Low-Viscosity Route to High-Molecular-Weight Water-Soluble Polymers: Exploiting the Salt Sensitivity of Poly(N-acryloylmorpholine)

We report a new one-pot low-viscosity synthetic route to high molecular weight non-ionic water-soluble polymers based on polymerization-induced self-assembly (PISA). The RAFT aqueous dispersion polymerization of N-acryloylmorpholine (NAM) is conducted at 30 °C using a suitable redox initiator and a poly(2-hydroxyethyl acrylamide) (PHEAC) precursor in the presence of 0.60 M ammonium sulfate. This relatively low level of added electrolyte is sufficient to salt out the PNAM block, while steric stabilization is conferred by the relatively short salt-tolerant PHEAC block. A mean degree of polymerization (DP) of up to 6000 was targeted for the PNAM block, and high NAM conversions (>96%) were obtained in all cases. On dilution with deionized water, the as-synthesized sterically stabilized particles undergo dissociation to afford molecularly dissolved chains, as judged by dynamic light scattering and 1H NMR spectroscopy studies. DMF GPC analysis confirmed a high chain extension efficiency for the PHEAC precursor, but relatively broad molecular weight distributions were observed for the PHEAC-PNAM diblock copolymer chains (Mw/Mn > 1.9). This has been observed for many other PISA formulations when targeting high core-forming block DPs and is tentatively attributed to chain transfer to polymer, which is well known for polyacrylamide-based polymers. In fact, relatively high dispersities are actually desirable if such copolymers are to be used as viscosity modifiers because solution viscosity correlates closely with Mw. Static light scattering studies were also conducted, with a Zimm plot indicating an absolute Mw of approximately 2.5 × 106 g mol-1 when targeting a PNAM DP of 6000. Finally, it is emphasized that targeting such high DPs leads to a sulfur content for this latter formulation of just 23 ppm, which minimizes the cost, color, and malodor associated with the organosulfur RAFT agent.

Synthesis and Characterization of All-Acrylic Tetrablock Copolymer Nanoparticles: Waterborne Thermoplastic Elastomers via One-Pot RAFT Aqueous Emulsion Polymerization

Reversible addition-fragmentation chain transfer (RAFT) aqueous emulsion polymerization is used to prepare well-defined ABCB tetrablock copolymer nanoparticles via sequential monomer addition at 30 °C. The A block comprises water-soluble poly(2-(N-acryloyloxy)ethyl pyrrolidone) (PNAEP), while the B and C blocks comprise poly(t-butyl acrylate) (PtBA) and poly(n-butyl acrylate) (PnBA), respectively. High conversions are achieved at each stage, and the final sterically stabilized spherical nanoparticles can be obtained at 20% w/w solids at pH 3 and at up to 40% w/w solids at pH 7. A relatively long PnBA block is targeted to ensure that the final tetrablock copolymer nanoparticles form highly transparent films on drying such aqueous dispersions at ambient temperature. The kinetics of polymerization and particle growth are studied using 1H nuclear magnetic resonance spectroscopy, dynamic light scattering, and transmission electron microscopy, while gel permeation chromatography analysis confirmed a high blocking efficiency for each stage of the polymerization. Differential scanning calorimetry and small-angle X-ray scattering studies confirm microphase separation between the hard PtBA and soft PnBA blocks, and preliminary mechanical property measurements indicate that such tetrablock copolymer films exhibit promising thermoplastic elastomeric behavior. Finally, it is emphasized that targeting an overall degree of polymerization of more than 1000 for such tetrablock copolymers mitigates the cost, color, and malodor conferred by the RAFT agent.

Chirality-controlled polymerization-induced self-assembly

Recent studies have shown that biodegradable nanoparticles can be efficiently prepared with polymerization of N-carboxyanhydrides-induced self-assembly (NCA-PISA). However, thus far, the effect of chiral monomer ratio on such NCA-PISA formulations and the resulting nanoparticles has not yet been fully explored. Herein, we show, for the first time, that the morphology, secondary structure, and biodegradation rate of PISA nanoparticles can be controlled by altering the chiral ratio of the core-forming monomers. This chirality-controlled PISA (CC-PISA) method allowed the preparation of nanoparticles that are more adjustable and applicable for future biomedical applications. Additionally, the complex secondary peptide structure (ratio of α-helix to β-sheet) and π-π stacking affect the polymer self-assembly process. More specifically, a PEG45 macro-initiator was chain-extended with l- and d-phenylalanine (l- and d-Phe-NCA) in various molar ratios in dry THF at 15 wt%. This ring-opening polymerization (ROP) allowed the preparation of homo- and hetero-chiral Phe-peptide block copolymers that self-assembled in situ into nanoparticles. For homo-chiral formulations, polymers self-assembled into vesicles once a sufficiently high phenylalanine degree of polymerization (DP) was obtained. Hetero-chiral formulations formed larger nanoparticles with various morphologies and, much to our surprise, using an equal enantiomer ratio inhibited PISA and led to a polymer solution instead. Finally, it was shown that the enzymatic biodegradation rate of such PISA particles is greatly affected by the polymer chirality. This PISA approach could be of great value to fabricate nanoparticles that exploit chirality in disease treatment.

