Surface-initiated Living Radical Polymerization Research Articles

Colloidal Crystals of Charged-Polymer-Brush-Decorated Hybrid Particles in Low-Polarity Solvents

Colloidal crystals are self-assembled systems that are suitable as models for studying crystallization; they are also attractive as nanostructures with a periodic arrangement of materials that have different refractive indices. Here, we present a method of constructing colloidal crystals in an organic solvent using charged-polymer-brush-decorated core-shell-type hybrid particles synthesized by surface-initiated living radical polymerization. The core-shell-type hybrid particles consisted of a silica particle core surrounded by a shell of polymer brushes obtained by the polymerization of methyl methacrylate and a small amount of a cationic monomer, [2-(methacryloyloxy)ethyl]trimethylammonium chloride. When the core-shell-type hybrid particles were dispersed in a low-polarity solvent with a dielectric constant of ∼11, colloidal crystals formed when the particle volume fraction exceeded a certain threshold, and remarkably, the interparticle distance in the colloidal crystal reached more than several micrometers under certain colloidal crystallization conditions. The colloidal crystallization behavior depended upon the surface charge density of the hybrid particles, ionic strength of the suspension, and dielectric constant of the solvent. The proposed method to construct colloidal crystals using electrostatic interactions between charged polymer brushes will promote the development of systems exhibiting particle self-assembly.

A tumor microenvironment responsive nanosystem for chemodynamic/chemical synergistic theranostics of colorectal cancer.

Rationale: The synergism of new modalities alongside chemodynamic therapy into common chemotherapy has shown promising potential in clinical applications. This paper reports a tumor microenvironment-responsive nanosystem for chemodynamic/chemical synergistic therapy and magnetic resonance imaging (MRI).Methods: The biodegradable nanosystem is synthesized using a surface-modified chain transfer agent for surface-initiated living radical polymerization of the chemotherapeutic drug.Results: In this nanosystem, named CAMNSN@PSN38, the cycling time and solubility of the chemotherapeutic drug are improved. The nanoparticles delivered to tumor tissues gradually release the chemotherapeutic drug and Mn2+ through glutathione (GSH)-triggered biodegradation in the tumor microenvironment. SN38, the released chemotherapeutic drug, not only shows excellent chemical therapy effects but also improves the generation of H2O2. Furthermore, with the Fenton-like agent Mn2+, the generation of reactive oxygen species (ROS) is improved markedly. Finally, CAMNSN@PSN38 shows excellent inhibition of tumor growth in three colorectal cancer tumor models, with an improved accumulation of ROS and controlled release of SN38.Conclusions: The CAMNSN@PSN38-mediated chemodynamic/chemical synergistic therapy provides a promising paradigm for the treatment and MRI-guided therapy of colorectal cancer.

(Invited) Designed Polymers for Antifouling Surfaces

There is a strong demand for antifouling surface. To create such antifouling surface, there are some approaches such as chemical reaction at the surface, coatings, internal additives, etc. We are developing polymers that can modify the surface property by easy treatments based on precise design and synthesis of the polymers. Biomedical synthetic materials, such as poly(ethylene terephthalate) (PET) and polypropylene (PP), are prone to adhesion of proteins, cells, and bacteria, causing functional failures in implants, artificial organs, catheters, and diagnostic devices, and increasing the risk of secondary infections. One of the promising answers is a hydrophilic polymer brush surface on which many polymer chains are connected in high density to inhibit adsorption of proteins and following adhesion of related biomolecules and cells. Such high density polymer brush is usually built by surface-initiated living radical polymerization through several chemical steps and polymers applied for medical devices are rather inert to chemical modification than inorganic materials. We thus created a new star shaped heteroarm polymer consisting of hydrophilic chains, as an antifouling block, and hydrophobic chains, as a substrate attaching block, for a biocompatible coating material, which is supposed to form polymer brush-like structure by simple coating procedure. The coating surface exhibited very good resistance to adhesion of platelets, bacteria, fibroblasts, and proteins although the corresponding linear polymer exhibited moderate resistance. This result indicates utilization of unique architecture of the star polymer is effective for building condensed polymer brush structure at the surface. Perfluoroalkyl group-containing polymer shows water/oil repellency. Although long perfluoroalkyl groups (longer than C8) exhibit high repellency, they can not be employed because of the environmental reason, nowadays. Shorter perfluoroalkyl groups (less than C8) give less effect to environment but they have less repellency. We thud designed block copolymers consisting of fluorine and non-fluorine containing monomers and evaluated the sequence effects as an internal additive to PP resin to improve oil repellency. By controlling fluorinated/non-fluorinated monomer sequence, the oil repellency of the fluorinated polymer-added polypropylene (PP) was dramatically improved compared to that modified with the randomly sequenced fluoropolymer. For instance, random copolymer consisting of 2-(perfluorohexyl)ethyl methacrylate (C6FMA) and stearyl methacrylate (StMA) gave slightly higher contact angle (13º) than that of non-additive PP (6º). Compared to the random copolymer, the diblock and triblock copolymer lead much higher contact angles (23º and 38º, respectively) when only 1wt% of the fluoropolymer was mixed into PP. This result indicates sequence regulation of fluorinated/non-fluorinated monomer in the fluoropolymer is effective to concentrate fluorine groups at the resin surface resulting in improvement of water/oil repellency.

