Emerging Synthetic Strategies for Core Cross‐Linked Star (CCS) Polymers and Applications as Interfacial Stabilizers: Bridging Linear Polymers and Nanoparticles

Core cross‐linked star (CCS) polymers become increasingly important in polymer science and are evaluated in many value‐added applications. However, limitations exist to varied degrees for different synthetic methods. It is clear that improvement in synthetic efficiency is fundamental in driving this field moving even further. Here, the most recent advances are highlighted in synthetic strategies, including cross‐linking with cross‐linkers of low solubility, polymerization‐induced self‐assembly in aqueous‐based heterogeneous media, and cross‐linking via dynamic covalent bonds. The understanding of CCS polymers is also further refined to advocate their role as an intermediate between linear polymers and polymeric nanoparticles, and their use as interfacial stabilizers is rationalized within this context.

     

Highly Efficient Synthesis of Low Polydispersity Core Cross‐Linked Star Polymers by Ru‐Catalyzed Living Radical Polymerization

The efficient formation of low polydispersity core cross‐linked star (CCS) polymers via controlled/living radical polymerization (LRP) and the arm‐first approach was found to be dependant on the mediating catalyst system. The Ru catalyst, Ru(Ind)Cl(PPh₃)₂ Cat. 1 , and tertiary amine co‐catalyst were used to synthesize highly living poly(methyl methacrylate) (PMMA) macroinitiators, which were then linked together with ethylene glycol dimethacrylate (EGDMA) to form PMMA_armPEGDMA_core CCS polymers. The quantitative and near‐quantitative synthesis of CCS polymers were observed for low to moderate molecular weight macroinitiators ( = 8 and 20 kDa), respectively. Lower conversions were observed for high‐molecular weight macroinitiators ( ≥ 60 kDa). Overall, an improvement of between 10 and 20% was observed when comparing the Cat. 1 system to a conventional Cu‐catalyzed system. This significant improvement in macroinitiator‐to‐star conversion is explained in the context of catalyst system selection and CCS polymer formation.

     

In Situ Generated Janus Fabrics for the Rapid and Efficient Separation of Oil from Oil‐in‐Water Emulsions

A cotton fabric was coated with a polymer that contains both poly(dimethyl siloxane) (PDMS) and poly(N,N‐dimethylaminoethyl methacrylate) (PDMAEMA). When the repeat unit number of PDMS is about three‐fold that of PDMAEMA and the fabric is exposed to air, the fabric is superhydrophobic because PDMS in the coating covers the PDMAEMA chains. Upon contact with an oil‐in‐water emulsion, the water‐soluble PDMAEMA rises to the top and the side in contact with the emulsion becomes hydrophilic. The emerged PDMAEMA chains then cause the emulsion droplets to coagulate, and the aggregated oil fills the pores on the superhydrophobic side of the fabric. The oil‐impregnated side remains hydrophobic even upon prolonged contact with water. Thus, a Janus fabric is elegantly generated in situ and sustained. This easy‐to‐prepare Janus fabric rapidly and efficiently separates oil from emulsions and may find practical applications.     

Salt‐Induced Fabrication of Superhydrophilic and Underwater Superoleophobic PAA‐g‐PVDF Membranes for Effective Separation of Oil‐in‐Water Emulsions

Conventional polymer membranes suffer from low flux and serious fouling when used for treating emulsified oil/water mixtures. Reported herein is the fabrication of a novel superhydrophilic and underwater superoleophobic poly(acrylic acid)‐grafted PVDF filtration membrane using a salt‐induced phase‐inversion approach. A hierarchical micro/nanoscale structure is constructed on the membrane surface and endows it with a superhydrophilic/underwater superoleophobic property. The membrane separates both surfactant‐free and surfactant‐stabilized oil‐in‐water emulsions under either a small applied pressure (<0.3 bar) or gravity, with high separation efficiency and high flux, which is one to two orders of magnitude higher than those of commercial filtration membranes having a similar permeation property. The membrane exhibits an excellent antifouling property and is easily recycled for long‐term use. The outstanding performance of the membrane and the efficient, energy and cost‐effective preparation process highlight its potential for practical applications.     

Rapid and Efficient Separation of Oil from Oil‐in‐Water Emulsions Using a Janus Cotton Fabric

A novel bi‐functional Janus cotton fabric is used to separate oil from oil‐in‐water emulsions. This fabric is superhydrophobic on one surface and polyamine‐bearing on the other. When used as a filter, the polyamine‐bearing side causes the micrometer‐sized oil droplets to coalesce. The coalesced oil then fills fabric pores on the superhydrophobic side and selectively permeates it. Oil separation using this method is rapid and the separated oil is pure. Furthermore, the content of the model oil hexadecane (HD) in water after a separation can be reduced to less than 0.03±0.03 vol %. These features demonstrate the practical potential of this technology.     

