PNIPAAm Copolymers Research Articles

Palladium-Catalyzed Mizoroki-Heck and Copper-Free Sonogashira Coupling Reactions in Water Using Thermoresponsive Polymer Micelles.

A few kinds of thermoresponsive diblock copolymers have been synthesized and utilized for palladium-catalyzed coupling reactions in water. Poly(N-isopropylacrylamide) (PNIPAAm) and poly(N,N-diethylacrylamide) (PDEAAm) are employed for thermoresponsive segments and poly(sodium 4-styrenesulfonate) (PSSNa) and poly(sodium 2-acrylamido-methylpropanesulfonate) (PAMPSNa) are employed for hydrophilic segments. Palladium-catalyzed Mizoroki–Heck reactions are performed in water and the efficiency of the extraction process is studied. More efficient extraction was observed for the PDEAAm copolymers when compared with the PNIPAAm copolymers and conventional surfactants. In the study of the Sonogashira coupling reactions in water, aggregative precipitation of the products was observed. Washing the precipitate with water gave the product with satisfactory purity with a good yield.

Fabrication and characterization of thermoresponsive composite carriers: PNIPAAm-grafted glass spheres

Abstract Processing capacity and product yield of three-dimensional (3D) smart responsive carriers are markedly superior to those of two-dimensional substrates with the same compositions due to the special structure; therefore, more attempts have been made to develop the 3D intelligent systems in recent decades. A novel preparation strategy of thermoresponsive glass sphere-based composite carriers was reported in this study. First, PNIPAAm copolymers were synthesized by free-radical polymerization of N-isopropylacrylamide (NIPAAm), hydroxypropyl methacrylate (HPM), and 3-trimethoxysilypropyl methacrylate (TMSPM). Then, the copolymer solution was sprayed on the surfaces of glass spheres using a self-made bottom-spray fluidized bed reactor, and the bonding between copolymers and glass spheres was fabricated by thermal annealing to form PNIPAAm copolymer/glass sphere composite carriers. The coating effects of PNIPAAm copolymers on sphere surfaces were investigated, including characteristic functional groups, surface microstructure, grafting density, equilibrium swelling, as well as biocompatibility and potential application for cell culture. The results show that the temperature-responsive PNIPAAm copolymers can be linked to the surfaces of glass spheres by bottom-spray coating technology, and the copolymer layers can be formed on the sphere surfaces. The composite carriers have excellent thermosensitivity and favorable biocompatibility, and they are available for effective cell adhesion and spontaneous cell detachment by the use of smart responsiveness.

Simultaneous analysis of multiple oligonucleotides by temperature-responsive chromatography using a poly(N-isopropylacrylamide)-based stationary phase

Oligonucleotide therapeutics have contributed remarkably to healthcare, being well suited for the treatment of intractable diseases that are difficult to approach using conventional drug modalities. However, as common techniques of oligonucleotide analysis rely on reversed-phase or ion-exchange liquid chromatography and thus employ toxic organic solvents and/or ion-pairing reagents, better alternatives are highly sought after. Poly(N-isopropylacrylamide) (PNIPAAm) is widely used in temperature-responsive chromatography (TRC), which relies on column temperature variation to control the physical properties of the stationary phase and, unlike conventional reversed-phase liquid chromatography, avoids the use of toxic organic solvents and complicated gradient methods. Herein, PNIPAAm copolymer hydrogel-modified silica beads were used for the simultaneous analysis of multiple synthetic oligonucleotides by TRC to recognize differences in the length of single nucleotides, single bases, and the number of phosphorothioated sites. Temperature-responsive elution was observed in all cases. Each separation of all combinations of multiple oligonucleotides was better at higher temperatures above the lower critical solution temperature and was performed without the use of organic solvents and gradient methods. In the case of multiply phosphorothioated oligonucleotides, good separation was achieved using an aqueous solvent and isocratic elution in the absence of ion-pairing reagents. Thus, the developed procedure was concluded to be well suited for oligonucleotide analysis.Graphical abstract

RAFT polymerization and dually responsive behaviors of terpyridine-containing PNIPAAm copolymers in dilute solutions

Introduction of new components is an effective way to gain new functionalities and to regulate the properties of responsive polymers. In the present work, a RAFT polymerization approach to terpyridine-containing poly(N-isopropyl acrylamide) (PNIPAAm) copolymers was developed and the dually responsive behavior of the obtained polymer to temperature variation and metal ions in dilute solutions were investigated. By utilizing RAFT polymerization, the molecular weight polydispersity index of the polymer can be reduced compared with the traditional free radical polymerization method. Due to the coexistence of thermosensitive component and terpyridine, the synthesized polymer is thermosensitive and metal ion sensitive. Further titration investigation revealed that this polymer may be utilized as a water-soluble and colorimetric sensor for Fe2+ and Fe3+ and the obvious color change from a colorless state to a purple state makes the polymer a potential sensor to detect Fe2+ and Fe3+ macroscopically by naked eye. Overall, the terpyridine-containing poly(N-isopropyl acrylamide) copolymers may enrich the family of terpyridine-containing polymers and provide some clues for developing new multi-responsive and multi-functional polymers.

