Preparation and Aqueous Solution Properties of Thermoresponsive Biocompatible AB Diblock Copolymers

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).

Pickering Emulsifiers Based on Block Copolymer Nanoparticles Prepared by Polymerization-Induced Self-Assembly

Block copolymer nanoparticles prepared via polymerization-induced self-assembly (PISA) represent an emerging class of organic Pickering emulsifiers. Such nanoparticles are readily prepared by chain-extending a soluble homopolymer precursor using a carefully selected second monomer that forms an insoluble block in the chosen solvent. As the second block grows, it undergoes phase separation that drives in situ self-assembly to form sterically stabilized nanoparticles. Conducting such PISA syntheses in aqueous solution leads to hydrophilic nanoparticles that enable the formation of oil-in-water emulsions. Alternatively, hydrophobic nanoparticles can be prepared in non-polar media (e.g., n-alkanes), which enables water-in-oil emulsions to be produced. In this review, the specific advantages of using PISA to prepare such bespoke Pickering emulsifiers are highlighted, which include fine control over particle size, copolymer morphology, and surface wettability. This has enabled various fundamental scientific questions regarding Pickering emulsions to be addressed. Moreover, block copolymer nanoparticles can be used to prepare Pickering emulsions over various length scales, with mean droplet diameters ranging from millimeters to less than 200 nm.

The Effect of Number of Arms on the Aggregation Behavior of Thermoresponsive Poly(N-isopropylacrylamide) Star Polymers

The thermoresponsive nature of aqueous solutions of poly(N-isopropylacrylamide) (PNIPAAM) star polymers containing 2, 3, 4, and 6 arms has been investigated by turbidity, dynamic light scattering, rheology, and rheo-SALS. Simulations of the thermosensitive nature of the single star polymers have also been conducted. Some of the samples form aggregates even at temperatures significantly below the lower critical solution temperature (LCST) of PNIPAAM. Increasing concentration and number of arms promotes associations at low temperatures. When the temperature is raised, there is a competition between size increase due to enhanced aggregation and a size reduction caused by contraction. Monte Carlo simulations show that the single stars contract with increasing temperature, and that this contraction is more pronounced when the number of arms is increased. Some samples exhibit a minimum in the turbidity data after the initial increase at the cloud point. The combined rheology and rheo-SALS data suggest that this is due to a fragmentation of the aggregates followed by re-aggregation at even higher temperatures. Although the 6-arm star polymer aggregates more than the other stars at low temperatures, the more compact structure renders it less prone to aggregation at temperatures above the cloud point.

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.

Aqueous solution behavior of stimulus-responsive poly(methacrylic acid)-poly(2-hydroxypropyl methacrylate) diblock copolymer nanoparticles

RAFT aqueous dispersion polymerization is used to prepare poly(methacrylic acid)-poly(2-hydroxypropyl methacrylate) diblock copolymer nanoparticles, which exhibit stimulus-responsive behaviour on adjusting the solution temperature and/or solution pH.

Critical Dependence of Molecular Weight on Thermoresponsive Behavior of Diblock Copolymer Worm Gels in Aqueous Solution

