol, we were able to construct a detailed phase diagram by systematically varying the target DP for the core-forming PBzMA block from 50 to 300 (see Figure 3; details of the individual polymerizations are provided in Table 1). The corresponding GPC traces for all of these PMP–PBzMA syntheses are shown in Figure S4 and clearly indicate an evolution of the molecular weight with increasing target PBzMA DP. High blocking efficiencies and relatively low Mw/Mn values were obtained, where these are similar to the data discussed earlier for the kinetic study conducted when targeting PMP24–PBzMA300. However, a relatively low concentration of unreacted macro-CTA can be detected for the PMP42 macro-CTA, which suggests that some degree of premature termination occurred in this particular case. Moreover, the final molecular weight distribution was significantly broader (Mw/Mn = 1.53) for PMP42–PBzMA558 compared to all other polymerizations (Mw/Mn < 1.21). As discussed earlier, a high molecular weight shoulder becomes discernible when targeting PBzMA DPs greater than 100 for all PMP macro-CTAs. In previous studies involving a poly(2-hydroxypropyl methacrylate) core-forming block, a similar shoulder was explained in terms of a relatively low level of dimethacrylate impurity in the monomer, which inevitably leads to light branching when targeting higher DPs. (42) However, this explanation seems to be unlikely for benzyl methacrylate, since there is no reason for such a monomer to contain a dimethacrylate impurity. An alternative explanation may be some degree of termination by combination, which is not unknown for methacrylic monomers. (82)
For all three sets of PMP macro-CTAs the initially formed spheres were relatively small (Dh < 25 nm), as previously observed for a PGMA–PHPMA PISA formulation. (42) Inspecting the phase diagram shown in Figure 3, it is clear that only spherical nanoparticles can be obtained when using the PMP42 macro-CTA. Presumably, this longer block confers additional steric stabilization during the BzMA polymerization and hence prevents sphere–sphere fusion, which is the key first step in the generation of worms and vesicles. Systematically increasing the target PBzMA DP from 50 to 558 leads to a monotonic increase in particle size, with hydrodynamic diameters ranging from 16 to 76 nm, as evidenced by both DLS and TEM (see Table 1 and Figure S5). The larger spheres are believed to be kinetically trapped morphologies. (39)

Figure 3

Figure 3. Phase diagram constructed for PMPx–PBzMAy diblock copolymer nano-objects prepared in methanol at 64 °C at 20% w/w solids. Representative TEM images obtained for the three pure copolymer morphologies: (a) PMP24–PBzMA50 (small spheres), (b) PMP32–PBzMA79 (worms), (c) PMP42–PBzMA300 (large spheres), and (d) PMP24–PBzMA200 (vesicles).

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We recently reported similar findings for a PHPMA–PBzMA RAFT alcoholic dispersion polymerization formulation, when employing a PHPMA macro-CTA with a DP of 63. (50) However, using shorter PMP stabilizer blocks (DP = 24 or 32) enables access to higher order morphologies as well as spheres. Such rich phase behavior was also observed by Zehm and co-workers for the PHPMA–PBzMA PISA formulation on reducing the mean DP of the stabilizer block. (50) In the present study, using the PMP24 macro-CTA only enabled access to pure spheres, vesicles, or mixed phases (Figure 3a,d). However, pure worms occupying a very narrow phase region could be obtained when employing the PMP32 macro-CTA (Figure 3b). Representative TEM images obtained for the entire PMP32–PBzMAy series are shown in Figure S6. Increasing y from 48 to 61 led to partial fusion of spherical nanoparticles to form a mixture of spheres and short worms, with the latter having a distinctive “pearl necklace” appearance. Increasing y by a further seven BzMA units produced longer worms, with only a few spheres present. When y = 79, a free-standing gel comprising well-defined worms was obtained. Longer core-forming blocks (89 < y < 135) produced mixed phases, with vesicular phases becoming increasingly common. Finally, a pure vesicular phase was attained for PMP32–PBzMA146 (and higher y values). DLS studies suggest that increasing the DP of the PBzMA core induces some contraction in the mean vesicle dimensions (from Dh = 158 nm for y = 146 to Dh = 136 nm for y = 300). A similar size reduction was observed for the PMP24–PBzMAy series, where y = 100–300, with hydrodynamic diameters decreasing from 352 to 181 nm (see Table 1). This may be caused by thickening of the vesicle membranes exclusively via inward growth. TEM images recorded for the various PMP32–PBzMAy PISA formulations provide some evidence for this hypothesis. For mixed phases consisting of worms and vesicles, the vesicle membrane is clearly visible, and the vesicles are rather polydisperse (see Figure S6; y = 100, 120, and 135). On further increasing y, the vesicles become somewhat less polydisperse, and their associated lumen volumes are reduced (see Figure S6; y = 146, 200, and 300). Minimization of the vesicular interfacial area (and hence free energy) is the driving force for this phenomenon, which was recently described in detail for an aqueous PISA formulation. (83)
Table 1. Summary of Monomer Conversions, Intensity-Average Particle Diameters, and GPC Data Obtained for a Series of Non-Ionic PMPx–PBzMAy Diblock Copolymers Synthesized at 20% w/w Solids via RAFT Alcoholic Dispersion Polymerization of BzMA in Methanol at 64 °C for 24 Hours (S = Spheres, W = Worms, V = Vesicles)
   GPCDLSTEM
target compositionBzMA % convBzMA DPMn [kg mol–1]Mw/MnDh [nm]PDImorphology
PMP24–PBzMA5099509.81.17240.29S
PMP24–PBzMA58995711.01.15350.43S + W
PMP24–PBzMA65>996511.71.158120.99S + W
PMP24–PBzMA72>997212.41.153120.55S + W + V
PMP24–PBzMA80>998013.31.142380.30W + V
PMP24–PBzMA100>9910015.81.143520.12V
PMP24–PBzMA200>9920028.11.112010.20V
PMP24–PBzMA300>9930034.71.211810.07V
PMP32–PBzMA50964810.61.19170.09S
PMP32–PBzMA65946112.01.20290.26S + W
PMP32–PBzMA70976812.61.17840.28S + W
PMP32–PBzMA80997913.71.182490.31W
PMP32–PBzMA90998914.41.194990.47W + V
PMP32–PBzMA100>9910016.01.153560.44W + V
PMP32–PBzMA120>9912019.31.161670.09W + V
PMP32–PBzMA135>9913520.61.181750.17W + V
PMP32–PBzMA1509714622.51.151580.07V
PMP32–PBzMA200>9920027.71.161520.03V
PMP32–PBzMA300>9930039.21.201360.03V
PMP42–PBzMA50>995014.71.18160.16S
PMP42–PBzMA80>998022.71.17230.12S
PMP42–PBzMA100>9910025.91.18270.13S
PMP42–PBzMA2009819644.11.17530.08S
PMP42–PBzMA300>9930063.11.19630.11S
PMP42–PBzMA60093558109.51.53760.06S
On the other hand, it is not understood yet why vesicles of apparently increasing diameter were observed after approximately 6 h during the kinetic studies (inset in Figure 2a).
We then examined the use of anionic PMPA macro-CTAs as putative steric stabilizers for the RAFT aqueous dispersion polymerization of HPMA. According to Semsarilar and co-workers, strong electrostatic repulsion between adjacent polyelectrolytic stabilizer blocks can impede in situ self-assembly during attempted aqueous PISA syntheses. (67-69) We investigated this hypothesis for our system by targeting a HPMA DP of 300; such an asymmetric diblock copolymer might be expected to form vesicles. However, it proved impossible to obtain pure vesicles for various PISA formulations at pH 5, even when utilizing binary mixtures of non-ionic poly(glycerol monomethacrylate) (PGMA45) with an anionic PMPAx macro-CTA in the presence of added salt to screen the unfavorable electrostatics. Table S2 and Figure S7 summarize this series of experiments, for which a detailed protocol is given in the Supporting Information.
In view of these negative results, RAFT alcoholic dispersion polymerization of BzMA was performed in methanol at 20% w/w solids using each of the PMPA macro-CTAs in turn (see Scheme 1). For this series of experiments, it was envisaged that the significantly lower dielectric constant of methanol (ε = 32.6 at 298 K) compared to water (ε = 78.5 at 298 K) (84) might reduce the unfavorable electrostatic repulsive forces and hence facilitate PISA. A PMPA macro-CTA with a mean DP of 24 was chosen for a kinetic study of the dispersion polymerization of BzMA at 64 °C. When targeting a DP of 300 for the PBzMA block, greater than 99% conversion was achieved after 24 h, as judged by 1H NMR (Figure 4a). A significant rate enhancement was observed, again not at the onset of micellar nucleation but instead when spherical micelles first begin to fuse to form higher order morphologies (see inset in Figure 4a). The apparent first-order rate constant, kapp, for this second stage was a little higher when using the anionic PMPA24 stabilizer (kapp = 8.4 × 10–5 s–1) compared to the non-ionic PMP24 macro-CTA (kapp = 5.5 × 10–5 s–1). This suggests higher partitioning of the nonpolar BzMA monomer within the growing micelle cores in the former case. DLS data are shown for all samples in Figure 4a (see inset) and summarized in Table S3. A representative TEM image of the final vesicular morphology is depicted in Figure 4b: the estimated mean vesicle diameter of approximately 510 nm (based on analysis of 80 particles) is in reasonably good agreement with that indicated by DLS, bearing in mind that the latter technique reports a hydrodynamic diameter and is biased toward larger particles. Unfortunately, DMF GPC analysis of the various PMP24–PBzMAy diblock copolymers was not feasible because of unfavorable copolymer–column interactions. Moreover, the hydrophobic character of the PBzMA block precluded aqueous GPC analysis. In view of these problems, chain extension experiments were conducted for each of the three PMPA macro-CTAs in turn using GMA to produce a water-soluble PGMA block, simply in order to assess blocking efficiencies when polymerizing a second methacrylic monomer. Aqueous GPC traces obtained for these PMPAx–PGMAy diblock copolymers are compared to the respective PMPA macro-CTAs in Figure S2. In each case symmetric monomodal curves were obtained, with pronounced shifts to higher molecular weight relative to the original PMPA macro CTA. All three PMPA–PGMA diblock copolymers exhibited relatively narrow molecular weight distributions (Mw/Mn < 1.07). This indicates high blocking efficiencies for the solution polymerization of GMA. Given that the pseudo-living character of RAFT dispersion polymerization is enhanced compared to solution polymerization, (85) at least comparable (and most likely higher) blocking efficiencies are anticipated under the former conditions.

Figure 4

Figure 4. (a) Kinetic data derived from 1H NMR studies of the RAFT alcoholic dispersion polymerization of BzMA (target DP = 300) using an anionic PMPA24 macro-CTA at 20% w/w solids in methanol at 64 °C. The inset shows the evolution of the hydrodynamic diameter Dh and the first-order kinetic plot with respect to monomer concentration. (b) Representative TEM image of the final vesicular morphology obtained after 24 h (>99% conversion).

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Using the three PMPA macro-CTAs (DP = 24, 32, or 42) to target a range of core-forming PBzMA blocks enables the construction of a predictive phase diagram (Figure 5). A summary of the individual PISA syntheses conducted at 20% w/w solids in methanol at 64 °C is shown in Table 2. In contrast to the phase diagram for the PMP–PBzMA formulation, nonspherical morphologies can be observed even for the PMPA42 macro-CTA. At first sight, this might seem surprising given the anionic nature of the stabilizer chains, which might be expected to impede fusion of the initial spheres (and indeed hinder PISA itself). (68, 69) However, electrostatic repulsive forces are substantially reduced in methanol compared to aqueous media. This hypothesis is consistent with the results obtained for the corresponding PMPA–PHPMA aqueous PISA formulations (see Table S2), which do not readily provide access to pure copolymer phases even when employing binary mixtures of anionic and non-ionic macro-CTAs. The phase diagram for the PMPAx–PBzMAy formulations shown in Figure 5 contains a relatively broad vesicle phase and a somewhat narrower mixed phase region.
Spheres are accessible up to a target PBzMA DP of 35 (Figure 5b), while pure worms can only be obtained when using the PMPA42 macro-CTA (Figure 5a). Furthermore, the anionic PMPA24–PBzMA300 vesicles shown in Figure 5d (Dh from DLS = 529 nm) are significantly larger than the corresponding non-ionic PMP24–PBzMA300 vesicles (Dh from DLS = 181 nm). In principle, a higher packing parameter might be expected to favor the formation of larger vesicles. (86) This therefore suggests that the hydrodynamic volume occupied by an anionic PMPA24 chain in methanol is less than that of a non-ionic PMP24 chain, indicating reduced solvation of the former stabilizer under these conditions. Similarly, increasing the PMPA stabilizer DP for a given core-forming PBzMA DP reduces the mean vesicle dimensions, presumably because of the larger hydrodynamic volume occupied by the PMPA chains (Figure 5c). The PMPA42–PBzMAy series provides access to all three copolymer morphologies, with representative TEM images being shown in Figure S8. As for the non-ionic PMP–PBzMA formulations, there is an apparent reduction in vesicle dimensions when targeting higher degrees of polymerization of the PBzMA block, and these observations are supported by DLS studies (see corresponding entries in Table 2).