Synthesis and characterization of bead-like poly(N-isopropylacrylamide) copolymers with double decker silsesquioxane in the main chains

A bead-like PNIPAAm copolymer with double-decker silsesquioxane (DDSQ) in the main chain was synthesized via a reversible addition–fragmentation chain transfer polymerization (RAFT) approach.

Thermoresponsive sodium alginate‐g‐poly(N‐isopropylacrylamide) copolymers III. Solution properties

AbstractStimuli‐responsive biocompatible and biodegradable materials can be obtained by combining polysaccharides with polymers exhibiting lower critical solution temperature (LCST) phase behavior, such as poly(N‐isopropylacrylamide) (PNIPAAm). The behavior of aqueous solutions of sodium alginate (NaAl) grafted with PNIPAAm (NaAl‐g‐PNIPAAm) copolymers as a function of composition and temperature is presented. The products obtained exhibit a remarkable thermothickening behavior in aqueous solutions if the degree of grafting, the concentration, and the temperature are higher than some critical values. The sol–gel‐phase transition temperatures have been determined. It was found that at temperatures below LCST the systems behave like a solution, whereas at temperatures above LCST, the solutions behave like a stiff gel, because of PNIPAAm segregation. This behavior is reversible and could find applications in tissue engineering and drug delivery systems. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013

Interactions between bovine serum albumin and Langmuir films composed of charged and uncharged poly(N-isopropylacrylamide) block copolymers

The thermoresponsive poly(N-isopropylacrylamide) (PNIPAAM) and NIPAAM block copolymer derivatives are attractive for drug delivery applications as they contract reversibly at lower critical solution temperatures (LCST) close to physiological conditions. In order to investigate biomaterial-protein compatibility, we have studied the interaction between PNIPAAM copolymer films spread at the air–water surface and bovine serum albumin (BSA) injected below the precompressed polymer films, using the Langmuir technique coupled with Brewster angle microscopy (BAM). A PNIPAAM homopolymer was applied together with a number of PNIPAAM-based di- and triblock copolymers, to assess effects of e.g., charge and hydrophobicity on protein–polymer interactions. The nature and strength of protein–polymer interaction was found to be tunable, ranging from complex formation (PNIPAAM homopolymer) to mixed monolayers and electrostatic cross-linking, according to the nature of the co-monomer. Results show that intercalation versus adsorption can be controlled through polymer composition.

Comparison of mesenchymal stem cells released from poly(N-isopropylacrylamide) copolymer film and by trypsinization

Temperature-responsive platforms containing poly(N-isopropylacrylamide) (PNIPAAm) have been developed as an effective substitute for enzymatic treatment to recover adherent cells, but it remains unclear whether this alternative harvesting method tends to support stem cells preserving them being primitive. This study mainly investigated the biological properties of mesenchymal stem cells derived from rat bone marrow and human adipose tissue (BM-MSCs and AT-MSCs) after being cultured on PNIPAAm copolymer films and recovered by temperature drop, and compared the cells harvested from glass coverslips with trypsinization as controls. The experimental results demonstrated that after three serial passages, the released MSCs from thermal liftoff showed no significant differences in cell morphology, immunophenotype and osteogenesis for BM-MSCs or adipogenesis for AT-MSCs, but had higher viability, stronger proliferation and higher adipogenic differentiation for BM-MSCs or higher osteogenic differentiation for AT-MSCs compared with the trypsinization group. Besides, more proteins remained around or within the cell membranes upon temperature drop. It is concluded that cell detachment with more extracellular matrix proteins facilitates the maintenance of membrane proteins, and accordingly preserves MSC properties related to viability, proliferation and differentiation to some extent. This indicates that the PNIPAAm copolymer films and their matching cooling treatment can be used as effective alternatives to the existing culture substrates and traditional enzymatic digestion for MSCs.