1,1‐Diphenyl Ethylene‐Mediated Radical Polymerisation: A General Non‐Metal‐Based Technique For The Synthesis Of Precise Core Cross‐Linked Star Polymers

This communication details the successful synthesis of low polydispersity core cross‐linked star (CCS) polymers via DPE‐mediated polymerisation. We demonstrate the ability to produce poly(methyl methacrylate) and poly(acrylonitrile) CCS polymers that are currently inaccessible via the two most common non‐metal‐based controlled radical polymerisation techniques (NMP and RAFT polymerisations).

     

Preparation of Hybrid Thiol‐Acrylate Emulsion‐Templated Porous Polymers by Interfacial Copolymerization of High Internal Phase Emulsions

Emulsion‐templated highly porous polymers (polyHIPEs), containing distinct regions differing in composition, morphology, and/or properties, are prepared by the simultaneous polymerization of two high internal phase emulsions (HIPEs) contained within the same mould. The HIPEs are placed together in the mould and subjected to thiol‐acrylate photopolymerization. The resulting polyHIPE material is found to contain two distinct semicircular regions, reflecting the composition of each HIPE. The original interface between the two emulsions becomes a copolymerized band between 100 and 300 μm wide, which is found to be mechanically robust. The separate polyHIPE layers are distinguished from one another by their differing average void diameter, chemical composition, and extent of contraction upon drying.

     

Synthesis of Novel Thermo‐ and pH‐Responsive Poly(L‐lysine)‐Based Copolymer and its Micellization in Water

A series of novel temperature and pH responsive block copolymers composed of poly(N‐isopropylacrylamide) (PNIPAM) and poly(L ‐lysine) (PLL) were synthesized. The effect of pH and the length of PLL on the lower critical solution temperature (LCST) of PNIPAM, and the self‐assembly of these PLL‐based copolymers induced by temperature and pH changes were investigated by the cloud point method, dynamic light scattering (DLS) and environmental scanning electron microscopy (ESEM). These PNIPAM‐b‐PLL copolymers can self‐assemble into micelle‐like aggregates with PNIPAM as the hydrophobic block at acidic pH and high temperatures; and at alkaline pH and low temperatures, they can self‐assemble into particles with PLL as the hydrophobic block. The copolymers may have potential applications in biotechnological and biomedical areas as drug release carriers.

     

Accelerated Combinatorial High Throughput Star Polymer Synthesis via a Rapid One‐Pot Sequential Aqueous RAFT (rosa‐RAFT) Polymerization Scheme

Advanced polymerization methodologies, such as reversible addition‐fragmentation transfer (RAFT), allow unprecedented control over star polymer composition, topology, and functionality. However, using RAFT to produce high throughput (HTP) combinatorial star polymer libraries remains, to date, impracticable due to several technical limitations. Herein, the methodology “rapid one‐pot sequential aqueous RAFT” or “rosa‐RAFT,” in which well‐defined homo‐, copolymer, and mikto‐arm star polymers can be prepared in very low to medium reaction volumes (50 µL to 2 mL) via an “arm‐first” approach in air within minutes, is reported. Due to the high conversion of a variety of acrylamide/acrylate monomers achieved during each successive short reaction step (each taking 3 min), the requirement for intermediary purification is avoided, drastically facilitating and accelerating the star synthesis process. The presented methodology enables RAFT to be applied to HTP polymeric bio/nanomaterials discovery pipelines, in which hundreds of complex polymeric formulations can be rapidly produced, screened, and scaled up for assessment in a wide range of applications.

     

A Strategy for Synthesis of Ion‐Bonded Supramolecular Star Polymers by Reversible Addition‐Fragmentation Chain Transfer (RAFT) Polymerization

We have developed a novel strategy for the preparation of ion‐bonded supramolecular star polymers by RAFT polymerization. An ion‐bonded star supramolecule with six functional groups was prepared from a triphenylene derivative containing tertiary amino groups and trithiocarbonate carboxylic acid, and used as the RAFT agent in polymerizations of tert‐butyl acrylate (tBA) and styrene (St). Molecular weights and structures of the polymers were characterized by ¹H NMR and GPC. The results show that the polymerization possesses the character of living free‐radical polymerization and the ion‐bonded supramolecular star polymers PSt, PtBA, and PSt‐b‐PtBA, with six well‐defined arms, were successfully synthesized.

     

Synthesis and Self‐Assembly of pH‐Responsive Amphiphilic Poly(dimethylaminoethyl methacrylate)‐block‐Poly(pentafluorostyrene) Block Copolymer in Aqueous Solution

We report the synthesis of a novel pH‐responsive amphiphilic block copolymer poly(dimethylaminoethyl methacrylate)‐block‐poly(pentafluorostyrene) (PDMAEMA‐b‐PPFS) using RAFT‐mediated living radical polymerization. Copolymer micelle formation, in aqueous solution, was investigated using fluorescence spectroscopy, static and dynamic light scattering (SLS and DLS), and transmission electron microscopy (TEM). DLS and SLS measurements revealed that the diblock copolymers form spherical micelles with large aggregation numbers, N_agg ≈ 30 where the dense PPFS core is surrounded by dangling PDMAEMA chains as the micelle corona. The hydrodynamic radii, R_h of these micelles is large, at pH 2–5 as the protonated PDMAEMA segments swell the micelle corona. Above pH 5, the PDMAEMA segments are gradually deprotonated, resulting in a lower osmotic pressure and enhanced hydrophobicity within the micelle, thus decreasing the R_h. However, the radius of gyration, R_g remains independent of pH as the dense PPFS cores predominate.

