Synthesis of star thermoresponsive amphiphilic block copolymer nano-assemblies and the effect of topology on their thermoresponse

Well-defined multi-arm star thermoresponsive block copolymer nano-assemblies of [poly(N-isopropylacrylamide)-block-polystyrene]_n [(PNIPAM-b-PS)_n] with n = 1, 2, 3 and 4 arms were synthesized by RAFT dispersion polymerization via polymerization-induced self-assembly.

Thermoreversible crystallization-driven aggregation of diblock copolymer nanoparticles in mineral oil

A poly(behenyl methacrylate)37 (PBeMA37) macromolecular chain transfer agent is utilized for the reversible addition-fragmentation chain transfer (RAFT) dispersion polymerization of benzyl methacrylate (BzMA) directly in mineral oil at 90 °C. Polymerization-induced self-assembly (PISA) occurs under these conditions, yielding a series of sterically-stabilized PBeMA37-PBzMA x diblock copolymer spheres of tunable diameter as confirmed by dynamic light scattering (DLS) and transmission electron microscopy (TEM) studies. Rheological studies indicate that a relatively transparent, free-flowing, concentrated dispersion of non-interacting 32 nm PBeMA37-PBzMA100 spheres at 50 °C forms a turbid, paste-like dispersion on cooling to 20 °C. Turbidimetry and differential scanning calorimetry (DSC) studies conducted on solutions of PBeMA37 homopolymer in mineral oil suggest that this switchable colloidal stability is linked to crystallization-induced phase separation exhibited by this stabilizer block. Indeed, variable-temperature small-angle X-ray scattering (SAXS) indicates that a loose mass fractal network of strongly interacting spheres is formed on cooling to 20 °C, which accounts for this thermoreversible sol-gel transition. Moreover, SAXS, DSC and wide-angle X-ray scattering (WAXS) analyses indicate that the behenyl (C22H45) side-chains first form crystalline domains comprising adjacent stabilizer chains within individual spherical nanoparticles, with subsequent crystallization between neighboring nanoparticles leading to the formation of the mass fractal aggregates.

Influence of Grafted Block Copolymer Structure on Thermoresponsiveness of Superparamagnetic Core-Shell Nanoparticles

The morphology and topology of thermoresponsive polymers have a strong impact on their responsive properties. Grafting onto spherical particles has been shown to reduce responsiveness and transition temperatures; grafting of block copolymers has shown that switchable or retained wettability of a surface or particle during desolvation of one block can take place. Here, doubly thermoresponsive block copolymers were grafted onto spherical, monodisperse, and superparamagnetic iron oxide nanoparticles to investigate the effect of thermal desolvation on spherical brushes of block copolymers. By inverting the block order, the influence of core proximity on the responsive properties of the individual blocks could be studied as well as their relative influence on the nanoparticle colloidal stability. The inner block was shown to experience a stronger reduction in transition temperature and transition enthalpy compared to the outer block. Still, the outer block also experiences a significant reduction in responsiveness due to the restricted environment in the nanoparticle shell compared to that of the free polymer state. The demonstrated pronounced distance dependence importantly implies the possibility, but also the necessity, to radially tailor polymer hydration transitions for applications such as drug delivery, hyperthermia, and biotechnological separation for which thermally responsive nanoparticles are being developed.

Micellization and gelatinization in aqueous media of pH- and thermo-responsive amphiphilic ABC (PMMA82-b-PDMAEMA150-b-PNIPAM65) triblock copolymer synthesized by consecutive RAFT polymerization

The ABC triblock copolymer molecularly displays diverse properties in dilute solution and concentrated solution at different pH with elevated temperatures.

RAFT synthesis and micellization of a photo-, temperature- and pH-responsive diblock copolymer based on spiropyran

A photo-, temperature- and pH-responsive diblock copolymer containing a rigid spiropyran moiety was synthesized and its micellization was investigated.