Figure 5

Figure 5. Phase diagram constructed for PMPAx–PBzMAy diblock copolymer nano-objects prepared in methanol at 64 °C at 20% w/w solids. TEM images for pure copolymer morphologies: (a) PMPA42–PBzMA55 (worms), (b) PMPA24–PBzMA35 (spheres), (c) PMPA42–PBzMA300 (small vesicles), and (d) PMPA24–PBzMA300 (large vesicles).

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Generation of Organic–Inorganic Hybrid Materials

It is well recognized that naturally occurring biopolymers are intimately involved in the formation of biominerals, (87) where these can not only direct features such as polymorph and morphology but also enhance mechanical properties through occlusion within the crystal lattice. (88, 89) Importantly, this behavior is not restricted to biomacromolecules, and recent work has shown that a range of additives, including submicron latex particles, various anionic diblock copolymer nano-objects, and even small molecules, can become occluded within single crystals of calcite (CaCO3). (90-95, 98) In all cases, successful occlusion is dependent on the additives binding to the surfaces of the growing crystals but remaining dispersed in the crystal growth solution. (93) The PMPA–PBzMA nano-objects synthesized here provide interesting candidates for controlling CaCO3 precipitation.
Table 2. Summary of Monomer Conversions and Intensity-Average Particle Diameters Obtained for a Series of Anionic PMPAx–PBzMAy Diblock Copolymers Conducted at 20% w/w Solids via RAFT Alcoholic Dispersion Polymerization in Methanol at 64 °C for 24 h (S = Spheres, W = Worms, L = Lamellae, and V = Vesicles)
   DLSTEM
target compositionBzMA % convBzMA DPDh [nm]PDImorphology
PMPA24–PBzMA35>9935150.13S
PMPA24–PBzMA459643610.29S + W
PMPA24–PBzMA50>99504210.61S + W + V
PMPA24–PBzMA6094565560.29S + L + V
PMPA24–PBzMA70>99704290.91S + L + V
PMPA24–PBzMA100>991004590.18V
PMPA24–PBzMA200>992004390.14V
PMPA24–PBzMA300>993005290.06V
PMPA32–PBzMA35>9935170.10S
PMPA32–PBzMA429640380.23S + W
PMPA32–PBzMA50>99502270.57S + W
PMPA32–PBzMA56>99565170.68W + L + V
PMPA32–PBzMA7091644050.16W + V
PMPA32–PBzMA72>99723550.24W + V
PMPA32–PBzMA10098983090.23V
PMPA32–PBzMA200>992002820.14V
PMPA32–PBzMA300>993002120.07V
PMPA42–PBzMA359834210.08S
PMPA42–PBzMA5090451740.29W
PMPA42–PBzMA55>99552670.29W
PMPA42–PBzMA7093652480.22W + V
PMPA42–PBzMA72>99722460.21W + V
PMPA42–PBzMA10093931940.16V
PMPA42–PBzMA200>992002640.10V
PMPA42–PBzMA300>993002140.02V
Both phosphate- and phosphonic acid-functionalized surfactants and polymers are well-known to interact strongly with CaCO3 and are widely used as crystal growth inhibitors. (96, 97) The efficiency of occlusion of these additives within calcite was therefore investigated, and the results compared with the corresponding non-ionic PMP–PBzMA nano-objects.

Figure 6

Figure 6. (a) Representative TEM images recorded for non-ionic PMP24–PBzMA300 vesicles and anionic PMPAx–PBzMAy worms and vesicles, obtained via RAFT dispersion polymerization of BzMA at 20% w/w solids in methanol at 64 °C. (b) TEM images for the same copolymer nanoparticles after their transfer from methanol to water via exhaustive dialysis at 20 °C. In all cases the copolymer concentration used to prepare the TEM grids was 0.2% w/w.

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The nano-objects were first transferred from the methanol continuous phase to water via dialysis. TEM analysis of selected nano-objects before and after dialysis indicated no adverse effect on the copolymer morphology (Figure 6), where these findings were supported by DLS studies which showed no change in the mean particle diameter before and after dialysis. These observations are consistent with the relatively high Tg of around 55 °C for the core-forming PBzMA block, which suppresses exchange of copolymer chains between kinetically frozen nano-objects.

Figure 7

Figure 7. Aqueous electrophoresis curves and DLS data obtained for PMPAx–PBzMAy worms and vesicles prepared by RAFT alcoholic dispersion polymerization of BzMA in methanol at 20% w/w solids. Measurements were conducted using 0.01% w/v dispersions in the presence of 10 mM NaCl as background electrolyte.

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In addition to the successful transfer of vesicles into aqueous media (PMPA32–PBzMA300, PMPA24–PBzMA300, and PMP24–PBzMA300), an aqueous worm dispersion was also obtained after dilution of a methanolic worm gel followed by dialysis (PMPA42–PBzMA45, see Figure 6). Aqueous electrophoresis studies of these anionic nano-objects confirmed highly anionic zeta-potentials of −35 to −45 mV across a wide pH range (see Figure 7). Only in relatively acidic media (pH 2) was a modest reduction in zeta potential observed, as expected for a steric stabilizer based on a strong polyacid such as PMPA. Concomitant DLS studies indicated no significant change in particle dimensions from pH 2 to 10, suggesting that colloidal stability was maintained (see Figure 7).
Occlusion experiments were conducted by precipitating CaCO3 in the presence of non-ionic PMP24–PBzMA300 vesicles and the various anionic PMPAx–PBzMAy nano-objects at pH ≈ 9 via a one-pot protocol using the ammonia diffusion method. (79) All experiments were conducted using a stock solution of 1.5 mM CaCl2 and a copolymer concentration of either 0.010% w/w (Figure 8a) or 0.005% w/w (Figure 8b). All dispersions remained colloidally stable in the presence of the CaCl2 solution, which was added at an initial pH of less than 6. Rhombohedral calcite crystals of 10–15 μm were prepared in control experiments performed in the absence of diblock copolymer nano-objects. Figure S9 shows a typical Raman spectrum recorded for calcite crystals grown under these conditions, with the characteristic calcite bands being observed at 155 and 282 (lattice modes), 710 (ν4), and 1085 cm–1 (ν1). (93) Formation of calcite in the presence of non-ionic PMP24–PBzMA300 vesicles at both additive concentrations produced crystals with comparable habits and dimensions to those formed in the control experiments. SEM studies confirmed that the crystals were perfect rhombohedra with well-defined edges, with just a few weakly interacting vesicles located on the crystal surface. These findings were not unexpected, since anionic character is believed to be a prerequisite for successful occlusion. Raman microscopy studies of such crystals formed when using 0.010% w/w copolymer confirmed that the polymorph was indeed calcite (Figure S9).
In contrast, the anionic nano-objects had a stronger influence on the morphology and dimensions of the calcite crystals. Well-defined rhombohedral crystals of 15–20 μm were formed in the presence of 0.005% w/w PMPA24–PBzMA300 and PMPA32–PBzMA300 vesicles (DLS studies indicated mean diameters of approximately 520 and 210 nm, respectively).

Figure 8

Figure 8. Optical micrographs and corresponding SEM images obtained for a series of CaCO3 crystals prepared using a 1.5 mM CaCl2 solution and either (a) 0.010% w/w or (b) 0.005% w/w PMPA–PBzMA or PMP–PBzMA diblock copolymer nano-objects. The scale bar corresponds to 20 and 5 μm for the optical micrographs and SEM images, respectively. Optical micrograph and SEM images of the reference calcite crystals precipitated in the absence of diblock copolymer nano-objects are shown at the left-hand side.

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At the higher copolymer concentration, 20–25 μm crystals with increasingly rough faces and edge truncations were obtained. Again, Raman studies indicated calcite formation under these conditions (Figure S9). Anionic PMPA42–PBzMA45 worms had an even more pronounced effect, generating a bimodal distribution of small ∼7 μm calcite crystals and larger ∼20 μm polycrystalline vaterite particles at a copolymer concentration of 0.010% w/w, as confirmed by Raman microscopy (see Figure S9). At an additive concentration of 0.050% w/w, copolymer overgrown calcite rhombohedra with somewhat curved edges and roughened faces are produced. This indicates a strong interaction between the anionic nano-objects and the crystal lattice.
The crystals were also fractured to give a rough estimate of the relative degrees of occlusion of these contrasting nano-objects. Figure 9 shows both intact and fractured crystals prepared at the two different additive concentrations. The non-ionic PMP24–PBzMA300 vesicles are not occluded at all, whereas a moderate degree of occlusion can be observed for both PMPA32–PBzMA300 and PMPA24–PBzMA300anionic vesicles. This is particularly interesting for the latter vesicles, which have mean diameters of ∼520 nm and are thus significantly larger than the 200 nm latex particles occluded in earlier work. (95) These vesicles are of potential interest as carriers of functional additives. However, the degree of vesicle occlusion appears to be significantly higher at the periphery of the crystals for both additive concentrations. This may derive from an increase in incorporation efficiency at the lower supersaturation encountered toward the end of the crystallization experiments (79) or could suggest a mass-transport-limited crystallization mechanism in which vesicle diffusion to the surface of the growing calcite crystals limits occlusion. This would again promote occlusion during the later, slower stages of growth. In contrast, the ∼200 nm anionic PMPA32–PBzMA300 vesicles are incorporated to much greater extents at both polymer concentrations. Furthermore, homogeneous occlusion throughout the crystal is observed. This suggests that the incorporation of larger particles may be more dependent on the reaction conditions, and in particular supersaturation, than that of smaller particles. Finally, examination of the distribution of the PMPA42–PBzMA45 worms within the crystals precipitated at the lower polymer concentration showed that they were predominantly located in the outer 1 μm region of the crystal. In contrast, for the small calcite crystals precipitated at the higher additive concentration the worms appear to be occluded rather more homogeneously throughout the crystal.

Figure 9

Figure 9. SEM images obtained for a series of fractured CaCO3 crystals, prepared using a 1.5 mM CaCl2 solution and (a) 0.010% w/w or (b) 0.005% w/w PMPA- or PMP-stabilized PBzMA diblock copolymer nano-objects. The scale bar represents 1.5 μm in all SEM images.

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Conclusions

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A new PISA formulation has been explored for the generation of both phosphonate- and phosphonic acid-stabilized spheres, worms, and vesicles. More specifically, three poly(methacryloyloxymethyl dimethylphosphonate) [PMP] macro-CTAs with mean degrees of polymerization of 24, 32, and 42 were synthesized via RAFT solution polymerization and converted into the corresponding poly(methacryloxymethyl phosphonic acid) [PMPA] macro-CTAs via ester hydrolysis. Subsequent chain extension with BzMA via RAFT methanolic dispersion polymerization at 20% w/w solids enabled construction of detailed phase diagrams for both types of macro-CTAs. Systematic variation of the target degree of polymerization of the PBzMA block provides access to diblock copolymer spheres, worms, and vesicles, as well as mixed phases. Comparing the performance of the same macro-CTA in its non-ionic and anionic forms allowed the effect of stabilizer charge density on self-assembly in nonaqueous media (i.e., methanol) to be assessed for the first time. GPC studies indicate high blocking efficiencies and low final polydispersities for all non-ionic PMPx–PBzMAy diblock copolymers. However, for the anionic PMPAx macro-CTAs, blocking efficiencies could only be assessed indirectly via chain extension experiments conducted using a water-soluble monomer (glycerol monomethacrylate) as the second block. Transfer of selected diblock copolymer nano-objects from methanol to water via dialysis did not result in any discernible changes in copolymer morphology, as judged by TEM and DLS. Aqueous electrophoresis studies confirmed the highly anionic character of the phosphonic acid-stabilized nano-objects in aqueous media. Examination of both sets of copolymer nano-objects as crystal growth additives showed that anionic worms and vesicles were occluded within calcite crystals during the crsytal growth, while the corresponding non-ionic phosphonate-stabilized vesicles were not. These findings emphasize the importance of appropriate surface chemistry in promoting occlusion of organic nanoparticles within inorganic host crystals.

Supporting Information

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.5b02212.