Surface and interface designs in side-chain liquid crystalline polymer systems for photoalignment

In side-chain liquid-crystal (SCLC) polymer systems, the liquid crystalline (LC) mesogenic groups preferentially orient normal to the substrate plane due to the excluded volume effect (homeotropic alignment). Photoresponsive azobenzene (Az) mesogens have a transition moment parallel to the molecular long axis. Light irradiation for photoreactions is generally applied perpendicular to the film surface. Therefore, a homeotropic orientation inhibits efficient photoreactions and photoalignments in Az SCLC polymer systems. This review focuses on new approaches to induce a random planar orientation in Az SCLC polymer systems by interface and surface molecular design. The mesogens in a high-density SCLC polymer brush formed by surface-initiated living radical polymerization adopt a random planar orientation. In the film of an SCLC block copolymer with an amorphous block, a random planar orientation is induced via surface segregation of either of the blocks. The random planar orientations of SCLC polymer systems are thermally stable and offer efficient in-plane photoalignment and photoswitching with hierarchical LC molecular architectures, forming, e.g., microphase-separated (MPS) SCLC block copolymers and layered polymer LC systems. These surface and interface molecular designs are expected to provide new concepts and possibilities for LC polymer devices. The liquid crystalline (LC) rod-like mesogens preferentially orient normal to the substrate plane due to the excluded volume effect (homeotropic alignment) in side-chain liquid-crystal (SCLC) polymers free-standing films. For in-plane alignment, a homeotropic orientation is unsuitable because the mesogens are in a direction opposite to the in-plane directions. This review focuses on new approaches to induce a random planar orientation in SCLC polymer systems by interface and surface molecular design utilizing a high-density polymer brush structure and surface segregation structures of block copolymers.

Semisoft Colloidal Crystals in Ionic Liquids.

Here, we introduce a technique for the direct formation of semisoft colloidal crystals of hybrid particles in nonvolatile ionic liquid (IL) solvents. The hybrid particles are comprised of a silica core and a densely grafted polymer brush shell, which were synthesized by surface-initiated living radical polymerization. A phase transition of the suspensions from a disordered fluid to a crystallized system was observed within a narrow concentration range. Confocal laser scanning microscopy observation and ultraviolet-visible light (UV-vis) spectrometry confirmed the highly ordered structure of the hybrid particles in ionic liquids. The effect of the hybrid particle structure on the photonic band gap of the colloidal crystals was investigated, and the band gaps varied by changing graft chain lengths. In addition, the colloidal crystal suspensions were successfully immobilized in ILs.

Glycopolymer-Grafted Polymer Particles for Lectin Recognition.

Glycopolymers bearing carbohydrates have an advantage in protein recognition that is attributable to the multivalent effect (cluster effect) of side-chain carbohydrates. A variety of surface-modified polymer particles have been prepared concurrently with the development of new synthetic technology. Here we describe a synthetic method of glycopolymer-grafted polymer particles by surface-initiated living radical polymerization, i.e., atom-transfer radical polymerization (ATRP) and photoiniferter polymerization, for specific lectin recognition.