Pyridine-functional diblock copolymer nanoparticles synthesized via RAFT-mediated polymerization-induced self-assembly: effect of solution pH

Polymerization-induced self-assembly (PISA) via reversible addition-fragmentation chain transfer (RAFT) polymerization has become widely recognized as a versatile and efficient strategy to prepare complex block copolymer nanoparticles with controlled morphology, size, and surface functionality. In this article, we report the preparation of cationic sterically-stabilized poly(2-vinylpyridine)-poly(benzyl methacrylate) (P2VP-PBzMA) diblock copolymer nanoparticles via RAFT-mediated PISA under aqueous emulsion polymerization conditions. It is demonstrated that the solution pH during PISA has a dramatic effect on the resulting P2VP-PBzMA nanoparticles, as judged by dynamic light scattering (DLS), disc centrifuge photosedimentometry (DCP) and transmission electron microscopy (TEM). Varying the solution pH results in the P2VP stabilizer having different solubilities due to protonation/deprotonation of the pyridine groups. This allows P2VP-PBzMA nanoparticles with tunable diameters to be prepared by altering the DP of the stabilizer (P2VP) and/or core-forming block (PBzMA), or simply by changing the solution pH for a fixed copolymer composition. For example, P2VP-PBzMA nanoparticles with larger diameters can be obtained at higher solution pH as the protonation degree of the P2VP stabilizer has a large effect on both the aggregation of polymer chains during the PISA process, and the resulting behavior of the diblock copolymer nanoparticles. Changing the dispersion pH post-polymerization has a relatively limited effect on particle diameter. Furthermore, aqueous electrophoresis studies indicate that these P2VP-PBzMA nanoparticles had good colloidal stability and high cationic charge (>30 mV) below pH 5 and can be dispersed readily over a wide pH range.

RAFT aqueous dispersion polymerization of 4-hydroxybutyl acrylate: effect of end-group ionization on the formation and colloidal stability of sterically-stabilized diblock copolymer nanoparticles

Poly(2-hydroxyethyl acrylate)-poly(4-hydroxybutyl acrylate) nano-objects are prepared by aqueous polymerization-induced self-assembly (PISA) using an ionic RAFT agent.

Reverse sequence polymerization-induced self-assembly in aqueous media: a counter-intuitive approach to sterically-stabilized diblock copolymer nano-objects

A 500 nm charge-stabilized latex is converted into 40 nm sterically-stabilized nanoparticles via reverse sequence polymerization-induced self-assembly (PISA).

pH-responsive pickering foam created from self-aggregate polymer using dynamic covalent bond

HYPOTHESIS

Responsive surfactant systems based on dynamic covalent bond exhibit an unsatisfactory foamability and foam stability, despite their documented functionality in emulsions. As such we anticipate that the foaming performance should be improved by introducing Pickering effect, which is possible when the responsiveness of the dynamic covenant bonds controls not only the hydrophobicity of polymers but also their aggregation behavior (to form nanoparticles).

EXPERIMENTS

Here we created surface active nanoparticles made from self-aggregated polymers consisting of PAH (polyallylamine hydrochloride)-BA (benzaldehyde). The covalent imine bonds between originally hydrophilic PAH and hydrophobic BA are dynamic in that their formation and breakage is a function of solution pH, confirmed by ¹H NMR and dynamic interfacial tension measurement.

FINDINGS

At pH 7.4, a stable foam is achieved in the PAH-BA (amino to aldehyde ratio at 1:0.2) solution; while at pH 2.5, it defoams due to breakage of dynamic bonds corresponding to the measured diminishing surface activity. The reversibility of foaming-defoaming has been demonstrated by alternatively changing pH for multiple cycles, with the foaming performance persistent. The foam stability can be improved by more hydrophobic compounds e.g. at a lower amino to aldehyde ratio or using PAH-cinnamaldehyde (CA). The reversible and responsive foaming demonstrated in a Pickering system provides a new method to create novel foaming systems with properties desirable to many applications.