  • 31P NMR spectra and GPC data for the three PMPA macro-CTAs, GPC and DLS data for kinetics of dispersion polymerization experiments, GPC data for all PMPx–PBzMAy diblock copolymers, additional TEM images, DLS data for aqueous dispersion polymerizations conducted using binary mixtures of macro-CTAs, and Raman spectra for selected CaCO3 occlusion experiments (PDF)

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Author Information

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  • Corresponding Author
    • Steven P. Armes - Dainton Building, Department of Chemistry, The University of Sheffield, Brook Hill, Sheffield, South Yorkshire S3 7HF, U.K.;  Email: [email protected]
  • Authors
    • Andreas Hanisch - Dainton Building, Department of Chemistry, The University of Sheffield, Brook Hill, Sheffield, South Yorkshire S3 7HF, U.K.
    • Pengcheng Yang - Dainton Building, Department of Chemistry, The University of Sheffield, Brook Hill, Sheffield, South Yorkshire S3 7HF, U.K.
    • Alexander N. Kulak - School of Chemistry, University of Leeds, Woodhouse Lane, Leeds LS2 9JT, U.K.
    • Lee A. Fielding - Dainton Building, Department of Chemistry, The University of Sheffield, Brook Hill, Sheffield, South Yorkshire S3 7HF, U.K.
    • Fiona C. Meldrum - School of Chemistry, University of Leeds, Woodhouse Lane, Leeds LS2 9JT, U.K.
  • Notes
    The authors declare no competing financial interest.

Acknowledgment

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EPSRC is acknowledged for postdoctoral support of A.H. (EP/K006290/1) and A.N.K. (EP/K006304/1). The authors thank André H. Gröschel for the preparation of the nano-object illustrations.

Added in Proof

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The following reference has been added by the author. Reference 98 was inserted at galley stage. Y. Ning, L. A. Fielding, T. S. Andrews, D. J. Growney and S. P. Armes Sulfate-based anionic diblock copolymer nanoparticles for efficient occlusion within zinc oxide. Nanoscale, 2015, 7, 6691−6702.

