Exploring the Upper Size Limit for Sterically Stabilized Diblock Copolymer Nanoparticles Prepared by Polymerization-Induced Self-Assembly in Non-Polar Media

Controlling Adsorption of Diblock Copolymer Nanoparticles onto an Aldehyde-Functionalized Hydrophilic Polymer Brush via pH Modulation

Sterically stabilized diblock copolymer nanoparticles with a well-defined spherical morphology and tunable diameter were prepared by RAFT aqueous emulsion polymerization of benzyl methacrylate at 70 °C. The steric stabilizer precursor used for these syntheses contained pendent cis-diol groups, which means that such nanoparticles can react with a suitable aldehyde-functional surface via acetal bond formation. This principle is examined herein by growing an aldehyde-functionalized polymer brush from a planar silicon wafer and studying the extent of nanoparticle adsorption onto this model substrate from aqueous solution at 25 °C using a quartz crystal microbalance (QCM). The adsorbed amount, Γ, depends on both the nanoparticle diameter and the solution pH, with minimal adsorption observed at pH 7 or 10 and substantial adsorption achieved at pH 4. Variable-temperature QCM studies provide strong evidence for chemical adsorption, while scanning electron microscopy images recorded for the nanoparticle-coated brush surface after drying indicate mean surface coverages of up to 62%. This fundamental study extends our understanding of the chemical adsorption of nanoparticles on soft substrates.

Recent Advances in Polymerization-Induced Self-Assembly (PISA) Syntheses in Non-Polar Media

It is well-known that polymerization-induced self-assembly (PISA) is a powerful and highly versatile technique for the rational synthesis of colloidal dispersions of diblock copolymer nanoparticles, including spheres, worms or vesicles. PISA can be conducted in water, polar solvents or non-polar media. In principle, the latter formulations offer a wide range of potential commercial applications. However, there has been just one review focused on PISA syntheses in non-polar media and this prior article was published in 2016. The purpose of the current review article is to summarize the various advances that have been reported since then. In particular, PISA syntheses conducted using reversible addition-fragmentation chain-transfer (RAFT) polymerization in various n-alkanes, poly(α-olefins), mineral oil, low-viscosity silicone oils or supercritical CO2 are discussed in detail. Selected formulations exhibit thermally induced worm-to-sphere or vesicle-to-worm morphological transitions and the rheological properties of various examples of worm gels in non-polar media are summarized. Finally, visible absorption spectroscopy and small-angle X-ray scattering (SAXS) enable in situ monitoring of nanoparticle formation, while small-angle neutron scattering (SANS) can be used to examine micelle fusion/fission and chain exchange mechanisms.

Group-Transfer Polymerization-Induced Self-Assembly (GTPISA) in Non-polar Media: An Organocatalyzed Route to Block Copolymer Nanoparticles at Room Temperature

Polymerization-induced self-assembly (PISA) enables the synthesis at large scale of a wide variety of functional nanoparticles. However, a large number of works are related to controlled radical polymerization (CRP) methods and are generally undertaken at elevated temperatures (>50 °C). Here is the first report on methacrylate-based nanoparticles fabricated by group transfer polymerization-induced self-assembly (GTPISA) in non-polar media (n-heptane). This GTPISA process is achieved at room temperature (RT) using 1-methoxy-1-(trimethylsiloxy)-2-methylprop-1-ene (MTS) and tetrabutylammonium bis-benzoate (TBABB) as initiator and organic catalyst, respectively. Under these conditions, well-defined metal-free and colorless diblock copolymers are produced with efficient crossover from the non-polar stabilizing poly(lauryl methacrylate) (PLMA) block to the non-soluble poly(benzyl methacrylate) (PBzMA) segment. The resulting PLMA-b-PBzMA block copolymers simultaneously self-assemble into nanostructures of various sizes and morphologies. GTPISA in non-polar solvent proceeds rapidly at RT and avoids the use of sulfur or halogenated compounds or metallic catalysts associated with the implementation of CRP methods, thus expanding the potential of PISA formulations for applications in non-polar environments.