Reversible addition-fragmentation chain transfer (RAFT) aqueous dispersion polymerization of 2-hydroxypropyl methacrylate was used to prepare three poly(glycerol monomethacrylate) x -poly(2-hydroxypropyl methacrylate) y (denoted G x -H y  or PGMA-PHPMA) diblock copolymers, namely G37-H80, G54-H140, and G71-H200. A master phase diagram was used to select each copolymer composition to ensure that a pure worm phase was obtained in each case, as confirmed by transmission electron microscopy (TEM) and small-angle x-ray scattering (SAXS) studies. The latter technique indicated a mean worm cross-sectional diameter (or worm width) ranging from 11 to 20 nm as the mean degree of polymerization (DP) of the hydrophobic PHPMA block was increased from 80 to 200. These copolymer worms form soft hydrogels at 20 °C that undergo degelation on cooling. This thermoresponsive behavior was examined using variable temperature DLS, oscillatory rheology, and SAXS. A 10% w/w G37-H80 worm dispersion dissociated to afford an aqueous solution of molecularly dissolved copolymer chains at 2 °C; on returning to ambient temperature, these chains aggregated to form first spheres and then worms, with the original gel strength being recovered. In contrast, the G54-H140 and G71-H200 worms each only formed spheres on cooling to 2 °C, with thermoreversible (de)gelation being observed in the former case. The sphere-to-worm transition for G54-H140 was monitored by variable temperature SAXS: these experiments indicated the gradual formation of longer worms at higher temperature, with a concomitant reduction in the number of spheres, suggesting worm growth via multiple 1D sphere-sphere fusion events. DLS studies indicated that a 0.1% w/w aqueous dispersion of G71-H200 worms underwent an irreversible worm-to-sphere transition on cooling to 2 °C. Furthermore, irreversible degelation over the time scale of the experiment was also observed during rheological studies of a 10% w/w G71-H200 worm dispersion. Shear-induced polarized light imaging (SIPLI) studies revealed qualitatively different thermoreversible behavior for these three copolymer worm dispersions, although worm alignment was observed at a shear rate of 10 s-1 in each case. Subsequently conducting this technique at a lower shear rate of 1 s-1 combined with ultra small-angle x-ray scattering (USAXS) also indicated that worm branching occurred at a certain critical temperature since an upturn in viscosity, distortion in the birefringence, and a characteristic feature in the USAXS pattern were observed. Finally, SIPLI studies indicated that the characteristic relaxation times required for loss of worm alignment after cessation of shear depended markedly on the copolymer molecular weight.

Thermoresponsive gels directly obtained via visible light-mediated polymerization-induced self-assembly with oxygen tolerance

A thermoresponsive reversible hydrogel is developed by PET-RAFT mediated PISA in aqueous solution at room temperature without deoxygenation.

Dual-responsive aggregation-induced emission-active supramolecular nanoparticles for gene delivery and bioimaging

Dual-responsive aggregation-induced emission-active supramolecular nanoparticles exhibit a unique morphological transition with fluorescence variation, further achieving efficient gene delivery and bioimaging.

Bespoke cationic nano-objects via RAFT aqueous dispersion polymerisation

Cationic diblock copolymer spheres, worms and vesicles are synthesised via polymerisation-induced self-assembly; worms form soft, thermoresponsive anti-bacterial gels at 20 °C.

Galactosylated Polymer Nano-objects by Polymerization-Induced Self-Assembly, Potential Drug Nanocarriers

Glycopolymer-based nanostructures are invaluable tools to both study biological phenomena and to design future targeted drug delivery systems. Polymerization-induced self-assembly, especially RAFT aqueous dispersion polymerization is a unique method to prepare such polymer nanostructures, as it enables the preparation of very-well-defined morphologies at very high concentrations. Here we describe the implementation of PISA to the synthesis of galactosylated spheres, wormlike micelles and vesicles, and the preliminary results of cell toxicity, cell uptake, and cargo delivering capacity of galactose-decorated vesicles.

Polysulfobetaine-based diblock copolymer nano-objects via polymerization-induced self-assembly

A zwitterionic polysulfobetaine-based macro-CTA is used for the synthesis of spheres, worms or vesiclesviaaqueous RAFT dispersion polymerization of 2-hydroxypropyl methacrylate (HPMA). These new diblock copolymer nano-objects exhibit high tolerance of added salt.

Polymerization-induced self-assembly of block copolymer nano-objects via RAFT aqueous dispersion polymerization

In this Perspective, we discuss the recent development of polymerization-induced self-assembly mediated by reversible addition-fragmentation chain transfer (RAFT) aqueous dispersion polymerization. This approach has quickly become a powerful and versatile technique for the synthesis of a wide range of bespoke organic diblock copolymer nano-objects of controllable size, morphology, 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.