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    While it has been shown that phosphates can target mols. and nanocarriers to bone we herein demonstrate the prepn. of polyphosphate nanoparticles loaded with paclitaxel using a simple miniemulsion/solvent-evapn. technique as a model for chemotherapeutic delivery. Polyphosphates exhibit much higher structural versatility, relying on the pentavalence of the phosphorus center compared to conventional polyesters. This versatility allows for the development of new degradable polymeric carriers with inherent bone adhesion ability by the interaction of the nanoparticles with a calcium phosphate material used for bone regeneration. The novel polyphosphate nanoparticles were investigated in detail with respect to their size distribution, zeta-potential, thermal and morphol. properties and were further proven to be efficiently loaded with a hydrophobic drug (up to 15 wt%). The in vitro cytotoxicity was assessed against human cancer cell lines (HeLa and Saos-2), and the paclitaxel-loaded nanoparticles showed a similar cytotoxicity profile similar to the com. available formulation Taxomedac and the pure paclitaxel for loading ratios of 10 wt% but addnl. proved efficient adhesion on calcium phosphate granules allowing drug delivery to bone. This first report demonstrates that polyphosphate nanoparticles are promising materials for the development of systemic or local bone cancer treatment, even by direct application or by formation of composites with calcium phosphate cements.
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    Flame retardance and shrinkage reduction of polystyrene modified with acrylate-containing phosphorus and crosslinkable spiro-orthoester moieties
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    Polymer Degradation and Stability (2007), 92 (8), 1482-1490CODEN: PDSTDW; ISSN:0141-3910. (Elsevier Ltd.)
    Styrene was radically copolymd. with a spiro-orthoester with an acrylate group (SOE-AC), and terpolymd. with SOE-AC and diethyl(methacryloyloxymethyl)phosphonate (DEMMP). This was done for several different feed ratios, to obtain polymers with spiro-orthoester moieties in the side chain. These polymers were then crosslinked with ytterbium triflate, as cationic initiator, via the double ring-opening polymn. The thermal stability and fire retardant properties of these materials were evaluated by TGA and LOI. The DEMMP-contg. polymers give materials which were significantly more flame retardant than the nonphosphorus-contg. materials, as indicated by the LOI measurements. The vol. changes measured upon crosslinking of the polymers were evaluated by d. measurements with a gas pycnometer. In all the cases, expansion was obsd. This indicates that SOE-AC is an effective monomer for crosslinkable polymers without vol. changes.
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    RAFT polymerization of dimethyl(methacryloyloxy)methyl phosphonate and its phosphonic acid derivative: a new opportunity for phosphorus-based materials
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    Polymer Chemistry (2013), 4 (13), 3676-3685CODEN: PCOHC2; ISSN:1759-9962. (Royal Society of Chemistry)
    The RAFT polymn. of dimethyl(methacryloyloxy)methyl phosphonate (MAPC1) using dithioester chain transfer agents is reported for the first time to our knowledge in the literature. Poly(dimethyl(methacryloyloxy)methyl phosphonate) (PMAPC1) was synthesized in DMF at 70° with a good control over the mol. wt., the latter ranging from 8000 to 24 000 g mol-1. Polymers were characterized by 1H and 31P NMR, and size exclusion chromatog. using both refractive index and triple detection. The effect of the solvent was also investigated. We demonstrated that polarity had an effect on the control of the polymn. as low polarity led to low polymn. rate and termination reactions whereas very high polarity resulted in high polymn. rate but also transfer reactions. Addnl., we showed that it was possible to restart the RAFT polymn. from a PMAPC1 macrochain transfer agent. PMAPC1 was hydrolyzed to afford poly((methacryloyloxy)methyl phosphonic acid) (hPMAPC1), whose pKa values were detd. Finally, we achieved the controlled RAFT polymn. of the (methacryloyloxy)methyl phosphonic acid (hMAPC1). The easy obtaining of PMAPC1 and hPMAPC1 opens the way to the synthesis of complex polymer architectures.
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    Synthesis by RAFT of innovative well-defined (co)polymers from a novel phosphorus-based acrylamide monomer
    Graillot, Alain; Monge, Sophie; Faur, Catherine; Bouyer, Denis; Robin, Jean-Jacques
    Polymer Chemistry (2013), 4 (3), 795-803CODEN: PCOHC2; ISSN:1759-9962. (Royal Society of Chemistry)
    The present contribution reports on the synthesis and controlled polymn. of a novel acrylamide monomer contg. phosphonated moieties, namely diethyl-2-(acrylamido)ethylphosphonate. This monomer appears to be of great interest due to the phosphonated moieties, which can lead to a wide range of applications, assocd. with the chem. stability of the acrylamide compared to more common (meth)acrylate monomers. Reversible Addn.-Fragmentation Transfer (RAFT) polymn. of this monomer was studied using two different trithiocarbonate chain transfer agents, and it allowed the synthesis of poly(diethyl-2-(acrylamido)ethylphosphonate) with controlled mol. wt. and low dispersity. Addnl., a diblock copolymer was successfully prepd. by a similar RAFT procedure using thermosensitive poly(N-n-propylacrylamide) as a macro-chain transfer agent. A combination of both stimuli-responsive and phosphonated ester or phosphonic diacid (after hydrolysis) contg. blocks appears valuable for drug delivery or water treatment, for instance.
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    Three arylene difluoride monomers contg. phosphine oxide (1), phosphinic acid (2), or phosphinate ester (3) groups were prepd. and polymd. with bisphenol A to give novel poly(arylene ether)s (4, 5, and 6). The polymers obtained had moderate mol. wts. (ηinh: 0.14-0.30 dL g-1 in N-methylpyrrolidinone) and glass-transition temps. (Tg: 102-200°), depending on the phosphine group in the main chain. Using bis(4-fluorophenyl)sulfone as a comonomer improved the polymn. to give copolymers with higher soln. viscosities. The stoichiometric investigation revealed that 7 mol % excess of fluoride monomer gave the highest mol. wt. copolymer 8 with ηinh of 0.78 dL g-1, which had a Tg of 176 °C, a Td10% of 432°, and formed a hard film by casting from soln.
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    A review. Current status and future perspectives in atom transfer radical polymn. (ATRP) are presented. Special emphasis is placed on mechanistic understanding of ATRP, recent synthetic and process development, and new controlled polymer architectures enabled by ATRP. New hybrid materials based on org./inorg. systems and natural/synthetic polymers are presented. Some current and forthcoming applications are described.
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    Aggregation behavior in water of amphiphilic diblock copolymers bearing biocompatible phosphorylcholine and cholesteryl groups
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    Polymer Journal (Tokyo, Japan) (2015), 47 (1), 71-76CODEN: POLJB8; ISSN:0032-3896. (NPG Nature Asia-Pacific)
    Poly(2-(methacryloyloxy)ethyl phosphorylcholine)-block-poly(cholesteryl 6-methacryloyloxyhexanoate) (PMPC82-b-PChMn) copolymers with different PChM block lengths were prepd. via reversible addn.-fragmentation chain transfer controlled/living radical polymn. using a PMPC-based macro-chain transfer agent. The subscript no. and n (=3 and 6) refer to the d.p. of the PMPC and PChM blocks, resp. PMPC82-b-PChMn cannot dissolve in water directly due to the strong hydrophobic nature of the PChM block. To prep. the aq. soln., the diblock copolymer was dissolved in an org. solvent and then dialyzed against pure water. These diblock copolymers formed spherical and rod-like micelles in water, depending on the compn. of cholesteryl (Chol) group in the polymer. The prepd. aggregates were characterized using static light scattering, dynamic light scattering, transmission electron microscopy and fluorescence probe techniques. The characterization results suggest that the morphol. of the polymer aggregates can be controlled from spherical to rod-like micelles by increasing the no. of Chol groups in the polymer.
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    Aqueous RAFT/MADIX polymerization of vinylphosphonic acid
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    Polymer Chemistry (2012), 3 (3), 609-612CODEN: PCOHC2; ISSN:1759-9962. (Royal Society of Chemistry)
    RAFT/MADIX polymn. of vinylphosphonic acid (VPA) was controlled in water with an O-Et xanthate transfer agent. This represents the first example of reversible deactivation radical polymn. of a monomer bearing an unprotected phosphonic acid function. Hence, macromol. engineering of polyphosphonates can now be envisioned by directly polymg. VPA in water.
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    Poly(MMA)-b-poly(monophosphonic acrylate) diblock copolymers obtained by ATRP and used as additives for anticorrosive coatings
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    Journal of Applied Polymer Science (2009), 114 (4), 2213-2220CODEN: JAPNAB; ISSN:0021-8995. (John Wiley & Sons, Inc.)
    Atom transfer radical polymn. (ATRP) of dimethyl(methacryloyloxy)methyl phosphonate (MAC1P) was investigated in toluene, in the presence of Me 2-bromoisobutyrate as the initiator, and using different metal and ligand systems. Polymn. proceeded with very low monomer conversion, which was attributed to the ability of phosphorus to complex the copper ions, removing copper ions from original ligand, and then stopping the MAC1P polymn. Poly(MMA)-b-poly(phosphonate acrylate) diblock copolymer structure was efficiently obtained by the ATRP process, based on a four-step reaction. Poly(MMA)-b-poly(tert-Bu acrylate) diblock copolymer was first obtained by ATRP, then the tert-Bu groups were removed and phosphonate functions were incorporated by esterification reaction, using 4-dimethylaminopyridine as the catalyst. This new diblock copolymer was used as an additive for anticorrosive coating; however, no improvement (using the salt spray test technique) was obsd. comparatively with the statistical copolymer with the same acid content. P-contg. additive led to the increased corrosion resistance. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009.
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    Atom transfer radical polymerization of dimethyl(methacryloyloxymethyl) phosphonate
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    European Polymer Journal (2014), 56 (), 11-16CODEN: EUPJAG; ISSN:0014-3057. (Elsevier Ltd.)
    Atom transfer radical polymn. (ATRP) of dimethyl(methacryloyloxymethyl) phosphonate (DMMAMP, also referred to as MAPC1) with a copper/2,2'-bipyridine based catalytic system in DMSO resulted in a homopolymer with narrow mol. wt. distribution. Initiators for continuous activator regeneration (ICAR) ATRP of this monomer was also successful when using tris[(2-pyridyl)methyl]amine as ligand and azobisisobutyronitrile as supplementary initiator for the regeneration of the CuI activator. Normal ATRP provided better control compared to ICAR ATRP. Differential scanning calorimetry measurements of the resulting poly(DMMAMP) revealed a glass transition temp. (Tg = 63 °C), lower than that of poly(Me methacrylate). Chain extension of the poly(dimethyl(methacryloyloxymethyl) phosphonate) macroinitiator with styrene was successfully achieved with a high fraction of block copolymer.
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    Journal of the American Chemical Society (2014), 136 (29), 10174-10185CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    A review. In this Perspective, we discuss the recent development of polymn.-induced self-assembly mediated by reversible addn.-fragmentation chain transfer (RAFT) aq. dispersion polymn. This approach has quickly become a powerful and versatile technique for the synthesis of a wide range of bespoke org. diblock copolymer nano-objects of controllable size, morphol., and surface functionality. Given its potential scalability, such environmentally-friendly formulations are expected to offer many potential applications, such as novel Pickering emulsifiers, efficient microencapsulation vehicles, and sterilizable thermo-responsive hydrogels for the cost-effective long-term storage of mammalian cells.
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    Polymer Chemistry (2013), 4 (4), 873-881CODEN: PCOHC2; ISSN:1759-9962. (Royal Society of Chemistry)
    A review. Differently from bulk, soln., suspension, emulsion, and miniemulsion polymns., the controlled radical dispersion polymn. (CRDP) demonstrates self-assembly of the block copolymers formed in the homogeneous system, forming various kinds of micelles or vesicles. Thus, this technol. can prep. both the block copolymers and the polymeric aggregates directly. Among CRDP, the reversible addn.-fragmentation chain transfer (RAFT) dispersion polymn. has been studied in relative detail and has been successfully developed to prep. a diverse range of assemblies. Several typical systems for RAFT dispersion polymn. are presented in detail and the factors influencing the polymn. and the in situ self-assembly are also highlighted in this minireview.
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    A straightforward approach was developed to synthesize pegylated thermo-responsive core-shell nanoparticles in a min. of steps, directly in water. It is based on RAFT-controlled radical crosslinking copolymn. of N,N-diethylacrylamide (DEAAm) and N,N'-methylene bisacrylamide (MBA) in aq. dispersion polymn. Because DEAAm is water-sol. and poly(N,N-diethylacrylamide) (PDEAAm) exhibits a lower crit. soln. temp. at 32°, the initial medium was homogeneous, whereas the polymer formed a sep. phase at the reaction temp. The first macroRAFT agent was a surface-active trithiocarbonate based on a hydrophilic poly(ethylene oxide) block and a hydrophobic dodecyl chain. It was further extended with N,N-dimethylacrylamide (DMAAm) to target macroRAFT agents with increasing chain length. All macroRAFT agents provided excellent control over the aq. dispersion homopolymn. of DEAAm. When they were used in the radical crosslinking copolymn. of DEAAm and MBA, the stability and size of the resulting gel particles were found to depend strongly on the chain length of the macroRAFT agent, on the concns. of both the monomer and the crosslinker, and on the process (one step or two steps). The best-suited exptl. conditions to reach thermosensitive hydrogels with nanometric size and well-defined surface properties were detd.
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    Polymerization-induced thermal self-assembly (PITSA)
    Figg, C. Adrian; Simula, Alexandre; Gebre, Kalkidan A.; Tucker, Bryan S.; Haddleton, David M.; Sumerlin, Brent S.
    Chemical Science (2015), 6 (2), 1230-1236CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)