Thermoresponsive Copolymer Nanofilms for Controlling Cell Adhesion, Growth, and Detachment

This study reports the development and use of a novel thermoresponsive polymeric nanofilm for controlling cell adhesion and growth at 37 °C, and then cell detachment for cell recovery by subsequent temperature drop to the ambient temperature, without enzymatic cleavage or mechanical scraping. A copolymer, poly(N-isopropylacrylamide-co-hydroxypropyl methacrylate-co-3-(trimethoxysilyl)propyl methacrylate) (abbreviated PNIPAAm copolymer), was synthesized by free radical polymerization. The thermoresponses of the copolymer in aqueous solution were demonstrated by dynamic light scattering (DLS) through detecting the sensitive changes of copolymer aggregation against temperature. The DLS measurements revealed the lower critical solution temperature (LCST) at approximately 30 °C. The PNIPAAm film stability and robustness was provided through silyl cross-linking within the film and with the hydroxyl groups on the substrate surface. Film thickness, stability, and reversibility with respect to temperature switches were examined by spectroscopic ellipsometry (SE), atomic force microscopy (AFM), and contact angle measurements. The results confirmed the high extent of thermosensitivity and structural restoration based on the alterations of film thickness and surface wettability. The effective control of adhesion, growth, and detachment of HeLa and HEK293 cells demonstrated the physical controllability and cellular compatibility of the copolymer nanofilms. These PNIPAAm copolymer nanofilms could open up a convenient interfacial mediation for cell film production and cell expansion by nonenzymatic and nonmechanical cell recovery.

Intact endothelial cell sheet harvesting from thermoresponsive surfaces coated with cell adhesion promoters

Recently, with the development of smart polymers, research has looked to using thermoresponsive polymers as cell culture substrates. These novel surfaces allow the cultivation of cells without enzymes using the thermoresponsive phase transition property of poly(N-isopropylacrylamide) (PNIPAAm). However, this requires expensive techniques to generate a sufficiently thin film that allows cell adhesion. In this study, we looked at simple solvent cast films which normally show poor cell adhesion, but here the films are coated with cell adhesion promoters (CAPs) to improve cell growth without altering the copolymer thermoresponsive behaviour.A copolymer of PNIPAAm and N-tert-butylacrylamide (NtBAm) with a ratio of 85:15, respectively, was synthesized and solvent cast. The copolymer films were coated with CAPs, such as collagen, fibronectin and laminin, to increase their cell adhesion and growth properties. Cell activity measured by the alamarBlue assay showed similar results for coated copolymer films and standard tissue culture plastic controls. Deposition of CAPs on to the copolymer films was characterized by scanning electron microscopy and atomic force microscopy. Cell detachment from the copolymer films is not affected by the surface coatings of CAPs, and endothelial cells are recovered as an intact sheet, which has great potential for uses in tissue engineering applications. The results demonstrate a versatile method for the cultivation of cells while eliminating the need for the use of digestive enzymes such as trypsin. This study shows that cultivation on physically bonded PNIPAAm copolymers is viable and achievable by relatively simple methods.

Thermo‐Responsive PNIPAAm Copolymers with Hydrophobic Spacers

AbstractWe have synthesised and characterised three sets of acryloyl(amino‐acid)/NIPAAm co‐polymers. Three different co‐monomers were synthesised and copolymerized with N‐isopropylacrylamide (NIPAAm) by free radical polymerization in different molar ratios (12:1, 9:1, and 6:1). All series contain aliphatic spacers and are carboxylic terminated. The effect of co‐monomer nature and content on the glass transition and the LCST transition in solution has been investigated. The main factor affecting the glass transition and the LCST of the polymer was found to be the side chain length of the co‐monomer. The introduction of the co‐monomer resulted in an overall increase in the polymer hydrophobicity which led to a decrease of both the polymer glass transition and the LCST transition observed in solution.

Aqueous chromatography system using temperature‐responsive polymer‐modified stationary phases

Extensive research has been carried out on functional polymers which are currently playing important roles in various fields such as medicine and engineering. Such functional polymers which respond to various kinds of stimuli are termed 'intelligent materials'. Poly(N-isopropylacrylamide) (PNIPAAm), a temperature-responsive polymer, was utilized as a chromatography column matrix modifier for a novel chromatographic approach in which only aqueous media are used as a mobile phase. The ability of the developed temperature-responsive chromatography system to separate solutes without using an organic solvent is advantageous from the point of view of maintaining the structure and activity of bioactive compounds. Recently, we designed and synthesized a new pH- and temperature-responsive copolymer as a representative of such environment-responsive polymers and grafted it onto aminopropyl silica beads. The products were evaluated as HPLC packing materials for separation systems based on a new concept, according to which the properties of the stationary phase surface are altered by external stimuli such as pH and temperature. This chromatography system utilizing the PNIPAAm copolymer is very useful for the separation of bioactive substances, such as proteins and peptides, because separation in the aqueous mobile phase is controlled solely by changing the temperature. This analytical system reduces organic waste because no organic solvent is used to separate the solutes and can therefore be classified as environmentally friendly. Future medical and pharmaceutical applications are expected.