Synthesis of well-defined diblock copolymer nano-objects by RAFT non-aqueous emulsion polymerization of N-(2-acryloyloxy)ethyl pyrrolidone in non-polar media

RAFT non-aqueous emulsion polymerization of N-(2-acryloyloxy)ethyl pyrrolidone in n-dodecane using a poly(stearyl methacrylate) precursor is used to prepare sterically-stabilized nanoparticles, which are evaluated as a putative Pickering emulsifier.

Time-resolved small-angle neutron scattering studies of the thermally-induced exchange of copolymer chains between spherical diblock copolymer nanoparticles prepared via polymerization-induced self-assembly

Sterically-stabilized diblock copolymer nanoparticles (a.k.a. micelles) are prepared directly in non-polar media via polymerization-induced self-assembly (PISA). More specifically, a poly(lauryl methacrylate) chain transfer agent is chain-extended via reversible addition-fragmentation chain transfer (RAFT) dispersion polymerization of methyl methacrylate (MMA) to form sterically-stabilized spheres at 20% w/w solids in n-dodecane at 90 °C. Both fully hydrogenous (PLMA39-PMMA55 and PLMA39-PMMA94) and core-deuterated (PLMA39-d8PMMA57 and PLMA39-d8PMMA96) spherical nanoparticles with mean core diameters of approximately 20 nm were prepared using this protocol. After diluting each dispersion in turn to 1.0% w/w with n-dodecane, small-angle X-ray scattering studies confirmed essentially no change in spherical nanoparticle diameter after thermal annealing at 150 °C. Time-resolved small angle neutron scattering was used to examine whether copolymer chain exchange occurs between such nanoparticles at elevated temperatures. Copolymer chain exchange for a binary mixture of PLMA39-PMMA55 and PLMA39-d8PMMA57 nanoparticles produced hybrid (mixed) cores containing both PMMA55 and d8PMMA57 blocks within 3 min at 150 °C. In contrast, a binary mixture of PLMA39-PMMA94 and PLMA39-d8PMMA96 nanoparticles required 8 min at this temperature before no further reduction in neutron scattering intensity could be observed. These observations suggest that the rate of copolymer chain exchange depends on the degree of polymerization of the core-forming block. Relatively slow copolymer chain exchange was also observed at 80 °C, which is below the Tg of the core-forming PMMA block as determined by DSC studies. These observations confirm rapid exchange of individual copolymer chains between sterically-stabilized nanoparticles at elevated temperature. The implications of these findings are briefly discussed in the context of PISA, which is a powerful technique for the synthesis of sterically-stabilized nanoparticles.

How Do Charged End-Groups on the Steric Stabilizer Block Influence the Formation and Long-Term Stability of Pickering Nanoemulsions Prepared Using Sterically Stabilized Diblock Copolymer Nanoparticles?

Reversible addition-fragmentation chain transfer (RAFT) solution polymerization is used to prepare well-defined poly(glycerol monomethacrylate) (PGMA) chains bearing carboxylic acid, tertiary amine, or neutral end-groups. Each of these PGMA precursors was then chain-extended in turn via RAFT aqueous emulsion polymerization of 2,2,2-trifluoroethyl methacrylate to form spherical nanoparticles as confirmed by transmission electron microscopy (TEM) analysis. Dynamic light scattering studies indicated an intensity-average diameter of approximately 25 nm. Aqueous electrophoresis measurements confirmed that the amine-functional nanoparticles became cationic at low pH owing to end-group protonation. In contrast, carboxylic acid-functional nanoparticles became appreciably anionic at pH 10 owing to end-group ionization. Finally, nanoparticles bearing neutral end-groups exhibited zeta potentials close to zero over a range of solution pH. High-shear homogenization of n-dodecane in the presence of such sterically stabilized nanoparticles led to the formation of oil-in-water Pickering macroemulsions with volume-average diameters of 20-30 μm. High-pressure microfluidization was then used to prepare the three corresponding Pickering nanoemulsions. Each Pickering nanoemulsion was characterized by analytical centrifugation and TEM studies of the dried nanoemulsion droplets confirmed their original nanoparticle superstructure. The nanoparticle adsorption efficiency at the oil-water interface was assessed by gel permeation chromatography (using a UV detector) for each nanoparticle type at both pH 3 and 7. Nanoparticles with charged end-groups exhibited relatively low adsorption efficiency, whereas up to 90% of the neutral nanoparticles were adsorbed onto the oil droplets. This observation was supported by small-angle X-ray scattering experiments, which indicated that the packing efficiency of neutral nanoparticles around oil droplets was higher than that of nanoparticles bearing charged end-groups. Analytical centrifugation was used to evaluate the colloidal stability of the aged Pickering nanoemulsions. Pickering nanoemulsions stabilized with nanoparticles bearing charged end-groups proved to be significantly less stable than those prepared using neutral end-groups.