Aqueous worm gels can be reconstituted from freeze-dried diblock copolymer powder

Worm-like diblock copolymer nanoparticles comprising poly(glycerol monomethacrylate) (PGMA) as a stabilizer block and poly(2-hydroxypropyl methacrylate) (PHPMA) as a core-forming block were readily synthesized at 10% w/w solids via aqueous dispersion polymerization at 70 °C using Reversible Addition-Fragmentation chain Transfer (RAFT) chemistry. On cooling to 20 °C, soft transparent free-standing gels are formed due to multiple inter-worm interactions. These aqueous PGMA-PHPMA diblock copolymer worms were freeze-dried, then redispersed in water with cooling to 3-5 °C before warming up to 20 °C; this protocol ensures molecular dissolution of the copolymer chains, which aids formation of a transparent aqueous gel. Rheology, SAXS and TEM studies confirm that such reconstituted gels comprise formed PGMA-PHPMA copolymer worms and they possess essentially the same physical properties determined for the original worm gels prior to freeze-drying. Such worm gel reconstitution is expected to be highly beneficial in the context of various biomedical applications, since it enables worm gels to be readily prepared using a wide range of cell growth media as the continuous aqueous phase.

RAFT aqueous dispersion polymerization yields poly(ethylene glycol)-based diblock copolymer nano-objects with predictable single phase morphologies

A poly(ethylene glycol) (PEG) macromolecular chain transfer agent (macro-CTA) is prepared in high yield (>95%) with 97% dithiobenzoate chain-end functionality in a three-step synthesis starting from a monohydroxy PEG₁₁₃ precursor. This PEG₁₁₃-dithiobenzoate is then used for the reversible addition–fragmentation chain transfer (RAFT) aqueous dispersion polymerization of 2-hydroxypropyl methacrylate (HPMA). Polymerizations conducted under optimized conditions at 50 °C led to high conversions as judged by ¹H NMR spectroscopy and relatively low diblock copolymer polydispersities (M_w/M_n < 1.25) as judged by GPC. The latter technique also indicated good blocking efficiencies, since there was minimal PEG₁₁₃ macro-CTA contamination. Systematic variation of the mean degree of polymerization of the core-forming PHPMA block allowed PEG₁₁₃-PHPMA_x diblock copolymer spheres, worms, or vesicles to be prepared at up to 17.5% w/w solids, as judged by dynamic light scattering and transmission electron microscopy studies. Small-angle X-ray scattering (SAXS) analysis revealed that more exotic oligolamellar vesicles were observed at 20% w/w solids when targeting highly asymmetric diblock compositions. Detailed analysis of SAXS curves indicated that the mean number of membranes per oligolamellar vesicle is approximately three. A PEG₁₁₃-PHPMA_x phase diagram was constructed to enable the reproducible targeting of pure phases, as opposed to mixed morphologies (e.g., spheres plus worms or worms plus vesicles). This new RAFT PISA formulation is expected to be important for the rational and efficient synthesis of a wide range of biocompatible, thermo-responsive PEGylated diblock copolymer nano-objects for various biomedical applications.

Tuning the critical gelation temperature of thermo-responsive diblock copolymer worm gels

Tuning the thermo-responsive behavior of statistical diblock copolymer worms.

Polymerization-induced self-assembly of galactose-functionalized biocompatible diblock copolymers for intracellular delivery

Recent advances in polymer science are enabling substantial progress in nanobiotechnology, particularly in the design of new tools for enhanced understanding of cell biology and for smart drug delivery formulations. Herein, a range of novel galactosylated diblock copolymer nano-objects is prepared directly in concentrated aqueous solution via reversible addition-fragmentation chain transfer polymerization using polymerization-induced self-assembly. The resulting nanospheres, worm-like micelles, or vesicles interact in vitro with galectins as judged by a turbidity assay. In addition, galactosylated vesicles are highly biocompatible and allow intracellular delivery of an encapsulated molecular cargo.