     

Star polymer synthesis using trithiocarbonate functional β‐cyclodextrin cores (reversible addition–fragmentation chain‐transfer polymerization)

Polystyrene stars were synthesized with reversible addition–fragmentation chain‐transfer (RAFT) polymerization. The core of the stars comprised a trithiocarbonate heptafunctional β‐cyclodextrin ring. Polymerizations were performed at 100 and 120 °C in the absence of an extraneous initiator and at 60 °C in the presence of a radical initiator. Monofunctional trithiocarbonate was also synthesized and used to make linear polystyrene to allow direct a comparison with the star synthesis. In all cases, the polymerization kinetics conformed to pseudo‐first‐order behavior. The measured molecular weights of the stars were found to deviate from those predicted on the basis of the monomer/trithiocarbonate group ratio. The extent of this deviation was dependent on the polymerization temperature, RAFT agent concentration, and conversion. Despite the low radical concentrations, termination reactions are suggested to play a significant role in the seven‐arm polystyrene star syntheses. The synthetic method was found to be suitable for generating star block structures. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 4498–4512, 2002     

Unusual Transformation from a Solvent‐Stabilized 1D Coordination Polymer to a Metal–Organic Framework (MOF)‐Like Cross‐Linked 3D Coordination Polymer

An unusual 1D‐to‐3D transformation of a coordination polymer based on organic linkers containing highly polar push–pull π‐conjugated side chains is reported. The coordination polymers are synthesized from zinc nitrate and an organic linker, namely, 2,5‐bis{4‐[1‐(4‐nitrophenyl)pyrrolidin‐2‐yl]butoxy}terephthalic acid, which possesses highly polar (4‐nitrophenyl)pyrrolidine groups, with high dipole moments of about 7 D. The coordination polymers exhibit an unusual transformation from a soluble, solvent‐stabilized 1D coordination polymer into an insoluble, metal–organic framework (MOF)‐like 3D coordination polymer. The coordination polymer exhibits good film‐forming ability, and the MOF‐like films are insoluble in conventional organic solvents.     

Star‐Shaped Polymer Consisting of a Porphyrin Core and Poly(L‐lysine) Dendron Arms: Synthesis,Drug Delivery,and In Vitro Chemo/Photodynamic Therapy

A novel star‐shaped polymer, porphyrin‐poly(L‐lysine) dendrons (PP‐PLLD), is synthesized by the click reaction between azido‐modified porphyrin and propargyl focal point poly(L‐lysine) dendrons. Its chemical structure is characterized by ¹H nuclear magnetic resonance, Fourier transform infrared spectroscopy, and gel permeation chromatography (GPC) is analyses etc. Due to its amphiphilic property, the obtained PP‐PLLD has a low critical micelle concentration in an aqueous solution, and can load doxorubicin (DOX) with a loading amount of 64 μg mg⁻¹. By in vitro toxicity assay, PP‐PLLD has no dark cytotoxicity but has significant phototoxicity. Moreover, DOX‐loaded PP‐PLLD shows a higher cytotoxicity under the light condition than PP‐PLLD or DOX alone, suggesting PP‐PLLD has a potential application in combined photodynamic therapy and chemotherapy.     

Control of the Optical Properties of a Star Copolymer with a Hyperbranched Conjugated Polymer Core and Poly(ethylene glycol) Arms by Self‐Assembly

A self‐assembly approach to tuning the optical properties of a star copolymer is reported herein. The star copolymer HCP‐star‐PEG with a hyperbranched conjugated polymer (HCP) core and many linear poly(ethylene glycol) (PEG) arms has been prepared successfully. The HCP core was synthesized by Wittig coupling of N‐(n‐hexyl)‐3,6‐diformylcarbazole and 1,3,5‐bis[(triphenylphosphonio)methyl]benzene tribromide. Subsequently, the linear PEG arms were grafted onto the HCP core by acylhydrazone connection. It was found that the optical properties of HCP‐star‐PEG in chloroform solution changed on addition of acid. Both ¹H NMR and UV/Vis spectroscopic investigations confirmed that the variation of the optical properties was related to the complexation of the acid and the imine bond in the acylhydrazone group. HCP‐star‐PEG self‐assembled into core–shell micelles in the mixed solvent of chloroform and acetonitrile, which affected the protonation of the imine bond. Therefore the optical properties of HCP‐star‐PEG can be readily controlled by self‐assembly.