Histidine-Functionalized Diblock Copolymer Nanoparticles Exhibit Enhanced Adsorption onto Planar Stainless Steel

RAFT aqueous emulsion polymerization of isopropylideneglycerol monomethacrylate (IPGMA) is used to prepare a series of PGEO5MA46 -PIPGMAy nanoparticles, where PGEO5MA is a hydrophilic methacrylic steric stabilizer block bearing pendent cis-diol groups. TEM studies confirm a spherical morphology while dynamic light scattering (DLS) analysis indicated that the z-average particle diameter can be adjusted by varying the target degree of polymerization for the core-forming PIPGMA block. Periodate oxidation is used to convert the cis-diol groups on PGEO5MA46 -PIPGMA500 and PGEO5MA46 -PIPGMA1000 nanoparticles into the analogous aldehyde-functionalized nanoparticles, which are then reacted with histidine via reductive amination. In each case, the extent of functionalization is more than 99% as determined by 1 H NMR spectroscopy. Aqueous electrophoresis studies indicate that such derivatization converts initially neutral nanoparticles into zwitterionic nanoparticles with an isoelectric point at pH 7. DLS studies confirm that such histidine-derivatized nanoparticles remain colloidally stable over a wide pH range. A quartz crystal microbalance is employed at 25°C to assess the adsorption of both the cis-diol- and histidine-functionalized nanoparticles onto planar stainless steel at pH 6. The histidine-bearing nanoparticles adsorb much more strongly than their cis-diol counterparts. For the highest adsorbed amount of 70.5 mg m-2 , SEM indicates a fractional surface coverage of 0.23 for the adsorbed nanoparticles.

Synthesis of High Molecular Weight Water-Soluble Polymers as Low-Viscosity Latex Particles by RAFT Aqueous Dispersion Polymerization in Highly Salty Media

We report the synthesis of sterically-stabilized diblock copolymer particles at 20% w/w solids via reversible addition–fragmentation chain transfer (RAFT) aqueous dispersion polymerization of N,N′-dimethylacrylamide (DMAC) in highly salty media (2.0 M (NH₄)₂SO₄). This is achieved by selecting a well-known zwitterionic water-soluble polymer, poly(2-(methacryloyloxy)ethyl phosphorylcholine) (PMPC), to act as the salt-tolerant soluble precursor block. A relatively high degree of polymerization (DP) can be targeted for the salt-insoluble PDMAC block, which leads to the formation of a turbid free-flowing dispersion of PDMAC-core particles by a steric stabilization mechanism. ¹H NMR spectroscopy studies indicate that relatively high DMAC conversions (>99%) can be achieved within a few hours at 30 °C. Aqueous GPC analysis indicates high blocking efficiencies and unimodal molecular weight distributions, although dispersities increase monotonically as higher degrees of polymerization (DPs) are targeted for the PDMAC block. Particle characterization techniques include dynamic light scattering (DLS) and electrophoretic light scattering (ELS) using a state-of-the-art instrument that enables accurate ζ potential measurements in a concentrated salt solution. ¹H NMR spectroscopy studies confirm that dilution of the as-synthesized dispersions using deionized water lowers the background salt concentration and hence causes in situ molecular dissolution of the salt-intolerant PDMAC chains, which leads to a substantial thickening effect and the formation of transparent gels. Thus, this new polymerization-induced self-assembly (PISA) formulation enables high molecular weight water-soluble polymers to be prepared in a highly convenient, low-viscosity form. In principle, such aqueous PISA formulations are highly attractive: there are various commercial applications for high molecular weight water-soluble polymers, while the well-known negative aspects of using a RAFT agent (i.e., its cost, color, and malodor) are minimized when targeting such high DPs.

Reversible Addition-Fragmentation Chain Transfer Aqueous Dispersion Polymerization of 4-Hydroxybutyl Acrylate Produces Highly Thermoresponsive Diblock Copolymer Nano-Objects

The reversible addition-fragmentation chain transfer (RAFT) aqueous dispersion polymerization of 2-hydroxypropyl methacrylate (HPMA) using a poly(glycerol monomethacrylate) (PGMA) precursor is an important prototypical example of polymerization-induced self-assembly. 4-Hydroxybutyl acrylate (HBA) is a structural isomer of HPMA, but the former monomer exhibits appreciably higher aqueous solubility. For the two corresponding homopolymers, PHBA is more weakly hydrophobic than PHPMA. Moreover, PHBA has a significantly lower glass transition temperature (T g) so it exhibits much higher chain mobility than PHPMA at around ambient temperature. In view of these striking differences, we have examined the RAFT aqueous dispersion polymerization of HBA using a PGMA precursor with the aim of producing a series of PGMA57-300-PHBA100-1580 diblock copolymer nano-objects by systematic variation of the mean degree of polymerization of each block. A pseudo-phase diagram is constructed using transmission electron microscopy to assign the copolymer morphology after employing glutaraldehyde to cross-link the PHBA chains and hence prevent film formation during grid preparation. The thermoresponsive character of the as-synthesized linear nano-objects is explored using dynamic light scattering and temperature-dependent rheological measurements. Comparison with the analogous PGMA x -PHPMA y formulation is made where appropriate. In particular, we demonstrate that replacing the structure-directing PHPMA block with PHBA leads to significantly greater thermoresponsive behavior over a much wider range of diblock copolymer compositions. Given that PGMA-PHPMA worm gels can induce stasis in human stem cells (see Canton et al., ACS Central Science, 2016, 2, 65-74), our findings are likely to have implications for the design of next-generation PGMA-PHBA worm gels for cell biology applications.