Cationic Cellulose Nanocrystals: Synthesis, Characterization and Cytotoxicity Studies

ABSTRACTCellulose nanocrystals (CNCs) have emerged as a new class of renewable material for various applications due to their remarkable properties and commercialization prospect. The relative low density, expected low cost, non-toxic character, uniform nanosize distribution, high aspect ratios, high surface area, thermal properties and high modulus of elasticity make CNCs attractive nanomaterials that recently prompted the industrial production of CNCs in North America. Surface functionalization of CNCs continues to be an exciting area of research for the design of novel CNC-based materials. In this work, we report the synthesis, characterization and cytotoxicity studies of novel cationic surface modified CNC derivatives. The negative surface of CNC was rendered positive after grafting with cationic polymers via surface-initiated living radical polymerization method. The modified CNCs were characterized by both spectroscopic and microscopic techniques. Their cytotoxicity effects were evaluated using MTT assay in two cell lines such as mouse macrophages (J774.A1) and human breast cancer (MCF7). Preliminary studies indicated that only one of the modified CNCs caused significant decrease in J774.A1 cell viability (50%), at the highest concentration tested (100 µg/mL). However this concentration is well above of what would be applicable for biomedical purposes. MCF7 cells were not affected by any of the cationic CNCs at any concentration. A detailed cytotoxicity study is currently underway to fully understand the interaction of these cationic CNCs with the biological systems for possible bio-inspired applications.

Immobilization of semisoft colloidal crystals formed by polymer-brush-afforded hybrid particles.

An immobilization technique for semisoft colloidal crystals, which are ordered arrays of polymer-brush-afforded hybrid particles synthesized by surface-initiated living radical polymerization (SI-LRP), is reported. Silica particles were first grafted with well-defined block copolymers of poly(methyl methacrylate-co-hydroxyethyl methacrylate)-b-poly(methyl methacrylate), P(MMA-co-HEMA)-b-PMMA by SI-LRP, which gave a graft density as high as 0.7 chains/nm(2). The HEMA units reacted with 2-isocyanatoethyl methacrylate to introduce vinyl groups at the outer layer of the polymer-brush shell. The modified hybrid particles formed a colloidal crystal in a solution containing a small amount of free polymers with vinyl groups. The colloidal crystal was photoirradiated in the presence of a photoradical initiator to immobilize it through a cross-linking reaction among the vinyl groups. The structural analyses of the colloidal crystals before and after the photoirradiation were carried out by confocal laser scanning microscopy; the results showed that the periodic structures of the crystals were maintained after immobilization.

Fabrication of Contrast Agents for Magnetic Resonance Imaging from Polymer-Brush-Afforded Iron Oxide Magnetic Nanoparticles Prepared by Surface-Initiated Living Radical Polymerization

The aim of this study is to fabricate a contrast agent for magnetic resonance imaging (MRI) by using hybrid particles composed of a core of iron oxide magnetite (Fe3O4) nanoparticles and a shell of hydrophilic polymer brush synthesized by surface-initiated (SI) living radical polymerization. To achieve this, Fe3O4 nanoparticles were surface-modified with initiating groups for atom transfer radical polymerization (ATRP) via a ligand-exchange reaction in the presence of a triethoxysilane derivative having an ATRP initiation site. The ATRP-initiator-functionalized Fe3O4 nanoparticles were used for performing the SI-ATRP of methyl methacrylate to demonstrate the ability of the synthesized nanoparticles to produce well-defined polymer brushes on their surfaces. The polymerization proceeded in a living fashion so as to produce graft polymers with targeted molecular weights and narrow molecular weight distribution. The average graft density was estimated to be as high as 0.7 chains/nm(2), which indicates the formation of so-called concentrated polymer brushes on the Fe3O4 nanoparticles. Dynamic light scattering and transmission electron microscope observations of the hybrid nanoparticles revealed their uniformity and dispersibility in solvents to be excellent. A similar polymerization process was conducted using a hydrophilic monomer, poly(ethylene glycol) methyl ether methacrylate (PEGMA), to prepare Fe3O4 nanoparticles grafted with poly(PEGMA) brushes. The resultant hybrid nanoparticles showed excellent dispersibility in aqueous media including physiological conditions without causing any aggregations. The blood clearance and biodistribution of the hybrid particles were investigated by intravenously injecting particles labeled with a radio isotope, (125)I, into mice. It was found that some hybrid particles exhibited an excellently prolonged circulation lifetime in the blood with a half-life of about 24 h. When such hybrid particles were injected intravenously into a tumor-bearing mouse, they preferentially accumulated in the tumor tissues owing to the so-called enhanced permeability and retention effect. The tumor-targeted delivery was visualized by a T2-enhaced MRI measurement.