    Polymn.-induced self-assembly (PISA) is a versatile technique to achieve a wide range of polymeric nanoparticle morphologies. Most previous examples of self-assembled soft nanoparticle synthesis by PISA rely on a growing solvophobic polymer block that leads to changes in nanoparticle architecture during polymn. in a selective solvent. However, synthesis of block copolymers with a growing stimuli-responsive block to form various nanoparticle shapes has yet to be reported. This new concept using thermo-responsive polymers is termed polymn.-induced thermal self-assembly (PITSA). A reversible addn.-fragmentation chain transfer (RAFT) polymn. of N-isopropylacrylamide from a hydrophilic chain transfer agent composed of N,N-dimethylacrylamide and acrylic acid was carried out in water above the known lower crit. soln. temp. (LCST) of poly(N-isopropylacrylamide) (PNIPAm). After reaching a certain chain length, the growing PNIPAm self-assembled, as induced by the LCST, into block copolymer aggregates within which dispersion polymn. continued. To characterize the nanoparticles at ambient temps. without their dissoln., the particles were crosslinked immediately following polymn. at elevated temps. via the reaction of the acid groups with a diamine in the presence of a carbodiimide. Size exclusion chromatog. was used to evaluate the unimer mol. wt. distributions and reaction kinetics. Dynamic light scattering and transmission electron microscopy provided insight into the size and morphologies of the nanoparticles. The resulting block copolymers formed polymeric nanoparticles with a range of morphologies (e.g., micelles, worms, and vesicles), which were a function of the PNIPAm block length.
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    In Situ Formation of Polymer-Gold Composite Nanoparticles with Tunable Morphologies
    Bleach, Richard; Karagoz, Bunyamin; Prakash, Shyam M.; Davis, Thomas P.; Boyer, Cyrille
    ACS Macro Letters (2014), 3 (7), 591-596CODEN: AMLCCD; ISSN:2161-1653. (American Chemical Society)
    A simple and efficient route to gold-polymer nanoparticle composites is described. Our versatile synthetic route exerts facile control over polymer nanoparticle morphol., including micelles, rod-like structures, and vesicles, all easily attainable from a single polymn. taken to different monomer conversions. Specifically, poly[oligo(ethylene glycol) methacrylate]-b-poly(dimethylaminoethyl methacrylate)-b-poly(styrene) (POEGMA-b-PDMAEMA-b-PST) triblock copolymers were synthesized using a polymn. induced self-assembly (PISA) approach. Subsequently, spherical gold nanoparticles (10 nm AuNPs) were formed at the hydrophilic-hydrophobic nexus of the assembled triblock copolymer nanoaggregates by the addn. of chloroauric acid (HAuCl4) followed by in situ redn. using NaBH4. After redn., the cloudy white nanoparticle dispersions turned to a red-purple color. The gold nanoparticles that formed were stabilized by the enveloping polymeric nanostructures, neither pptn. nor agglomeration occurred. We demonstrated that we were able to tune the gold nanoparticle compn. in these polymer-gold composites by varying the concn. of chloroauric acid. Morphol., particle size, mol. wt., AuNP content, and chem. structure of the polymer structures were characterized by transmittance electron microscopy (TEM), dynamic light scattering (DLS), size exclusion chromatog. (SEC), thermal gravimetric anal. (TGA), and 1H NMR. Finally, the formation of the AuNPs occurred without affecting the polymer nanoparticle morphol.
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    Poly(methacrylic acid)-based AB and ABC block copolymer nano-objects prepared via RAFT alcoholic dispersion polymerization
    Semsarilar, M.; Ladmiral, V.; Blanazs, A.; Armes, S. P.
    Polymer Chemistry (2014), 5 (10), 3466-3475CODEN: PCOHC2; ISSN:1759-9962. (Royal Society of Chemistry)
    A series of well-defined amphiphilic poly(methacrylic acid)-poly(benzyl methacrylate) (PMAA-PBzMA) diblock copolymers were synthesized via polymn.-induced self-assembly using an alc. dispersion polymn. formulation. Chain growth was mediated via reversible addn.-fragmentation chain transfer polymn. (RAFT) chem. using a trithiocarbonate-based chain transfer agent (CTA) at 70 °C. The poly(methacrylic acid) block was sol. in ethanol and acts as a steric stabilizer for the growing insol. PBzMA chains, resulting in the in situ generation of diblock copolymer nano-objects in the form of spheres, worms or vesicles, depending on the precise reaction conditions. Copolymer morphologies can be covalently stabilized via crosslinking to prevent their dissocn. when transferred into aq. soln., which leads to the formation of highly anionic nano-objects due to ionization of the PMAA stabilizer chains. ABC triblock copolymer nanoparticles can also be prepd. using this approach, where the third block was based on the semi-fluorinated monomer, 2,2,2-trifluoroethyl methacrylate (TFEMA). GPC studies confirm that chain extension was efficient and high TFEMA conversions can be achieved. Microphase sepn. between the mutually incompatible PBzMA and semi-fluorinated PTFEMA core-forming blocks occurs, producing a range of remarkably complex semi-fluorinated triblock copolymer morphologies.
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    Houillot, L.; Bui, C.; Farcet, C. l.; Moire, C.; Raust, J.-A.; Pasch, H.; Save, M.; Charleux, B. ACS Appl. Mater. Interfaces 2010, 2, 434– 442 DOI: 10.1021/am900695a
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    Deng, Y.; Yang, C.; Yuan, C.; Xu, Y.; Bernard, J.; Dai, L.; Gérard, J.-F. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 4558– 4564 DOI: 10.1002/pola.26872
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    Hybrid organic-inorganic block copolymer nano-objects from RAFT polymerization-induced self-assembly
    Deng, Yuanming; Yang, Cangjie; Yuan, Conghui; Xu, Yiting; Bernard, Julien; Dai, Lizong; Gerard, Jean-Francois
    Journal of Polymer Science, Part A: Polymer Chemistry (2013), 51 (21), 4558-4564CODEN: JPACEC; ISSN:0887-624X. (John Wiley & Sons, Inc.)
    A simple route to org.-inorg. (O/I) nano-objects with different morphologies through polymn.-induced block copolymer self-assembly is described. The synthetic strategy relies on the chain-extension of polyhedral oligomeric silsesquioxanes (POSS)-contg. macro-CTA (PMAiBuPOSS13 and PMAiBuPOSS19) with styrene at 120 °C in octane, a selective solvent of the POSS-contg. block. The polymn. system was proven to afford a plethora of O/I nano-objects, such as spherical micelles, cylindrical micelles, and vesicles depending on the resp. molar masses of the PMAiBuPOSS and polystyrene (PS) blocks. The cooling procedure was also proven to be a crucial step to generate particles with a unique morphol. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013.
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    Fielding, L. A.; Derry, M. J.; Ladmiral, V.; Rosselgong, J.; Rodrigues, A. M.; Ratcliffe, L. P. D.; Sugihara, S.; Armes, S. P. Chem. Sci. 2013, 4, 2081– 2087 DOI: 10.1039/c3sc50305d
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    RAFT dispersion polymerization in non-polar solvents: facile production of block copolymer spheres, worms and vesicles in n-alkanes
    Fielding, Lee A.; Derry, Matthew J.; Ladmiral, Vincent; Rosselgong, Julien; Rodrigues, Aurelie M.; Ratcliffe, Liam P. D.; Sugihara, Shinji; Armes, Steven P.
    Chemical Science (2013), 4 (5), 2081-2087CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)
    Well-defined poly(lauryl methacrylate-benzyl methacrylate) (PLMA-PBzMA) diblock copolymer nanoparticles are prepd. in n-heptane at 90° via reversible addn.-fragmentation chain transfer (RAFT) polymn. Under these conditions, the PLMA macromol. chain transfer agent (macro-CTA) is sol. in n-heptane, whereas the growing PBzMA block quickly becomes insol. Thus this dispersion polymn. formulation leads to polymn.-induced self-assembly (PISA). Using a relatively long PLMA macro-CTA with a mean d.p. (DP) of 37 or higher leads to the formation of well-defined spherical nanoparticles of 41 to 139 nm diam., depending on the DP targeted for the PBzMA block. In contrast, TEM studies confirm that using a relatively short PLMA macro-CTA (DP = 17) enables both worm-like and vesicular morphologies to be produced, in addn. to the spherical phase. A detailed phase diagram has been elucidated for this more asym. diblock copolymer formulation, which ensures that each pure phase can be targeted reproducibly. 1H NMR spectroscopy confirmed that high BzMA monomer conversions (>97%) were achieved within 5 h, while GPC studies indicated that reasonably good blocking efficiencies and relatively low diblock copolymer polydispersities (Mw/Mn < 1.30) were obtained in most cases. Compared to prior literature reports, this all-methacrylic PISA formulation is particularly novel because: (i) it is the first time that higher order morphologies (e.g. worms and vesicles) have been accessed in non-polar solvents and (ii) such diblock copolymer nano-objects are expected to have potential boundary lubrication applications for engine oils.
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    Dan, M.; Huo, F.; Zhang, X.; Wang, X.; Zhang, W. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 1573– 1584 DOI: 10.1002/pola.26527
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    Dispersion RAFT polymerization of 4-vinylpyridine in toluene mediated with the macro-RAFT agent of polystyrene dithiobenzoate: Effect of the macro-RAFT agent chain length and growth of the block copolymer nano-objects
    Dan, Meihan; Huo, Fei; Zhang, Xu; Wang, Xiaohui; Zhang, Wangqing
    Journal of Polymer Science, Part A: Polymer Chemistry (2013), 51 (7), 1573-1584CODEN: JPACEC; ISSN:0887-624X. (John Wiley & Sons, Inc.)
    The dispersion reversible addn.-fragmentation chain transfer (RAFT) polymn. of 4-vinylpyridine in toluene in the presence of the polystyrene dithiobenzoate (PS-CTA) macro-RAFT agent with different chain length is discussed. The RAFT polymn. undergoes an initial slow homogeneous polymn. and a subsequent fast heterogeneous one. The RAFT polymn. rate is dependent on the PS-CTA chain length, and short PS-CTA generally leads to fast RAFT polymn. The dispersion RAFT polymn. induces the self-assembly of the in situ synthesized polystyrene-b-poly(4-vinylpyridine) block copolymer into highly concd. block copolymer nano-objects. The PS-CTA chain length exerts great influence on the particle nucleation and the size and morphol. of the block copolymer nano-objects. It is found, short PS-CTA leads to fast particle nucleation and tends to produce large-sized vesicles or large-compd. micelles, and long PS-CTA leads to formation of small-sized nanospheres. Comparison between the polymn.-induced self-assembly and self-assembly of block copolymer in the block-selective solvent is made, and the great difference between the two methods is demonstrated. The present study is anticipated to be useful to reveal the chain extension and the particle growth of block copolymer during the RAFT polymn. under dispersion condition. © 2013 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013.
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    Warren, N. J.; Mykhaylyk, O. O.; Mahmood, D.; Ryan, A. J.; Armes, S. P. J. Am. Chem. Soc. 2014, 136, 1023– 1033 DOI: 10.1021/ja410593n
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    Optimization of the RAFT polymerization conditions for the in situ formation of nano-objects via dispersion polymerization in alcoholic medium
    Zhao, Wei; Gody, Guillaume; Dong, Siming; Zetterlund, Per B.; Perrier, Sebastien
    Polymer Chemistry (2014), 5 (24), 6990-7003CODEN: PCOHC2; ISSN:1759-9962. (Royal Society of Chemistry)
    Hydrophilic polymer brushes based on poly(ethylene glycol) Me ether acrylate (P(PEGA454)) or poly(ethylene glycol) Me ether methacrylate (P(PEGMA475)), both having a trithiocarbonate end group, were prepd. in water-dioxane (9 : 1) at 44 °C via RAFT polymn. and subsequently used in RAFT dispersion polymn. of styrene in isopropanol at 90 °C. RAFT reaction conditions were first optimized to prep. P(PEGA454) and P(PEGMA475) macro-RAFT agents at high monomer conversions (>90%) and very low fraction of dead chains (<1%). Both polymer brushes allowed the prepn. of well-defined amphiphilic diblock copolymers (P(PEGA454)-b-PS and P(PEGMA475)-b-PS) which self-assemble in situ into nano-objects with various morphologies. Using relatively long chain P(PEGA454) or P(PEGMA475) macro-RAFT agents (DP ~ 75) leads to the formation of near uniform spherical nanoparticles with diams. ranging from 30 to 140 nm, depending on the targeted DP of the PS block. In contrast, TEM and DLS studies demonstrated that using a shorter P(PEGA454) or P(PEGMA475) macro-RAFT agent (DP ~ 20) enables the formation of worm-like micelles, vesicles and large compd. vesicle morphologies in addn. to spheres. Cryo-TEM was used to confirm polymn. induced morphol. transition, rather than morphologies obtained via self-assembly driven by selective solvent or solvent evapn. during the prepn. of samples for characterization.
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    Ratcliffe, L. P. D.; Blanazs, A.; Williams, C. N.; Brown, S. L.; Armes, S. P. Polym. Chem. 2014, 5, 3643– 3655 DOI: 10.1039/c4py00203b
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    RAFT polymerization of hydroxy-functional methacrylic monomers under heterogeneous conditions: effect of varying the core-forming block
    Ratcliffe, L. P. D.; Blanazs, A.; Williams, C. N.; Brown, S. L.; Armes, S. P.
    Polymer Chemistry (2014), 5 (11), 3643-3655CODEN: PCOHC2; ISSN:1759-9962. (Royal Society of Chemistry)
    Statistical copolymn. of a 1:1 molar ratio of a water-miscible monomer (2-hydroxyethyl methacrylate, HEMA) with a water-immiscible monomer (4-hydroxybutyl methacrylate, HBMA) has been conducted in water via reversible addn.-fragmentation chain transfer (RAFT) polymn. using a water-sol. poly(glycerol monomethacrylate) macromol. chain transfer agent (PGMA macro-CTA). In principle, such a hybrid formulation might be expected to be intermediate between RAFT dispersion polymn. and RAFT emulsion polymn. Under such circumstances, it is of particular interest to examine whether both monomers are actually consumed and, if so, whether their rates of reaction are comparable. Given the water-soly. of both the PGMA macro-CTA and the free radical azo initiator, it is perhaps counter-intuitive that the water-immiscible HBMA is initially consumed significantly faster than the water-miscible HEMA, as judged by 1H NMR studies of this copolymn. However, both comonomers are eventually almost fully consumed at 70 °C. A detailed phase diagram has been constructed for this RAFT formulation that enables reproducible syntheses of various pure copolymer morphologies, including spheres, worms and vesicles. It is emphasized that utilizing a 1:1 HEMA/HBMA molar ratio produces a core-forming statistical copolymer block that is isomeric with the poly(2-hydroxypropyl methacrylate) (PHPMA) core-forming block previously synthesized via RAFT aq. dispersion polymn. (see A. Blanazs et al., Macromols., 2012, 45, 5099-5107). Hence it is rather remarkable that the thermo-responsive behavior of PGMA-P(HBMA-stat-HEMA) statistical block copolymer worm gels differs qual. from that exhibited by PGMA-PHPMA diblock copolymer worm gels.
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    Binauld, S.; Delafresnaye, L.; Charleux, B.; D’Agosto, F.; Lansalot, M. Macromolecules 2014, 47, 3461– 3472 DOI: 10.1021/ma402549x
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    Zhang, X.; Boisson, F.; Colombani, O.; Chassenieux, C.; Charleux, B. Macromolecules 2014, 47, 51– 60 DOI: 10.1021/ma402125r
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    Carlsson, L.; Fall, A.; Chaduc, I.; Wagberg, L.; Charleux, B.; Malmstrom, E.; D’Agosto, F.; Lansalot, M.; Carlmark, A. Polym. Chem. 2014, 5, 6076– 6086 DOI: 10.1039/C4PY00675E
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    Modification of cellulose model surfaces by cationic polymer latexes prepared by RAFT-mediated surfactant-free emulsion polymerization
    Carlsson, Linn; Fall, Andreas; Chaduc, Isabelle; Waagberg, Lars; Charleux, Bernadette; Malmstroem, Eva; D'Agosto, Franck; Lansalot, Muriel; Carlmark, Anna
    Polymer Chemistry (2014), 5 (20), 6076-6086CODEN: PCOHC2; ISSN:1759-9962. (Royal Society of Chemistry)
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Cite this: Macromolecules 2016, 49, 1, 192–204
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https://doi.org/10.1021/acs.macromol.5b02212
Published December 23, 2015

