Effect of Core Cross-linking on the Physical Properties of Poly(dimethylsiloxane)-Based Diblock Copolymer Worms Prepared in Silicone Oil

Degradable Block Copolymer Nanoparticles Synthesized by Polymerization-Induced Self-Assembly

Polymerization-induced self-assembly (PISA) combines polymerization and in situ self-assembly of block copolymers in one system and has become a widely used method to prepare block copolymer nanoparticles at high concentrations. The persistence of polymers in the environment poses a huge threat to the ecosystem and represents a significant waste of resources. There is an urgent need to develop novel chemical approaches to synthesize degradable polymers. To meet with this demand, it is crucial to install degradability into PISA nanoparticles. Most recently, degradable PISA nanoparticles have been synthesized by introducing degradation mechanisms into either shell-forming or core-forming blocks. This Minireview summarizes the development in degradable block copolymer nanoparticles synthesized by PISA, including shell-degradable, core-degradable, and all-degradable nanoparticles. Future development will benefit from expansion of polymerization techniques with new degradation mechanisms and adaptation of high-throughput approaches for both PISA syntheses and degradation studies.

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.

Well-shaped poly(dimethylsiloxane)-based copolymer nanowires from spherical micelles via kinetic shape evolution

The formation of self-assembled arrays or superstructures from copolymers has attracted intense research interest. Herein, we propose a kinetic approach to form self-assembled nanowires using a PDMS-based block copolymer consisting of poly(dimethylsiloxane)-b-poly[2-(cinnamoyloxy)ethyl methacrylate] (PDMS-b-PCEMA). The copolymer was synthesized by using the macroinitiator PDMS-Br to initiate 2-(trimethylsiloxy)ethyl methacrylate (HEMA-TMS) via ATRP, followed by hydrolysis of the TMS group and gradual esterification with cinnamoyl chloride. PDMS-b-PCEMA presented core-shell spherical micelles in tetrahydrofuran, which transformed into nanowires within 5 days self-assembly via a typical kinetic shape evolution. The diameter of the assembled nanowires with a PCEMA inner core and PDMS shell was about 25-35 nm. The formation of these nanowires reflected a balance between the PDMS and PCEMA components: the PDMS segment was soluble enough to form a corona block, which was beneficial for the transformation of the micellar shape. Meanwhile, the PCEMA segment was able to control the diameter of the nanowire micelles but had no decisive effect on their formation. The effect of solvents on the self-assembled micelles indicated that nanowires were formed in tetrahydrofuran and dichloromethane, while core-shell micelles were formed in acetone. This was due to the different permittivities of these solvents. The nanowires were fixed by cross-linking the PCEMA group under UV irradiation, which enhanced their stability. We believe that this work provides a new strategy for the formation of nanowires and offers a guide for the diversified self-assembly of nanostructures from copolymers.

Reverse Sequence Polymerization‐Induced Self‐Assembly in Aqueous Media

We report a new aqueous polymerization‐induced self‐assembly (PISA) formulation that enables the hydrophobic block to be prepared first when targeting diblock copolymer nano‐objects. This counter‐intuitive reverse sequence approach uses an ionic reversible addition–fragmentation chain transfer (RAFT) agent for the RAFT aqueous dispersion polymerization of 2‐hydroxypropyl methacrylate (HPMA) to produce charge‐stabilized latex particles. Chain extension using a water‐soluble methacrylic, acrylic or acrylamide comonomer then produces sterically stabilized diblock copolymer nanoparticles in an aqueous one‐pot formulation. In each case, the monomer diffuses into the PHPMA particles, which act as the locus for the polymerization. A remarkable change in morphology occurs as the ≈600 nm latex is converted into much smaller sterically stabilized diblock copolymer nanoparticles, which exhibit thermoresponsive behavior. Such reverse sequence PISA formulations enable the efficient synthesis of new functional diblock copolymer nanoparticles.

Sterically Stabilized Diblock Copolymer Nanoparticles Enable Convenient Preparation of Suspension Concentrates Comprising Various Agrochemical Actives