Synthesis of poly(stearyl methacrylate)-poly(2-hydroxypropyl methacrylate) diblock copolymer nanoparticles via RAFT dispersion polymerization of 2-hydroxypropyl methacrylate in mineral oil

RAFT dispersion polymerization of 2-hydroxypropyl methacrylate produces diblock copolymer spheres, worms or vesicles in mineral oil; the Pickering emulsifier performance of the spheres is examined.

RAFT dispersion polymerisation of lauryl methacrylate in ethanol–water binary mixtures: synthesis of diblock copolymer vesicles with deformable membranes

Diblock copolymer vesicles with deformable membranes are prepared via RAFT dispersion polymerisation of lauryl methacrylate in an 80 : 20 w/w ethanol–water mixture; visible light irradiation allows facile RAFT chain-end removal from these nano-objects.

Star amphiphilic block copolymers: synthesis via polymerization-induced self-assembly and crosslinking within nanoparticles, and solution and interfacial properties

The star amphiphilic block copolymer of star s-PNIPAM-b-PS is synthesized and it shows characteristics significantly different from those of the linear block copolymer counterpart.

In Situ Small-Angle X-ray Scattering Studies During Reversible Addition-Fragmentation Chain Transfer Aqueous Emulsion Polymerization

Polymerization-induced self-assembly (PISA) is a powerful platform technology for the rational and efficient synthesis of a wide range of block copolymer nano-objects (e.g., spheres, worms or vesicles) in various media. In situ small-angle X-ray scattering (SAXS) studies of reversible addition-fragmentation chain transfer (RAFT) dispersion polymerization have previously provided detailed structural information during self-assembly (see M. J. Derry et al., Chem. Sci. 2016 , 7 , 5078 - 5090 ). However, conducting the analogous in situ SAXS studies during RAFT aqueous emulsion polymerizations poses a formidable technical challenge because the inherently heterogeneous nature of such PISA formulations requires efficient stirring to generate sufficiently small monomer droplets. In the present study, the RAFT aqueous emulsion polymerization of 2-methoxyethyl methacrylate (MOEMA) has been explored for the first time. Chain extension of a relatively short non-ionic poly(glycerol monomethacrylate) (PGMA) precursor block leads to the formation of sterically-stabilized PGMA-PMOEMA spheres, worms or vesicles, depending on the precise reaction conditions. Construction of a suitable phase diagram enables each of these three morphologies to be reproducibly targeted at copolymer concentrations ranging from 10 to 30% w/w solids. High MOEMA conversions are achieved within 2 h at 70 °C, which makes this new PISA formulation well-suited for in situ SAXS studies using a new reaction cell. This bespoke cell enables efficient stirring and hence allows in situ monitoring during RAFT emulsion polymerization for the first time. For example, the onset of micellization and subsequent evolution in particle size can be studied when preparing PGMA29-PMOEMA30 spheres at 10% w/w solids. When targeting PGMA29-PMOEMA70 vesicles under the same conditions, both the micellar nucleation event and the subsequent evolution in the diblock copolymer morphology from spheres to worms to vesicles are observed. These new insights significantly enhance our understanding of the PISA mechanism during RAFT aqueous emulsion polymerization.

Emerging Trends in Polymerization-Induced Self-Assembly

In this Perspective, we summarize recent progress in polymerization-induced self-assembly (PISA) for the rational synthesis of block copolymer nanoparticles with various morphologies. Much of the PISA literature has been based on thermally initiated reversible addition-fragmentation chain transfer (RAFT) polymerization. Herein, we pay particular attention to alternative PISA protocols, which allow the preparation of nanoparticles with improved control over copolymer morphology and functionality. For example, initiation based on visible light, redox chemistry, or enzymes enables the incorporation of sensitive monomers and fragile biomolecules into block copolymer nanoparticles. Furthermore, PISA syntheses and postfunctionalization of the resulting nanoparticles (e.g., cross-linking) can be conducted sequentially without intermediate purification by using various external stimuli. Finally, PISA formulations have been optimized via high-throughput polymerization and recently evaluated within flow reactors for facile scale-up syntheses.

Combining the power of heat and light: temperature-programmed photoinitiated RAFT dispersion polymerization to tune polymerization-induced self-assembly

A novel temperature-programmed photo-PISA method which combines the power of heat and light is developed for the preparation of a diverse set of morphologies.