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  • Abstract

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    Scheme 1

    Scheme 1. RAFT Solution Polymerization of Methacryloyloxymethyl Dimethylphosphonate (MP) and Subsequent Hydrolysis of the PMP Precursor To Obtain Poly(methacryloyloxymethylphosphonic acid) (PMPA); RAFT Dispersion Polymerization of BzMA in Methanol Using Either the PMP or PMPA Macro-CTA as Steric Stabilizer Produced Spherical, Worm-like, or Vesicular Nano-Objects, Depending on the Precise PISA Formulation
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    Figure 1

    Figure 1. (a) DMF gel permeation chromatography traces obtained for the three PMP macro-CTAs used in this study. (b) 31P NMR spectra recorded for the PMP24 macro-CTA before (red spectrum, top) and after hydrolysis (black spectrum, bottom). This indicates complete hydrolysis of the dimethyl ester groups on the PMP precursor to produce a well-defined PMPA macro-CTA.

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    Figure 2

    Figure 2. (a) Kinetic data derived from 1H NMR studies of the RAFT alcoholic dispersion polymerization of BzMA (target DP = 300) at 64 °C using the PMP24 macro-CTA at 20% w/w solids in methanol. The inset shows the evolution of the hydrodynamic diameter Dh and the first-order kinetic plot with respect to monomer. (b) The corresponding DMF GPC traces and the associated Mn and Mw/Mn data obtained for selected copolymers on sampling this formulation, indicating its reasonably good controlled/living character (see Table S1 for a more complete set of GPC data).

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    Figure 3

    Figure 3. Phase diagram constructed for PMPx–PBzMAy diblock copolymer nano-objects prepared in methanol at 64 °C at 20% w/w solids. Representative TEM images obtained for the three pure copolymer morphologies: (a) PMP24–PBzMA50 (small spheres), (b) PMP32–PBzMA79 (worms), (c) PMP42–PBzMA300 (large spheres), and (d) PMP24–PBzMA200 (vesicles).

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    Figure 4

    Figure 4. (a) Kinetic data derived from 1H NMR studies of the RAFT alcoholic dispersion polymerization of BzMA (target DP = 300) using an anionic PMPA24 macro-CTA at 20% w/w solids in methanol at 64 °C. The inset shows the evolution of the hydrodynamic diameter Dh and the first-order kinetic plot with respect to monomer concentration. (b) Representative TEM image of the final vesicular morphology obtained after 24 h (>99% conversion).

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    Figure 5

    Figure 5. Phase diagram constructed for PMPAx–PBzMAy diblock copolymer nano-objects prepared in methanol at 64 °C at 20% w/w solids. TEM images for pure copolymer morphologies: (a) PMPA42–PBzMA55 (worms), (b) PMPA24–PBzMA35 (spheres), (c) PMPA42–PBzMA300 (small vesicles), and (d) PMPA24–PBzMA300 (large vesicles).

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    Figure 6

    Figure 6. (a) Representative TEM images recorded for non-ionic PMP24–PBzMA300 vesicles and anionic PMPAx–PBzMAy worms and vesicles, obtained via RAFT dispersion polymerization of BzMA at 20% w/w solids in methanol at 64 °C. (b) TEM images for the same copolymer nanoparticles after their transfer from methanol to water via exhaustive dialysis at 20 °C. In all cases the copolymer concentration used to prepare the TEM grids was 0.2% w/w.

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    Figure 7

    Figure 7. Aqueous electrophoresis curves and DLS data obtained for PMPAx–PBzMAy worms and vesicles prepared by RAFT alcoholic dispersion polymerization of BzMA in methanol at 20% w/w solids. Measurements were conducted using 0.01% w/v dispersions in the presence of 10 mM NaCl as background electrolyte.

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    Figure 8

    Figure 8. Optical micrographs and corresponding SEM images obtained for a series of CaCO3 crystals prepared using a 1.5 mM CaCl2 solution and either (a) 0.010% w/w or (b) 0.005% w/w PMPA–PBzMA or PMP–PBzMA diblock copolymer nano-objects. The scale bar corresponds to 20 and 5 μm for the optical micrographs and SEM images, respectively. Optical micrograph and SEM images of the reference calcite crystals precipitated in the absence of diblock copolymer nano-objects are shown at the left-hand side.

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    Figure 9

    Figure 9. SEM images obtained for a series of fractured CaCO3 crystals, prepared using a 1.5 mM CaCl2 solution and (a) 0.010% w/w or (b) 0.005% w/w PMPA- or PMP-stabilized PBzMA diblock copolymer nano-objects. The scale bar represents 1.5 μm in all SEM images.