It is well known that sterically stabilized diblock copolymer nanoparticles can be readily prepared using polymerization-induced self-assembly. Recently, we reported that such nanoparticles can be employed as a dispersant to prepare micron-sized particles of a widely used fungicide (azoxystrobin) via ball milling. In the present study, we examine the effect of varying the nature of the steric stabilizer block, the mean nanoparticle diameter, and the glass transition temperature (Tg) of the core-forming block on the particle size and colloidal stability of such azoxystrobin microparticles. In addition, the effect of crosslinking the nanoparticle cores is also investigated. Laser diffraction studies indicated the formation of azoxystrobin microparticles of approximately 2 μm diameter after milling for between 15 and 30 min at 6000 rpm. Diblock copolymer nanoparticles comprising a non-ionic steric stabilizer, rather than a cationic or anionic steric stabilizer, were determined to be more effective dispersants. Furthermore, nanoparticles of up to 51 nm diameter enabled efficient milling and ensured overall suspension concentrate stability. Moreover, crosslinking the nanoparticle cores and adjusting the Tg of the core-forming block had little effect on the milling of azoxystrobin. Finally, we show that this versatile approach is also applicable to five other organic crystalline agrochemicals, namely pinoxaden, cyproconazole, difenoconazole, isopyrazam and tebuconazole. TEM studies confirmed the adsorption of sterically stabilized nanoparticles at the surface of such agrochemical microparticles. The nanoparticles are characterized using TEM, DLS, aqueous electrophoresis and 1H NMR spectroscopy, while the final aqueous' suspension concentrates comprising microparticles of the above six agrochemical actives are characterized using optical microscopy, laser diffraction and electron microscopy.

Block Copolymer Vesicles with Tunable Membrane Thicknesses and Compositions Prepared by Aqueous Seeded Photoinitiated Polymerization-Induced Self-Assembly at Room Temperature

Block copolymer vesicles with diverse functionalities and intrinsic hollow structures have received considerable attention due to their broad applications in biomedical fields, including drug delivery, bioimaging, theranostics, gene therapy, etc. However, efficient preparation of block copolymer vesicles with tunable membrane thicknesses and compositions under mild conditions is still a challenge. Herein, we report an aqueous seeded photoinitiated polymerization-induced self-assembly (photo-PISA) for the precise preparation of block copolymer vesicles at room temperature. By changing the total degree of polymerization (DP) of the hydrophobic block in seeded photo-PISA, one can easily tune the membrane thickness without compromising the morphology of vesicles. Moreover, by adding different comonomers such as hydrophobic monomers, hydrophilic monomers, and cross-linkers into seeded photo-PISA, vesicles with different compositions could be prepared without compromising the morphology and colloidal stability. Polymerization kinetics show that seeded photo-PISA can skip the step of in situ self-assembly with a short homogeneous polymerization stage being observed. To demonstrate potential biological applications, enzymatic nanoreactors were constructed by loading horseradish peroxidase (HRP) inside vesicles via seeded photo-PISA. The enzymatic properties of these nanoreactors could be easily regulated by changing the membrane thickness and hydrophobicity. It is expected that this method can provide a facile platform for the precise preparation of block copolymer vesicles that may find applications in different fields.

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.

Shear-induced alignment of block copolymer worms in mineral oil

Poly(stearyl methacrylate)-poly(benzyl methacrylate) [PSMA-PBzMA] diblock copolymer worms were synthesized directly in mineral oil via reversible addition-fragmentation chain transfer (RAFT) dispersion polymerization at 90 °C. Free-standing gels were obtained from this polymerization-induced self-assembly (PISA) formulation when targeting PSMA₁₃-PBzMA₆₅ dispersions at 5% w/w to 20% w/w copolymer concentration. Gel permeation chromatography (GPC) studies indicated that almost identical copolymer chains were obtained in all cases, while transmission electron microscopy (TEM) and small-angle X-ray scattering (SAXS) studies confirmed that highly anisotropic worms were formed with mean cross-sectional diameters of 11.9-13.1 nm. These worms undergo a thermoreversible worm-to-sphere transition on heating up to 150 °C. Rheological studies were conducted to characterize the shear rate- and concentration-dependent behaviour caused by this change in copolymer morphology, where the initial shear-thinning worm gels form spheres (i.e. a Newtonian fluid) on heating up to 150 °C. Complementary shear-induced polarized light imaging (SIPLI) experiments confirmed the formation of aligned linear worms under applied shear between 80 °C and 110 °C, with high-viscosity dispersions of branched worms being obtained at 20-60 °C and low-viscosity spheres being produced at 150 °C. This study informs the use of such block copolymer worms as rheology modifiers for non-polar oils, which is of potential interest for the automotive industry.

Epoxy-functional diblock copolymer spheres, worms and vesicles via polymerization-induced self-assembly in mineral oil

Epoxy-functional poly(stearyl methacrylate)-poly(glycidyl methacrylate) spheres, worms or vesicles can be prepared by RAFT dispersion polymerization of glycidyl methacrylate in mineral oil at 70 °C.

Cross-linking approaches for block copolymer nano-assemblies via RAFT-mediated polymerization-induced self-assembly

This minireview summarizes the current cross-linking approaches to stabilize block copolymer nano-assemblies obtained via RAFT-mediated PISA process.