Study on the novel rare-earth nanocrystals/PNIPAM complex hydrogels prepared by surface-initiated living radical polymerization

A series of novel rare-earth nanocrystals/PNIPAM complex hydrogels were prepared by surface-initiated living radical polymerization with different contents of NaYF4:Eu3+ nanocrystals. The microstructure and performance of the NaYF4:Eu3+ nanocrystals and the complex hydrogels were characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD), Fourier transform infrared (FTIR), differential scanning calorimetry (DSC) and photoluminescence (PL). The thermosensitive fluorescence behaviors of the complex hydrogels and the mechanisms were investigated. The results suggested that the nanocrystals were incorporated into PNIPAM gels with covalent bonding and the fluorescence performance of complex hydrogels was influenced greatly by either the ambient temperature or the content of nanocrystals. The PL intensity of the complex hydrogels increased with increasing temperature. The higher content of nanocrystals in the complex hydrogels, the sharper the transitions of volume phase variation around low critical solution temperature (LCST).

Surface-initiated living radical polymerization from silica particles functionalized with poly(ethylene glycol)-carrying initiator

A novel yet versatile approach is described for surface-initiated living radical polymerization (SI-LRP) from silica particles (SiPs). Monodisperse SiPs were surface-modified with a newly designed surface-fixable initiator (BPEGE) having three components: a triethoxysilane moiety, a poly(ethylene glycol) (PEG) unit, and an initiation site for atom transfer radical polymerization (ATRP) in the form of a 2-bromoisobutyryl group. The surface-initiated ATRP of methyl methacrylate (MMA) mediated by a copper complex was carried out with the BPEGE-fixed SiPs. The polymerization proceeded in a living manner, producing SiPs coated with well-defined poly(MMA) of a target molecular weight with a graft density as high as 0.5 chains/nm2. Thanks to the amphiphilic property of PEG, the system was successfully applied for SI-ATRP of PEG methacrylate and sodium p-styrenesulfonate in aqueous media in which the BPEGE-fixed SiPs were highly dispersed without causing any aggregations. The formation of colloidal crystals with the polymer brush-afforded SiPs demonstrated the high uniformity and perfect dispersibility of the hybrid particles.

Blood Clearance and Biodistribution of Polymer Brush-Afforded Silica Particles Prepared by Surface-Initiated Living Radical Polymerization

The physiological properties of polymer brush-afforded silica particles prepared by surface-initiated living radical polymerization were investigated in terms of the circulation lifetime in the blood and distribution in tissues. Hydrophilic polymers consisting mainly of poly(poly(ethylene glycol) methyl ether methacrylate) were grafted onto silica particles by surface-initiated atom transfer radical polymerization that was mediated by a copper complex to produce hairy hybrid particles. A series of hybrid particles was synthesized by varying the diameter of the silica core and the chain length of the polymer brush to examine the relationship between their physicochemical and physiological properties. The hybrid particles were injected intravenously into mice to investigate systematically their blood clearance and body distribution. It was revealed that the structural features of the hybrid particles significantly affected their in vivo pharmacokinetics. Some hybrid particles exhibited an excellently prolonged circulation lifetime in the blood with a half life of ∼20 h. When such hybrid particles were injected intravenously into a tumor-bearing mouse, they preferentially accumulated in tumor tissue. The tumor-targeted delivery was optically visualized using hybrid particles grafted with fluorescence-labeled polymer brushes.