Synthesis and derivatization of epoxy-functional sterically-stabilized diblock copolymer spheres in non-polar media: does the spatial location of the epoxy groups matter?

Epoxy-functional sterically-stabilized diblock copolymer nanoparticles are prepared via PISA in mineral oil and then derivatized using various reagents and reaction conditions.

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).

In situ conversion from crew-cut to hairy micelles by surface-initiated polymerization

Whether spherical micelles of block copolymers have short or long coronas is intrinsically determined by the molecular weight of the corona-forming block with respect to that of the core block before the micelles are assembled. Because of the inherent conditions of packing copolymer chains into a micelle, the core diameter is altered when we assemble a micelle from a block copolymer having a long corona block, compared to that having a short corona block with the same length of the core block. However, micelles with the same core diameter but having various corona lengths can be guaranteed when the corona is extended upon surface-initiated polymerization on the micelles. Herein, we demonstrated in situ conversion from crew-cut to hairy micelles by selectively extending a corona block while maintaining the spherical shape of block copolymer micelles. We first synthesized block copolymers having a chain transfer agent (CTA) positioned at the end of the corona block and then assembled them into a crew-cut micelle. Employing this micelle as an assembly of macro-CTAs, we conducted surface-initiated polymerization on the micelle by photo-induced energy/electron transfer reversible addition-fragmentation chain transfer (PET-RAFT) polymerization. Since PET-RAFT enables the polymerization at room temperature, the corona block was selectively extended with preservation of the core diameter, thereby converting a crew-cut micelle to a hairy one. In addition, by applying the same polymerization protocol to a worm-like micelle, we could selectively extend the coronas, leading to the formation of a worm-like micelle with a long corona. If such copolymer chains were assembled into a micelle, we would obtain a spherical micelle instead of a worm-like micelle having a hairy corona, which is difficult to assess because of the inherent packing problem.

Influence of the Polymer Microstructure over the Phase Separation of Thermo-Responsive Nanoparticles

Thermo-responsive nanoparticles (NPs), i.e., colloids with a sharp and often reversible phase separation in response to thermal stimuli, are coming to the forefront due to their dynamic behavior, useful in applications ranging from biomedicine to advanced separations and smart optics. What is guiding the macroscopic behavior of these systems above their critical temperature is mainly the microstructure of the polymer chains of which these NPs are comprised. Therefore, a comprehensive understanding of the influence of the polymer properties over the thermal response is highly required to reproducibly target a specific behavior. In this study, we synthesized thermo-responsive NPs with different size, polymeric microstructure and hydrophilic-lipophilic balance (HLB) and investigated the role of these properties over their phase separation. We first synthesized four different thermo-responsive oligomers via Reversible Addition-Fragmentation Chain Transfer (RAFT) Polymerization of poly(ethylene glycol)methyl ether methacrylate. Then, exploiting the RAFT living character, we chain-extended these oligomers with butyl methacrylate obtaining a library of NPs. Finally, we investigated the NP thermo-responsive behavior, their physical state above the cloud point (Tcp) as well as their reversibility once the stimulus is removed. We concluded that the solid content plays a minor role compared to the relative length of the two blocks forming the polymer chains. In particular, the longer the stabilizer, the more favored the formation of a gel. At the same time, the reversibility is mainly achieved at high HLB, independently from the absolute lengths of the block copolymers.

Bromoform-assisted aqueous free radical polymerisation: a simple, inexpensive route for the preparation of block copolymers

Synthesis of ‘uncontrolled’ commercially-relevant block copolymers by metal- and sulfur-free, bromoform-assisted polymerisation.

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.