How Do Spherical Diblock Copolymer Nanoparticles Grow during RAFT Alcoholic Dispersion Polymerization?

A poly(2-(dimethylamino)ethyl methacrylate) (PDMA) chain transfer agent (CTA) is used for the reversible addition-fragmentation chain transfer (RAFT) alcoholic dispersion polymerization of benzyl methacrylate (BzMA) in ethanol at 70 °C. THF GPC analysis indicated a well-controlled polymerization with molecular weight increasing linearly with conversion. GPC traces also showed high blocking efficiency with no homopolymer contamination apparent and Mw/Mn values below 1.35 in all cases. 1H NMR studies confirmed greater than 98% BzMA conversion for a target PBzMA degree of polymerization (DP) of up to 600. The PBzMA block becomes insoluble as it grows, leading to the in situ formation of sterically stabilized diblock copolymer nanoparticles via polymerization-induced self-assembly (PISA). Fixing the mean DP of the PDMA stabilizer block at 94 units and systematically varying the DP of the PBzMA block enabled a series of spherical nanoparticles of tunable diameter to be obtained. These nanoparticles were characterized by TEM, DLS, MALLS, and SAXS, with mean diameters ranging from 35 to 100 nm. The latter technique was particularly informative: data fits to a spherical micelle model enabled calculation of the core diameter, surface area occupied per copolymer chain, and the mean aggregation number (Nagg). The scaling exponent derived from a double-logarithmic plot of core diameter vs PBzMA DP suggests that the conformation of the PBzMA chains is intermediate between the collapsed and fully extended state. This is in good agreement with 1H NMR studies, which suggest that only 5-13% of the BzMA residues of the core-forming chains are solvated. The Nagg values calculated from SAXS and MALLS are in good agreement and scale approximately linearly with PBzMA DP. This suggests that spherical micelles grow in size not only as a result of the increase in copolymer molecular weight during the PISA synthesis but also by exchange of individual copolymer chains between micelles and/or by sphere-sphere fusion events.

In situ synthesis of thermo-responsive ACB triblock terpolymer nanoparticles through seeded RAFT polymerization

ACB triblock terpolymer nanoparticles containing two thermo-responsive blocks are prepared by seeded RAFT polymerization and their thermo-response is checked.

Synthesis and characterisation of non-ionic AB-diblock nanoparticles prepared by RAFT dispersion polymerization with polymerization-induced self-assembly

The RAFT-PISA synthesis and characterization of non-ionic soft matter nanoparticles is described.

Polymeric micelles stabilized by polyethylenimine–copper (C2H5N–Cu) coordination for sustained drug release

Polymeric micelles stabilized by polyethylenimine–copper (C₂H₅N–Cu) coordination were described to improve the release property of water-insoluble anticancer drug.

Polymerization-induced thermal self-assembly (PITSA)

Polymerization-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 polymerization 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 thermoresponsive polymers is termed polymerization-induced thermal self-assembly (PITSA). A reversible addition-fragmentation chain transfer (RAFT) polymerization 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 critical solution temperature (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 polymerization continued. To characterize the nanoparticles at ambient temperatures without their dissolution, the particles were crosslinked immediately following polymerization at elevated temperatures via the reaction of the acid groups with a diamine in the presence of a carbodiimide. Size exclusion chromatography was used to evaluate the unimer molecular weight 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.

Doubly thermo-responsive nanoparticles constructed with two diblock copolymers prepared through the two macro-RAFT agents co-mediated dispersion RAFT polymerization

Doubly thermo-responsive nanoparticles constructed with two diblock copolymers were prepared, and the nanoparticles exhibit a two-step phase-transition with increasing temperature.

A new strategy to prepare thermo-responsive multicompartment nanoparticles constructed with two diblock copolymers

Thermo-responsive multicompartment nanoparticles containing a PS core, discrete PVEA nodules on the PS core and a PDMAEMA corona are prepared.