New directions in thermoresponsive polymers

Interest in thermoresponsive polymers has steadily grown over many decades, and a great deal of work has been dedicated to developing temperature sensitive macromolecules that can be crafted into new smart materials. However, the overwhelming majority of previously reported temperature-responsive polymers are based on poly(N-isopropylacrylamide) (PNIPAM), despite the fact that a wide range of other thermoresponsive polymers have demonstrated similar promise for the preparation of adaptive materials. Herein, we aim to highlight recent results that involve thermoresponsive systems that have not yet been as fully considered. Many of these (co)polymers represent clear opportunities for advancements in emerging biomedical and materials fields due to their increased biocompatibility and tuneable response. By highlighting recent examples of newly developed thermoresponsive polymer systems, we hope to promote the development of new generations of smart materials.

Supramolecular Fluorescent Nanoparticles for Targeted Cancer Imaging

By a combination of excellent fluorescent performance with smart targeting capability for cancer-specific delivery, a promising class of calcein-based supramolecular fluorescent nanoparticles has been successfully prepared via a "bricks and mortar" strategy. Through tuning the molar ratio of adamantane-functionalized calcein (CA-AD)/β-cyclodextrin-grafted branched polyethylenimine (PEI-CD), the size of these fluorescent nanoparticles can be effectively controlled. Importantly, the β-cyclodextrin/adamantane (β-CD/AD = 1/1) host-guest interaction dramatically suppresses the π-π stacking and fluorescence self-quenching of calcein chromophores in water, leading to the formation of highly fluorescent nanoparticles. By introduction of the folate receptor, these fluorescent nanoparticles exhibit excellent cancer imaging efficiency.

Micelles from amphiphilic block copolyphosphates for drug delivery

Amphiphilic block copolyphosphates (PEP-b-PIPPs) are synthesized by two-step ROP of cyclic phosphate monomers with different pedant groups. They can spontaneously self-assemble into approximately spherical micelles ranging in size between 89 and 198 nm in water. A typical hydrophobic anti-cancer drug DOX is encapsulated into the micelles. The release rate of DOX slows down with increasing hydrophobic block length of PIPP. DOX-loaded micelles are investigated for the proliferation inhibition of Hela cells and the DOX dose required for 50% cellular growth inhibition is found to be 0.8 µg mL(-1). It is demonstrated that PEP-b-PIPP micelles can be used as a safe and promising drug delivery system.

Optical-scattering method for the determination of the local polymer concentration inside nanoparticles

We have developed a method based on the Mie theory for determining the local polymer concentration inside spherical nanoparticles, thereby obtaining vital information about whether the particles are swelling in the solvent or if they are contracted into a more compact structure. In addition, this method can be used to calculate the number density of the particles, the molecular weight of the particles, and (if M(n) of the polymer is known) the aggregation number. The calculations are based on the relationship between the size of the nanoparticles and the turbidity of the sample.

Achieving highly effective nonfouling performance for surface-grafted poly(HPMA) via atom-transfer radical polymerization

Human blood plasma and serum pose significant challenges to implanted devices because of highly unfavorable nonspecific protein adsorption on the surface. In this work, we introduce an improved two-step method to immobilize initiator thiols on a gold substrate for the surface-initiated atom-transfer radical polymerization (SI-ATRP) of hydroxypropyl methacrylate (HPMA). We investigate protein adsorption from a single-protein solution, diluted (10%) and undiluted (100%) human blood plasma, and serum on the poly(HPMA) brushes with different film thicknesses using surface plasmon resonance (SPR) sensors. SPR results show a correlation between antifouling properties and film thickness; that is, the poly(HPMA) brushes exhibit high protein resistance at medium film thicknesses of ∼25-40 nm (e.g. <0.3 ng/cm(2) for single-protein adsorption and 10% human blood plasma and serum, ∼24.5 ng/cm(2) for 100% human serum, and ∼52.8 ng/cm(2) for 100% human plasma at a thickness of ∼29 nm). With an optimal film thickness and surface roughness, the poly(HPMA) brush also demonstrates its high resistance to fibroblast adhesion. This work provides an alternative surface polymerization approach to preparing effective antifouling poly(HPMA) materials for potential applications in blood-contacting medical devices.