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      Macromolecules (Washington, DC, United States) (2012), 45 (10), 4015-4039CODEN: MAMOBX; ISSN:0024-9297. (American Chemical Society)
      A review. Current status and future perspectives in atom transfer radical polymn. (ATRP) are presented. Special emphasis is placed on mechanistic understanding of ATRP, recent synthetic and process development, and new controlled polymer architectures enabled by ATRP. New hybrid materials based on org./inorg. systems and natural/synthetic polymers are presented. Some current and forthcoming applications are described.
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    24. 24
      Moad, G.; Rizzardo, E.; Thang, S. H. Aust. J. Chem. 2012, 65, 985– 1076 DOI: 10.1071/CH12295
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    25. 25
      Lobb, E. J.; Ma, I.; Billingham, N. C.; Armes, S. P.; Lewis, A. L. J. Am. Chem. Soc. 2001, 123, 7913– 7914 DOI: 10.1021/ja003906d
      There is no corresponding record for this reference.
    26. 26
      Madsen, J.; Canton, I.; Warren, N. J.; Themistou, E.; Blanazs, A.; Ustbas, B.; Tian, X.; Pearson, R.; Battaglia, G.; Lewis, A. L.; Armes, S. P. J. Am. Chem. Soc. 2013, 135, 14863– 14870 DOI: 10.1021/ja407380t
      There is no corresponding record for this reference.
    27. 27
      Yusa, S.-i.; Fukuda, K.; Yamamoto, T.; Ishihara, K.; Morishima, Y. Biomacromolecules 2005, 6, 663– 670 DOI: 10.1021/bm0495553
      There is no corresponding record for this reference.
    28. 28
      Ohno, S.; Hasegawa, S.; Liu, H.; Ishihara, K.; Yusa, S.-i. Polym. J. 2015, 47, 71 DOI: 10.1038/pj.2014.92
      28
      Aggregation behavior in water of amphiphilic diblock copolymers bearing biocompatible phosphorylcholine and cholesteryl groups
      Ohno, Sayaka; Hasegawa, Shoto; Liu, Huihua; Ishihara, Kazuhiko; Yusa, Shin-ichi
      Polymer Journal (Tokyo, Japan) (2015), 47 (1), 71-76CODEN: POLJB8; ISSN:0032-3896. (NPG Nature Asia-Pacific)
      Poly(2-(methacryloyloxy)ethyl phosphorylcholine)-block-poly(cholesteryl 6-methacryloyloxyhexanoate) (PMPC82-b-PChMn) copolymers with different PChM block lengths were prepd. via reversible addn.-fragmentation chain transfer controlled/living radical polymn. using a PMPC-based macro-chain transfer agent. The subscript no. and n (=3 and 6) refer to the d.p. of the PMPC and PChM blocks, resp. PMPC82-b-PChMn cannot dissolve in water directly due to the strong hydrophobic nature of the PChM block. To prep. the aq. soln., the diblock copolymer was dissolved in an org. solvent and then dialyzed against pure water. These diblock copolymers formed spherical and rod-like micelles in water, depending on the compn. of cholesteryl (Chol) group in the polymer. The prepd. aggregates were characterized using static light scattering, dynamic light scattering, transmission electron microscopy and fluorescence probe techniques. The characterization results suggest that the morphol. of the polymer aggregates can be controlled from spherical to rod-like micelles by increasing the no. of Chol groups in the polymer.
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    29. 29
      Huang, J.; Matyjaszewski, K. Macromolecules 2005, 38, 3577– 3583 DOI: 10.1021/ma047564y
      There is no corresponding record for this reference.
    30. 30
      Suzuki, S.; Whittaker, M. R.; Grøndahl, L.; Monteiro, M. J.; Wentrup-Byrne, E. Biomacromolecules 2006, 7, 3178– 3187 DOI: 10.1021/bm060583q
      There is no corresponding record for this reference.
    31. 31
      Blidi, I.; Geagea, R.; Coutelier, O.; Mazieres, S.; Violleau, F.; Destarac, M. Polym. Chem. 2012, 3, 609– 612 DOI: 10.1039/c2py00541g
      31
      Aqueous RAFT/MADIX polymerization of vinylphosphonic acid
      Blidi, Issam; Geagea, Roland; Coutelier, Olivier; Mazieres, Stephane; Violleau, Frederic; Destarac, Mathias
      Polymer Chemistry (2012), 3 (3), 609-612CODEN: PCOHC2; ISSN:1759-9962. (Royal Society of Chemistry)
      RAFT/MADIX polymn. of vinylphosphonic acid (VPA) was controlled in water with an O-Et xanthate transfer agent. This represents the first example of reversible deactivation radical polymn. of a monomer bearing an unprotected phosphonic acid function. Hence, macromol. engineering of polyphosphonates can now be envisioned by directly polymg. VPA in water.
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    32. 32
      Markova, D.; Kumar, A.; Klapper, M.; Müllen, K. Polymer 2009, 50, 3411– 3421 DOI: 10.1016/j.polymer.2009.06.011
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    33. 33
      David, G.; Negrell, C.; Manseri, A.; Boutevin, B. J. Appl. Polym. Sci. 2009, 114, 2213– 2220 DOI: 10.1002/app.30438
      33
      Poly(MMA)-b-poly(monophosphonic acrylate) diblock copolymers obtained by ATRP and used as additives for anticorrosive coatings
      David, Ghislain; Negrell, Claire; Manseri, Abdelatif; Boutevin, Bernard
      Journal of Applied Polymer Science (2009), 114 (4), 2213-2220CODEN: JAPNAB; ISSN:0021-8995. (John Wiley & Sons, Inc.)
      Atom transfer radical polymn. (ATRP) of dimethyl(methacryloyloxy)methyl phosphonate (MAC1P) was investigated in toluene, in the presence of Me 2-bromoisobutyrate as the initiator, and using different metal and ligand systems. Polymn. proceeded with very low monomer conversion, which was attributed to the ability of phosphorus to complex the copper ions, removing copper ions from original ligand, and then stopping the MAC1P polymn. Poly(MMA)-b-poly(phosphonate acrylate) diblock copolymer structure was efficiently obtained by the ATRP process, based on a four-step reaction. Poly(MMA)-b-poly(tert-Bu acrylate) diblock copolymer was first obtained by ATRP, then the tert-Bu groups were removed and phosphonate functions were incorporated by esterification reaction, using 4-dimethylaminopyridine as the catalyst. This new diblock copolymer was used as an additive for anticorrosive coating; however, no improvement (using the salt spray test technique) was obsd. comparatively with the statistical copolymer with the same acid content. P-contg. additive led to the increased corrosion resistance. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009.
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    34. 34
      Mukumoto, K.; Zhong, M.; Matyjaszewski, K. Eur. Polym. J. 2014, 56, 11– 16 DOI: 10.1016/j.eurpolymj.2014.03.029
      34
      Atom transfer radical polymerization of dimethyl(methacryloyloxymethyl) phosphonate
      Mukumoto, Kosuke; Zhong, Mingjiang; Matyjaszewski, Krzysztof
      European Polymer Journal (2014), 56 (), 11-16CODEN: EUPJAG; ISSN:0014-3057. (Elsevier Ltd.)
      Atom transfer radical polymn. (ATRP) of dimethyl(methacryloyloxymethyl) phosphonate (DMMAMP, also referred to as MAPC1) with a copper/2,2'-bipyridine based catalytic system in DMSO resulted in a homopolymer with narrow mol. wt. distribution. Initiators for continuous activator regeneration (ICAR) ATRP of this monomer was also successful when using tris[(2-pyridyl)methyl]amine as ligand and azobisisobutyronitrile as supplementary initiator for the regeneration of the CuI activator. Normal ATRP provided better control compared to ICAR ATRP. Differential scanning calorimetry measurements of the resulting poly(DMMAMP) revealed a glass transition temp. (Tg = 63 °C), lower than that of poly(Me methacrylate). Chain extension of the poly(dimethyl(methacryloyloxymethyl) phosphonate) macroinitiator with styrene was successfully achieved with a high fraction of block copolymer.
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    35. 35
      Charleux, B.; Delaittre, G.; Rieger, J.; D’Agosto, F. Macromolecules 2012, 45, 6753– 6765 DOI: 10.1021/ma300713f
      There is no corresponding record for this reference.
    36. 36
      Warren, N. J.; Armes, S. P. J. Am. Chem. Soc. 2014, 136, 10174– 10185 DOI: 10.1021/ja502843f
      36
      Polymerization-Induced Self-Assembly of Block Copolymer Nano-objects via RAFT Aqueous Dispersion Polymerization
      Warren, Nicholas J.; Armes, Steven P.
      Journal of the American Chemical Society (2014), 136 (29), 10174-10185CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      A review. In this Perspective, we discuss the recent development of polymn.-induced self-assembly mediated by reversible addn.-fragmentation chain transfer (RAFT) aq. dispersion polymn. This approach has quickly become a powerful and versatile technique for the synthesis of a wide range of bespoke org. diblock copolymer nano-objects of controllable size, morphol., and surface functionality. Given its potential scalability, such environmentally-friendly formulations are expected to offer many potential applications, such as novel Pickering emulsifiers, efficient microencapsulation vehicles, and sterilizable thermo-responsive hydrogels for the cost-effective long-term storage of mammalian cells.
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    37. 37
      Sun, J.-T.; Hong, C.-Y.; Pan, C.-Y. Polym. Chem. 2013, 4, 873– 881 DOI: 10.1039/C2PY20612A
      37
      Recent advances in RAFT dispersion polymerization for preparation of block copolymer aggregates
      Sun, Jiao-Tong; Hong, Chun-Yan; Pan, Cai-Yuan
      Polymer Chemistry (2013), 4 (4), 873-881CODEN: PCOHC2; ISSN:1759-9962. (Royal Society of Chemistry)
      A review. Differently from bulk, soln., suspension, emulsion, and miniemulsion polymns., the controlled radical dispersion polymn. (CRDP) demonstrates self-assembly of the block copolymers formed in the homogeneous system, forming various kinds of micelles or vesicles. Thus, this technol. can prep. both the block copolymers and the polymeric aggregates directly. Among CRDP, the reversible addn.-fragmentation chain transfer (RAFT) dispersion polymn. has been studied in relative detail and has been successfully developed to prep. a diverse range of assemblies. Several typical systems for RAFT dispersion polymn. are presented in detail and the factors influencing the polymn. and the in situ self-assembly are also highlighted in this minireview.
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    38. 38
      Cunningham, V. J.; Alswieleh, A. M.; Thompson, K. L.; Williams, M.; Leggett, G. J.; Armes, S. P.; Musa, O. M. Macromolecules 2014, 47, 5613– 5623 DOI: 10.1021/ma501140h
      There is no corresponding record for this reference.
    39. 39
      Blanazs, A.; Ryan, A. J.; Armes, S. P. Macromolecules 2012, 45, 5099– 5107 DOI: 10.1021/ma301059r
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    40. 40
      An, Z.; Shi, Q.; Tang, W.; Tsung, C.-K.; Hawker, C. J.; Stucky, G. D. J. Am. Chem. Soc. 2007, 129, 14493– 14499 DOI: 10.1021/ja0756974
      There is no corresponding record for this reference.
    41. 41
      Rieger, J.; Grazon, C.; Charleux, B.; Alaimo, D.; Jérôme, C. J. Polym. Sci., Part A: Polym. Chem. 2009, 47, 2373– 2390 DOI: 10.1002/pola.23329
      41
      Pegylated thermally responsive block copolymer micelles and nanogels via in situ RAFT aqueous dispersion polymerization
      Rieger, Jutta; Grazon, Chloe; Charleux, Bernadette; Alaimo, David; Jerome, Christine
      Journal of Polymer Science, Part A: Polymer Chemistry (2009), 47 (9), 2373-2390CODEN: JPACEC; ISSN:0887-624X. (John Wiley & Sons, Inc.)
      A straightforward approach was developed to synthesize pegylated thermo-responsive core-shell nanoparticles in a min. of steps, directly in water. It is based on RAFT-controlled radical crosslinking copolymn. of N,N-diethylacrylamide (DEAAm) and N,N'-methylene bisacrylamide (MBA) in aq. dispersion polymn. Because DEAAm is water-sol. and poly(N,N-diethylacrylamide) (PDEAAm) exhibits a lower crit. soln. temp. at 32°, the initial medium was homogeneous, whereas the polymer formed a sep. phase at the reaction temp. The first macroRAFT agent was a surface-active trithiocarbonate based on a hydrophilic poly(ethylene oxide) block and a hydrophobic dodecyl chain. It was further extended with N,N-dimethylacrylamide (DMAAm) to target macroRAFT agents with increasing chain length. All macroRAFT agents provided excellent control over the aq. dispersion homopolymn. of DEAAm. When they were used in the radical crosslinking copolymn. of DEAAm and MBA, the stability and size of the resulting gel particles were found to depend strongly on the chain length of the macroRAFT agent, on the concns. of both the monomer and the crosslinker, and on the process (one step or two steps). The best-suited exptl. conditions to reach thermosensitive hydrogels with nanometric size and well-defined surface properties were detd.
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    42. 42
      Li, Y.; Armes, S. P. Angew. Chem., Int. Ed. 2010, 49, 4042– 4046 DOI: 10.1002/anie.201001461
      There is no corresponding record for this reference.
    43. 43
      Sugihara, S.; Blanazs, A.; Armes, S. P.; Ryan, A. J.; Lewis, A. L. J. Am. Chem. Soc. 2011, 133, 15707– 15713 DOI: 10.1021/ja205887v
      There is no corresponding record for this reference.
    44. 44
      Liu, G.; Qiu, Q.; Shen, W.; An, Z. Macromolecules 2011, 44, 5237– 5245 DOI: 10.1021/ma200984h
      There is no corresponding record for this reference.
    45. 45
      Chambon, P.; Blanazs, A.; Battaglia, G.; Armes, S. P. Macromolecules 2012, 45, 5081– 5090 DOI: 10.1021/ma300816m
      There is no corresponding record for this reference.
    46. 46
      Ladmiral, V.; Semsarilar, M.; Canton, I.; Armes, S. P. J. Am. Chem. Soc. 2013, 135, 13574– 13581 DOI: 10.1021/ja407033x
      There is no corresponding record for this reference.
    47. 47
      Figg, C. A.; Simula, A.; Gebre, K. A.; Tucker, B. S.; Haddleton, D.; Sumerlin, B. S. Chem. Sci. 2015, 6, 1230 DOI: 10.1039/C4SC03334E
      47
      Polymerization-induced thermal self-assembly (PITSA)
      Figg, C. Adrian; Simula, Alexandre; Gebre, Kalkidan A.; Tucker, Bryan S.; Haddleton, David M.; Sumerlin, Brent S.
      Chemical Science (2015), 6 (2), 1230-1236CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)
      Polymn.-induced self-assembly (PISA) is a versatile technique to achieve a wide range of polymeric nanoparticle morphologies. Most previous examples of self-assembled soft nanoparticle synthesis by PISA rely on a growing solvophobic polymer block that leads to changes in nanoparticle architecture during polymn. in a selective solvent. However, synthesis of block copolymers with a growing stimuli-responsive block to form various nanoparticle shapes has yet to be reported. This new concept using thermo-responsive polymers is termed polymn.-induced thermal self-assembly (PITSA). A reversible addn.-fragmentation chain transfer (RAFT) polymn. of N-isopropylacrylamide from a hydrophilic chain transfer agent composed of N,N-dimethylacrylamide and acrylic acid was carried out in water above the known lower crit. soln. temp. (LCST) of poly(N-isopropylacrylamide) (PNIPAm). After reaching a certain chain length, the growing PNIPAm self-assembled, as induced by the LCST, into block copolymer aggregates within which dispersion polymn. continued. To characterize the nanoparticles at ambient temps. without their dissoln., the particles were crosslinked immediately following polymn. at elevated temps. via the reaction of the acid groups with a diamine in the presence of a carbodiimide. Size exclusion chromatog. was used to evaluate the unimer mol. wt. distributions and reaction kinetics. Dynamic light scattering and transmission electron microscopy provided insight into the size and morphologies of the nanoparticles. The resulting block copolymers formed polymeric nanoparticles with a range of morphologies (e.g., micelles, worms, and vesicles), which were a function of the PNIPAm block length.
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    48. 48
      Wan, W.-M.; Hong, C.-Y.; Pan, C.-Y. Chem. Commun. 2009, 5883– 5885 DOI: 10.1039/b912804b
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    49. 49
      Semsarilar, M.; Jones, E. R.; Blanazs, A.; Armes, S. P. Adv. Mater. 2012, 24, 3378– 3382 DOI: 10.1002/adma.201200925
      There is no corresponding record for this reference.
    50. 50
      Zehm, D.; Ratcliffe, L. P. D.; Armes, S. P. Macromolecules 2013, 46, 128– 139 DOI: 10.1021/ma301459y
      There is no corresponding record for this reference.
    51. 51
      Karagoz, B.; Boyer, C.; Davis, T. P. Macromol. Rapid Commun. 2014, 35, 417– 421 DOI: 10.1002/marc.201300730
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    52. 52
      Pei, Y.; Dharsana, N. C.; van Hensbergen, J. A.; Burford, R. P.; Roth, P. J.; Lowe, A. B. Soft Matter 2014, 10, 5787– 5796 DOI: 10.1039/C4SM00729H
      There is no corresponding record for this reference.
    53. 53
      Zhang, W.-J.; Hong, C.-Y.; Pan, C.-Y. Macromolecules 2014, 47, 1664– 1671 DOI: 10.1021/ma402497y
      There is no corresponding record for this reference.
    54. 54
      Bleach, R.; Karagoz, B.; Prakash, S. M.; Davis, T. P.; Boyer, C. ACS Macro Lett. 2014, 3, 591– 596 DOI: 10.1021/mz500195u
      54
      In Situ Formation of Polymer-Gold Composite Nanoparticles with Tunable Morphologies
      Bleach, Richard; Karagoz, Bunyamin; Prakash, Shyam M.; Davis, Thomas P.; Boyer, Cyrille
      ACS Macro Letters (2014), 3 (7), 591-596CODEN: AMLCCD; ISSN:2161-1653. (American Chemical Society)
      A simple and efficient route to gold-polymer nanoparticle composites is described. Our versatile synthetic route exerts facile control over polymer nanoparticle morphol., including micelles, rod-like structures, and vesicles, all easily attainable from a single polymn. taken to different monomer conversions. Specifically, poly[oligo(ethylene glycol) methacrylate]-b-poly(dimethylaminoethyl methacrylate)-b-poly(styrene) (POEGMA-b-PDMAEMA-b-PST) triblock copolymers were synthesized using a polymn. induced self-assembly (PISA) approach. Subsequently, spherical gold nanoparticles (10 nm AuNPs) were formed at the hydrophilic-hydrophobic nexus of the assembled triblock copolymer nanoaggregates by the addn. of chloroauric acid (HAuCl4) followed by in situ redn. using NaBH4. After redn., the cloudy white nanoparticle dispersions turned to a red-purple color. The gold nanoparticles that formed were stabilized by the enveloping polymeric nanostructures, neither pptn. nor agglomeration occurred. We demonstrated that we were able to tune the gold nanoparticle compn. in these polymer-gold composites by varying the concn. of chloroauric acid. Morphol., particle size, mol. wt., AuNP content, and chem. structure of the polymer structures were characterized by transmittance electron microscopy (TEM), dynamic light scattering (DLS), size exclusion chromatog. (SEC), thermal gravimetric anal. (TGA), and 1H NMR. Finally, the formation of the AuNPs occurred without affecting the polymer nanoparticle morphol.
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    55. 55
      Semsarilar, M.; Ladmiral, V.; Blanazs, A.; Armes, S. P. Polym. Chem. 2014, 5, 3466– 3475 DOI: 10.1039/c4py00201f
      55
      Poly(methacrylic acid)-based AB and ABC block copolymer nano-objects prepared via RAFT alcoholic dispersion polymerization
      Semsarilar, M.; Ladmiral, V.; Blanazs, A.; Armes, S. P.
      Polymer Chemistry (2014), 5 (10), 3466-3475CODEN: PCOHC2; ISSN:1759-9962. (Royal Society of Chemistry)