Molecularly imprinted beads with double thermosensitive gates for selective recognition of proteins

A new approach is reported on the use of poly(N-isopropylacrylamide) (PNIPAM)-coated molecularly imprinted beads (coated MIP beads) for controlling the release of protein. The coated MIP beads were composed of double layers, an internal thermosensitive lysozyme-imprinted layer, and an external PNIPAM layer. The coated MIP beads were prepared by two-step surface-initiated living-radical polymerization (SIP). In this systemic study, the coated MIP beads had good selectivity to the template protein (lysozyme) and temperature stimulus-responsive behavior, both of which were superior to those of MIP beads having a layer of thermosensitive lysozyme-imprinted polymer only. Using the coated MIP beads, reference proteins and the template lysozyme could be released separately at 38 °C and at 23 °C. The corresponding coated non-imprinted beads (coated NIP beads) did not have such double thermosensitive "gates" with specific selectivity for a particular protein. The proposed smart controlled imprinted system for protein is attractive for chemical carriers, drug-delivery system, and sensors.

Stereoregulation of Thermoresponsive Polymer Brushes by Surface-Initiated Living Radical Polymerization and the Effect of Tacticity on Surface Wettability.

In this study, stereocontrolled poly(N-isopropylacrylamide) (PIPAAm) brushes were grafted from surfaces by atom transfer radical polymerization (ATRP) in the presence of a Lewis acid, and the effect of PIPAAm brush tacticity on the thermoresponsive wettabiliy was investigated. PIPAAm grafted by ATRP in the presence of Y(OTf)(3) showed high isotacticity, while the control brush polymerized in the absence of Y(OTf)(3) was clearly atactic. The isotacticity and molecular weight of PIPAAm brushes were controlled by polymerization conditions. The wettability of isotactic PIPAAm-grafted surfaces decreased slightly below 10 °C, although the phase transition temperature of atactic surface was 30 °C, and the bulk isotactic polymer was water-insoluble between 5 and 45 °C.

Surface-Initiated Living Radical Polymerization from Narrowly Size-Distributed Silica Nanoparticles of Diameters Less Than 100 nm

Surface-initiated atom transfer radical polymerizations (ATRP) from narrowly size-distributed silica nanoparticles (SiNPs) of diameters less than 100 nm were investigated. Two methods were used for the preparation of the SiNP cores: one was the reverse-micelle technique, which gave monodisperse SiNPs of average diameter 55 nm, and the other was the lysine-addition technique, which gave nearly monodisperse SiNPs of average diameter 15 nm. These nanoparticles were surface-modified with a triethoxysilane derivative containing an ATRP-initiating group. The surface-initiated ATRP of methyl methacrylate (MMA) mediated by a copper complex was carried out with the initiator-fixed SiNPs in the presence of a “sacrificial” free initiator. Well-defined poly(methyl methacrylate) (PMMA) brushes of a target molecular weight were successfully grafted with a surface density as high as 0.4−0.8 chains/nm2. These core−shell hybrid particles were highly dispersible, without any aggregation, in various solvents for PMMA. Becau...

Responsive Plasmonic Assemblies of Amphiphilic Nanocrystals at Oil−Water Interfaces

We report a new class of amphiphilic nanocrystals with mixed polymer brush coatings, which can spontaneously assemble into two-dimensional arrays at oil-water interfaces. The plasmon coupling of gold nanoparticles and nanorods in the assembly can be reversibly modulated by conformational changes of the stimuli-responsive polymer brushes. Amphiphilic gold nanocrystals (nanoparticles and nanorods) with mixed polymer brushes were synthesized via sequential "grafting to" (ligand exchange) and "grafting from" (surface initiated atom transfer radical polymerization) reactions. The integration of "grafting to" and "grafting from" reactions offers great flexibility for the surface modification of the nanocrystal scaffolds, allowing the combination of polymers with distinctly different properties on well-defined nanocrystals. For nanocrystals with poly(ethylene glycol) and poly(methyl methacrylate) coatings, the collective plasmonic property of the assembly can be tailored by changing solvent quality. In the case of the amphiphilic nanocrystal with poly(ethylene glycol) and poly(2-(diethylamino)ethyl methacrylate), the pH-sensitivity of poly(2-(diethylamino)ethyl methacrylate) provides an additional means to reversibly tune the assembly by varying the pH. All of the components, including nanocrystals, materials for self-assembled monolayers (polymers and biomacromolecules) on nanocrystal surfaces, and monomers suitable for surface-initiated living radical polymerization, in this construct have a wealth of possibilities available, indicating the potential of our strategy for developing hybrid materials with integrated and collective functionalities.