      A series of well-defined amphiphilic poly(methacrylic acid)-poly(benzyl methacrylate) (PMAA-PBzMA) diblock copolymers were synthesized via polymn.-induced self-assembly using an alc. dispersion polymn. formulation. Chain growth was mediated via reversible addn.-fragmentation chain transfer polymn. (RAFT) chem. using a trithiocarbonate-based chain transfer agent (CTA) at 70 °C. The poly(methacrylic acid) block was sol. in ethanol and acts as a steric stabilizer for the growing insol. PBzMA chains, resulting in the in situ generation of diblock copolymer nano-objects in the form of spheres, worms or vesicles, depending on the precise reaction conditions. Copolymer morphologies can be covalently stabilized via crosslinking to prevent their dissocn. when transferred into aq. soln., which leads to the formation of highly anionic nano-objects due to ionization of the PMAA stabilizer chains. ABC triblock copolymer nanoparticles can also be prepd. using this approach, where the third block was based on the semi-fluorinated monomer, 2,2,2-trifluoroethyl methacrylate (TFEMA). GPC studies confirm that chain extension was efficient and high TFEMA conversions can be achieved. Microphase sepn. between the mutually incompatible PBzMA and semi-fluorinated PTFEMA core-forming blocks occurs, producing a range of remarkably complex semi-fluorinated triblock copolymer morphologies.
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    56. 56
      Gonzato, C.; Semsarilar, M.; Jones, E. R.; Li, F.; Krooshof, G. J. P.; Wyman, P.; Mykhaylyk, O. O.; Tuinier, R.; Armes, S. P. J. Am. Chem. Soc. 2014, 136, 11100– 11106 DOI: 10.1021/ja505406s
      There is no corresponding record for this reference.
    57. 57
      Houillot, L.; Bui, C.; Farcet, C. l.; Moire, C.; Raust, J.-A.; Pasch, H.; Save, M.; Charleux, B. ACS Appl. Mater. Interfaces 2010, 2, 434– 442 DOI: 10.1021/am900695a
      There is no corresponding record for this reference.
    58. 58
      Deng, Y.; Yang, C.; Yuan, C.; Xu, Y.; Bernard, J.; Dai, L.; Gérard, J.-F. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 4558– 4564 DOI: 10.1002/pola.26872
      58
      Hybrid organic-inorganic block copolymer nano-objects from RAFT polymerization-induced self-assembly
      Deng, Yuanming; Yang, Cangjie; Yuan, Conghui; Xu, Yiting; Bernard, Julien; Dai, Lizong; Gerard, Jean-Francois
      Journal of Polymer Science, Part A: Polymer Chemistry (2013), 51 (21), 4558-4564CODEN: JPACEC; ISSN:0887-624X. (John Wiley & Sons, Inc.)
      A simple route to org.-inorg. (O/I) nano-objects with different morphologies through polymn.-induced block copolymer self-assembly is described. The synthetic strategy relies on the chain-extension of polyhedral oligomeric silsesquioxanes (POSS)-contg. macro-CTA (PMAiBuPOSS13 and PMAiBuPOSS19) with styrene at 120 °C in octane, a selective solvent of the POSS-contg. block. The polymn. system was proven to afford a plethora of O/I nano-objects, such as spherical micelles, cylindrical micelles, and vesicles depending on the resp. molar masses of the PMAiBuPOSS and polystyrene (PS) blocks. The cooling procedure was also proven to be a crucial step to generate particles with a unique morphol. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013.
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    59. 59
      Fielding, L. A.; Derry, M. J.; Ladmiral, V.; Rosselgong, J.; Rodrigues, A. M.; Ratcliffe, L. P. D.; Sugihara, S.; Armes, S. P. Chem. Sci. 2013, 4, 2081– 2087 DOI: 10.1039/c3sc50305d
      59
      RAFT dispersion polymerization in non-polar solvents: facile production of block copolymer spheres, worms and vesicles in n-alkanes
      Fielding, Lee A.; Derry, Matthew J.; Ladmiral, Vincent; Rosselgong, Julien; Rodrigues, Aurelie M.; Ratcliffe, Liam P. D.; Sugihara, Shinji; Armes, Steven P.
      Chemical Science (2013), 4 (5), 2081-2087CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)
      Well-defined poly(lauryl methacrylate-benzyl methacrylate) (PLMA-PBzMA) diblock copolymer nanoparticles are prepd. in n-heptane at 90° via reversible addn.-fragmentation chain transfer (RAFT) polymn. Under these conditions, the PLMA macromol. chain transfer agent (macro-CTA) is sol. in n-heptane, whereas the growing PBzMA block quickly becomes insol. Thus this dispersion polymn. formulation leads to polymn.-induced self-assembly (PISA). Using a relatively long PLMA macro-CTA with a mean d.p. (DP) of 37 or higher leads to the formation of well-defined spherical nanoparticles of 41 to 139 nm diam., depending on the DP targeted for the PBzMA block. In contrast, TEM studies confirm that using a relatively short PLMA macro-CTA (DP = 17) enables both worm-like and vesicular morphologies to be produced, in addn. to the spherical phase. A detailed phase diagram has been elucidated for this more asym. diblock copolymer formulation, which ensures that each pure phase can be targeted reproducibly. 1H NMR spectroscopy confirmed that high BzMA monomer conversions (>97%) were achieved within 5 h, while GPC studies indicated that reasonably good blocking efficiencies and relatively low diblock copolymer polydispersities (Mw/Mn < 1.30) were obtained in most cases. Compared to prior literature reports, this all-methacrylic PISA formulation is particularly novel because: (i) it is the first time that higher order morphologies (e.g. worms and vesicles) have been accessed in non-polar solvents and (ii) such diblock copolymer nano-objects are expected to have potential boundary lubrication applications for engine oils.
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      Dan, M.; Huo, F.; Zhang, X.; Wang, X.; Zhang, W. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 1573– 1584 DOI: 10.1002/pola.26527
      60
      Dispersion RAFT polymerization of 4-vinylpyridine in toluene mediated with the macro-RAFT agent of polystyrene dithiobenzoate: Effect of the macro-RAFT agent chain length and growth of the block copolymer nano-objects
      Dan, Meihan; Huo, Fei; Zhang, Xu; Wang, Xiaohui; Zhang, Wangqing
      Journal of Polymer Science, Part A: Polymer Chemistry (2013), 51 (7), 1573-1584CODEN: JPACEC; ISSN:0887-624X. (John Wiley & Sons, Inc.)
      The dispersion reversible addn.-fragmentation chain transfer (RAFT) polymn. of 4-vinylpyridine in toluene in the presence of the polystyrene dithiobenzoate (PS-CTA) macro-RAFT agent with different chain length is discussed. The RAFT polymn. undergoes an initial slow homogeneous polymn. and a subsequent fast heterogeneous one. The RAFT polymn. rate is dependent on the PS-CTA chain length, and short PS-CTA generally leads to fast RAFT polymn. The dispersion RAFT polymn. induces the self-assembly of the in situ synthesized polystyrene-b-poly(4-vinylpyridine) block copolymer into highly concd. block copolymer nano-objects. The PS-CTA chain length exerts great influence on the particle nucleation and the size and morphol. of the block copolymer nano-objects. It is found, short PS-CTA leads to fast particle nucleation and tends to produce large-sized vesicles or large-compd. micelles, and long PS-CTA leads to formation of small-sized nanospheres. Comparison between the polymn.-induced self-assembly and self-assembly of block copolymer in the block-selective solvent is made, and the great difference between the two methods is demonstrated. The present study is anticipated to be useful to reveal the chain extension and the particle growth of block copolymer during the RAFT polymn. under dispersion condition. © 2013 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013.
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      Warren, N. J.; Mykhaylyk, O. O.; Mahmood, D.; Ryan, A. J.; Armes, S. P. J. Am. Chem. Soc. 2014, 136, 1023– 1033 DOI: 10.1021/ja410593n
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      Optimization of the RAFT polymerization conditions for the in situ formation of nano-objects via dispersion polymerization in alcoholic medium
      Zhao, Wei; Gody, Guillaume; Dong, Siming; Zetterlund, Per B.; Perrier, Sebastien
      Polymer Chemistry (2014), 5 (24), 6990-7003CODEN: PCOHC2; ISSN:1759-9962. (Royal Society of Chemistry)
      Hydrophilic polymer brushes based on poly(ethylene glycol) Me ether acrylate (P(PEGA454)) or poly(ethylene glycol) Me ether methacrylate (P(PEGMA475)), both having a trithiocarbonate end group, were prepd. in water-dioxane (9 : 1) at 44 °C via RAFT polymn. and subsequently used in RAFT dispersion polymn. of styrene in isopropanol at 90 °C. RAFT reaction conditions were first optimized to prep. P(PEGA454) and P(PEGMA475) macro-RAFT agents at high monomer conversions (>90%) and very low fraction of dead chains (<1%). Both polymer brushes allowed the prepn. of well-defined amphiphilic diblock copolymers (P(PEGA454)-b-PS and P(PEGMA475)-b-PS) which self-assemble in situ into nano-objects with various morphologies. Using relatively long chain P(PEGA454) or P(PEGMA475) macro-RAFT agents (DP ~ 75) leads to the formation of near uniform spherical nanoparticles with diams. ranging from 30 to 140 nm, depending on the targeted DP of the PS block. In contrast, TEM and DLS studies demonstrated that using a shorter P(PEGA454) or P(PEGMA475) macro-RAFT agent (DP ~ 20) enables the formation of worm-like micelles, vesicles and large compd. vesicle morphologies in addn. to spheres. Cryo-TEM was used to confirm polymn. induced morphol. transition, rather than morphologies obtained via self-assembly driven by selective solvent or solvent evapn. during the prepn. of samples for characterization.
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      Ratcliffe, L. P. D.; Blanazs, A.; Williams, C. N.; Brown, S. L.; Armes, S. P. Polym. Chem. 2014, 5, 3643– 3655 DOI: 10.1039/c4py00203b
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      RAFT polymerization of hydroxy-functional methacrylic monomers under heterogeneous conditions: effect of varying the core-forming block
      Ratcliffe, L. P. D.; Blanazs, A.; Williams, C. N.; Brown, S. L.; Armes, S. P.
      Polymer Chemistry (2014), 5 (11), 3643-3655CODEN: PCOHC2; ISSN:1759-9962. (Royal Society of Chemistry)
      Statistical copolymn. of a 1:1 molar ratio of a water-miscible monomer (2-hydroxyethyl methacrylate, HEMA) with a water-immiscible monomer (4-hydroxybutyl methacrylate, HBMA) has been conducted in water via reversible addn.-fragmentation chain transfer (RAFT) polymn. using a water-sol. poly(glycerol monomethacrylate) macromol. chain transfer agent (PGMA macro-CTA). In principle, such a hybrid formulation might be expected to be intermediate between RAFT dispersion polymn. and RAFT emulsion polymn. Under such circumstances, it is of particular interest to examine whether both monomers are actually consumed and, if so, whether their rates of reaction are comparable. Given the water-soly. of both the PGMA macro-CTA and the free radical azo initiator, it is perhaps counter-intuitive that the water-immiscible HBMA is initially consumed significantly faster than the water-miscible HEMA, as judged by 1H NMR studies of this copolymn. However, both comonomers are eventually almost fully consumed at 70 °C. A detailed phase diagram has been constructed for this RAFT formulation that enables reproducible syntheses of various pure copolymer morphologies, including spheres, worms and vesicles. It is emphasized that utilizing a 1:1 HEMA/HBMA molar ratio produces a core-forming statistical copolymer block that is isomeric with the poly(2-hydroxypropyl methacrylate) (PHPMA) core-forming block previously synthesized via RAFT aq. dispersion polymn. (see A. Blanazs et al., Macromols., 2012, 45, 5099-5107). Hence it is rather remarkable that the thermo-responsive behavior of PGMA-P(HBMA-stat-HEMA) statistical block copolymer worm gels differs qual. from that exhibited by PGMA-PHPMA diblock copolymer worm gels.
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      Binauld, S.; Delafresnaye, L.; Charleux, B.; D’Agosto, F.; Lansalot, M. Macromolecules 2014, 47, 3461– 3472 DOI: 10.1021/ma402549x
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      Carlsson, L.; Fall, A.; Chaduc, I.; Wagberg, L.; Charleux, B.; Malmstrom, E.; D’Agosto, F.; Lansalot, M.; Carlmark, A. Polym. Chem. 2014, 5, 6076– 6086 DOI: 10.1039/C4PY00675E
      77
      Modification of cellulose model surfaces by cationic polymer latexes prepared by RAFT-mediated surfactant-free emulsion polymerization
      Carlsson, Linn; Fall, Andreas; Chaduc, Isabelle; Waagberg, Lars; Charleux, Bernadette; Malmstroem, Eva; D'Agosto, Franck; Lansalot, Muriel; Carlmark, Anna
      Polymer Chemistry (2014), 5 (20), 6076-6086CODEN: PCOHC2; ISSN:1759-9962. (Royal Society of Chemistry)
      This paper presents the successful surface modification of a model cellulose substrate by the prepn. and subsequent phys. adsorption of cationic polymer latexes. The first part of the work introduces novel charged polymer nanoparticles constituted of amphiphilic block copolymers based on cationic P(DMAEMA-co-MAA) as the hydrophilic segment, and PMMA as the hydrophobic segment. First, RAFT polymn. of DMAEMA in water was performed at pH 7, below its pKa. The simultaneous hydrolysis of DMAEMA led to the formation of a statistical copolymer incorporating mainly protonated DMAEMA units and some deprotonated methacrylic acid units at pH 7. The following step was the RAFT-mediated surfactant-free emulsion polymn. of MMA using P(DMAEMA-co-MAA) as a hydrophilic macromol. RAFT agent. The formed amphiphilic block copolymers self-assembled into cationic latex nanoparticles by polymn.-induced self-assembly. The nanoparticles were found to increase in size with increasing molar mass of the hydrophobic block. The cationic latexes were subsequently adsorbed to cellulose model surfaces in a quartz crystal microbalance equipment with dissipation. The adsorbed amt., in mg m-2, increased with increasing size of the nanoparticles. This approach allows for phys. surface modification of cellulose, utilizing a water suspension of particles for which both the surface chem. and the surface structure can be altered in a well-defined way.
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      Zhang, X.; Cardozo, A. F.; Chen, S.; Zhang, W.; Julcour, C.; Lansalot, M.; Blanco, J.-F.; Gayet, F.; Delmas, H.; Charleux, B.; Manoury, E.; D’Agosto, F.; Poli, R. Chem. - Eur. J. 2014, 20, 15505– 15517 DOI: 10.1002/chem.201403819
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      Core-Shell Nanoreactors for Efficient Aqueous Biphasic Catalysis
      Zhang, Xuewei; Cardozo, Andres F.; Chen, Si; Zhang, Wenjing; Julcour, Carine; Lansalot, Muriel; Blanco, Jean-Francois; Gayet, Florence; Delmas, Henri; Charleux, Bernadette; Manoury, Eric; D'Agosto, Franck; Poli, Rinaldo
      Chemistry - A European Journal (2014), 20 (47), 15505-15517CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Water-borne phosphine-functionalized core-cross-linked micelles (CCM) consisting of a hydrophobic core and a hydrophilic shell were obtained as stable latexes by reversible addn.-fragmentation chain transfer (RAFT) in water in a one-pot, three-step process. Initial homogeneous aq.-phase copolymn. of methacrylic acid (MAA) and poly(ethylene oxide) Me ether methacrylate (PEOMA) is followed by copolymn. of styrene (S) and 4-diphenylphosphinostyrene (DPPS), yielding P(MAA-co-PEOMA)-b-P(S-co-DPPS) amphiphilic block copolymer micelles (M) by polymn.-induced self-assembly (PISA), and final micellar crosslinking with a mixt. of S and diethylene glycol dimethacrylate. The CCM were characterized by dynamic light scattering and NMR spectroscopy to evaluate size, dispersity, stability, and the swelling ability of various org. substrates. Coordination of [Rh(acac)(CO)2] (acac=acetylacetonate) to the core-confined phosphine groups was rapid and quant. The CCM and M latexes were then used, in combination with [Rh(acac)(CO)2], to catalyze the aq. biphasic hydroformylation of 1-octene, in which they showed high activity, recyclability, protection of the activated Rh center by the polymer scaffold, and low Rh leaching. The CCM latex gave slightly lower catalytic activity but significantly less Rh leaching than the M latex. A control expt. conducted in the presence of the sulfoxantphos ligand pointed to the action of the CCM as catalytic nanoreactors with substrate and product transport into and out of the polymer core, rather than as a surfactant in interfacial catalysis.
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      Comparison of pseudo-living character of RAFT polymerizations conducted under homogeneous and heterogeneous conditions
      Semsarilar, Mona; Jones, Elizabeth R.; Armes, Steven P.
      Polymer Chemistry (2014), 5 (1), 195-203CODEN: PCOHC2; ISSN:1759-9962. (Royal Society of Chemistry)
      RAFT dispersion polymn. of 2,2,2-trifluoroethyl methacrylate (TFEMA) is conducted in ethanol at 70 °C using either poly(2-(dimethylamino)ethyl methacrylate) or poly(methacrylic acid) as a macromol. chain transfer agent. If the diblock copolymer nanoparticles are not too large, the small refractive index difference between the PTFEMA cores and ethanol leads to minimal light scattering. This enables the pseudo-living character of RAFT formulations conducted under soln. and dispersion polymn. conditions to be compared by monitoring the loss of RAFT chain-ends via UV-visible absorption spectroscopy. Significantly fewer chain-ends are lost during RAFT dispersion polymn., suggesting that such heterogeneous formulations have greater pseudo-living character. Moreover, 19F NMR spectroscopy provides the first direct exptl. evidence that RAFT dispersion polymn. proceeds via monomer-swollen block copolymer micelles. The relatively low refractive index of PTFEMA complicates GPC anal., leading to apparent contamination of the diblock copolymer and erroneously high polydispersities. However, this artifact can be cor. by deconvolution of the GPC curves, followed by their reconstruction using appropriate refractive indexes.
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      Chemical Science (2014), 5 (2), 738-743CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)
      A facile one-pot method is described for the formation of novel heterostructures in which inorg. nanoparticles are homogeneously distributed throughout an inorg. single crystal matrix. Our strategy uses nanoparticles functionalised with a poly(sodium 4-styrenesulfonate)-poly(methacrylic acid) [PNaStS-PMAA] diblock copolymer as a sol. crystal growth additive. This copolymer plays a no. of essential roles. The PMAA anchor block is phys. adsorbed onto the inorg. nanoparticles, while the PNaStS block acts as an electrosteric stabilizer and ensures that the nanoparticles retain their colloidal stability in the crystal growth soln. In addn., this strong acid block promotes binding to both the nanoparticles and the host crystal, which controls nanoparticle incorporation within the host crystal lattice. We show that this approach can be used to achieve encapsulation loadings of at least 12 wt% copolymer-coated magnetite particles within calcite single crystals. Transmission electron microscopy shows that these nanoparticles are uniformly distributed throughout the calcite, and that the crystal lattice retains its continuity around the embedded magnetite particles. Characterization of these calcite/magnetite nanocomposites confirmed their magnetic properties. This new exptl. approach is expected to be quite general, such that a small family of block copolymers could be used to drive the incorporation of a wide range of pre-prepd. nanoparticles into host crystals, giving intimate mixing of phases with contrasting properties, while limiting nanoparticle aggregation and migration.
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      Ning, Y.; Fielding, L. A.; Andrews, T. S.; Growney, D. J.; Armes, S. P. Sulfate-based anionic diblock copolymer nanoparticles for efficient occlusion within zinc oxide Nanoscale 2015, 7, 6691– 6702
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  • Supporting Information

    Supporting Information

    The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.5b02212.

    • 31P NMR spectra and GPC data for the three PMPA macro-CTAs, GPC and DLS data for kinetics of dispersion polymerization experiments, GPC data for all PMPx–PBzMAy diblock copolymers, additional TEM images, DLS data for aqueous dispersion polymerizations conducted using binary mixtures of macro-CTAs, and Raman spectra for selected CaCO3 occlusion experiments (PDF)


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