A Versatile Method of Initiator Fixation for Surface-Initiated Living Radical Polymerization on Polymeric Substrates

A facile yet robust approach to chemically fix an initiating group for atom transfer radical polymerization (ATRP) on various polymeric substrates is described. Conventional free radical copolymerization was conducted with methyl methacrylate (MMA), 2-(2-bromoisobutyryloxy)ethyl methacrylate (an ATRP initiator-carrying monomer, BIEM), and 2-((4-azidobenzoyl)oxy)ethyl methacrylate (a photoreactive phenylazide-carrying monomer, ABEM) in N,N-dimethylformamide at 75 °C, giving a random (statistical) copolymer. Thin films of the obtained copolymer were fabricated on poly(ethylene terephthalate) (PET) film surface by spin-casting a toluene solution of the terpolymer and were immobilized on the PET substrate via UV-irradiation using the photoreactivity of the phenylazido units. The surface-initiated ATRP (SI-ATRP) of poly(ethylene glycol) methacrylate (PEGMA) mediated by a copper complex was carried out in water at 30 °C in the presence of a sacrificial (free) initiator and the initiator-immobilized PET film as the solid substrate. The polymerization proceeded in a living fashion. The molecular weight of free polymer increased with polymerization time while retaining low-polydispersity index, and more importantly, the thickness of the poly(PEGMA) graft layer increased as a function of polymerization time with a reduced graft density (surface occupancy) as high as 0.5 in all examined time. This initiator immobilization technique was applied to various polymeric substrates including polystyrene, polypropylene, polyethylene, and polylactide. The fabrication of micropatterned polymer-brush surfaces was also demonstrated by photopatterning the initiator layer followed by SI-ATRP.

Coating and dispersion of ceramic nanoparticles by UV-ozone etching assisted surface-initiated living radical polymerization

Commercially available unmodified ceramic nanoparticles (NPs) in dry powder state were surface-modified and dispersed in almost single-crystal size. The surface-initiated living radical polymerization after just UV-ozone soft etching enables one to graft polymers onto the surface of ceramic NPs and disperse them in solvents. Furthermore, a number of NPs were dispersed with single-crystal sizes. The technique developed here could be applied to almost all ceramic NPs including metal nitrides.

AFM studies on microtribology of concentrated polymer brushes in solvents

In concentrated polymer brushes (CPBs) prepared by surface-initiated living radical polymerization, graft chains in a good solvent are highly stretched, nearly to their full length, giving properties quite different and unpredictable from those of semi-dilute polymer brushes (SDPBs) previously studied. This paper reviews our recent studies on the microtribology of confronted CPBs in solvents. Atomic force microscopy revealed that the CPBs of poly(methyl methacrylate) and poly(styrene) showed dramatically low frictional coefficients in good solvents compared with those of the corresponding SDPBs. This was reasonably ascribed to confronted CPBs hardly mixing with each other due to their huge osmotic and elastic interactions. The lubrication mechanism of CPBs was discussed according to the sliding-speed dependency of frictional coefficient, and two different mechanisms, boundary and hydrodynamic lubrications, were suggested. Thermo-responsive lubrication was also demonstrated for the CPB of poly(N-isopropyl acrylamide). We believe that these studies will open up a new route to precision surface modification for "super" lubrication.

Synthesis of Hollow Nanospheres Consisting of Amphiphilic Block Copolymers

Abstract Hollow polymeric nanospheres with shells consisting of inner hydrophilic and outer hydrophobic polymer chains are prepared by a surface-initiated living radical polymerization of 2-trimethylsilyloxyethyl acrylate and methyl methacrylate from initiator-modified silica nanoparticles; these nanospheres are subsequently prepared by intraparticle cross-linking and by dissolution of silica matrices with HF. The translation behavior of hydrophilic substrates from an aqueous phase to an oil phase through the nanocapsules is investigated.