Photo-Cross-Linkable PNIPAAm Copolymers. 4. Effects of Copolymerization and Cross-Linking on the Volume-Phase Transition in Constrained Hydrogel Layers

Photo-cross-linkable temperature and pH-responsive polymers based on N-isopropylacrylamide and 2-(dimethylmaleimido)-N-ethyl-acrylamide were synthesized and spin-coated to produce uniform thin films of cross-linked, responsive hydrogels. Surface plasmon resonance and optical waveguide spectroscopy were used to determine the effect on the volume-phase transition when these materials are confined to a thin film. The film thicknesses ranged from 10 nm to 2.5 μm, and the volume-phase transition temperature and swelling ratio of the films fell into two overlapping regimes separated by at least one critical thickness. In the thick-film regime, greater than 100−760 nm, depending on the cross-linking density, ionizable comonomer concentration, and reference state, the films exhibited two transition temperatures. This can be explained by the stress imposed on the hydrogel as it swells perpendicular to the substrate. In the thin-film regime, less than 270−440 nm, depending on cross-linking density, the presence of a fixed substrate also limited the collapse of the gel at temperatures above the volume-phase transition temperature. Nonequilibrium chain conformations resulting from the spin-coating process may explain this effect. Two different mechanisms for constraint are likely in the thin- and thick-film regimes, and existing models for the anisotropic swelling of hydrogel layers were used to explain these trends. The spin-coated hydrogel layers were also compared to dip-coated and free-radical polymerized hydrogel layers to provide additional insight to the constraint mechanism. The elastic modulus of the hydrogel layers was measured with atomic force microcopy. The modulus in the swollen state was a function of both the cross-linking density and the ionizable comonomer concentration and ranged from 4.48 to 26.6 kPa. The modulus in the collapsed state was primarily a function of the cross-linking density and ranged from 466 to 1540 kPa. The effect of the ionizable comonomer concentration on the modulus also suggests the presence of non-Gaussian chain conformations in the swollen network.

Temperature and pH‐Dependent Swelling Behavior of Poly(N‐isopropylacrylamide) Copolymer Hydrogels and Their Use in Flow Control

AbstractPoly(N‐isopropylacrylamide) (PNIPAAm) copolymer gels with acidic and basic comonomers of various molar ratios were prepared by radical polymerization. The relationship between the swelling behavior of the gels and the copolymerization ratio was studied experimentally at different temperatures and in various pH value buffer solutions. The results of the experiments revealed that the transition temperatures of the PNIPAAm copolymer gels were changed in proportion to the monomer ratio used in copolymerization. The pH value of the buffer solution strongly affected the swelling ratio and some of the transition temperatures of the gels. The PNIPAAm copolymer gels were used in a chemo‐mechanical valve. The liquid flows directly through a gel actuator, which consists of a cylindrical actuator chamber filled with small particles of the sensitive cross‐linked polymer. The flow rate as well as the pressure drop was measured in dependence on the solvent properties. With the presented experimental arrangement it could be shown that sensitive polymers can be used for controlling the flow in dependence on temperature and pH.pH‐Dependence of the pressure in a chemo‐mechanical valve.magnified imagepH‐Dependence of the pressure in a chemo‐mechanical valve.

Photo-Cross-Linkable PNIPAAm Copolymers. 2. Effects of Constraint on Temperature and pH-Responsive Hydrogel Layers

Photo-cross-linkable co- and terpolymers of N-isopropylacrylamide, 2-(dimethylmaleimido)-N-ethyl-acrylamide as the photosensitive component, and 3-acryloylaminopropionic acid or N-(2-(dimethylamino)ethyl)acrylamide as ionizable comonomers were prepared by free radical polymerization. Aqueous solutions of the linear un-cross-linked co- and terpolymers showed lower critical solution temperature behavior. The phase transition temperature, which was detected by differential scanning calorimetry, ranged from 23.1 to 39.2 °C depending on the pH of the solution and the composition of the polymer. Surface plasmon resonance and optical waveguide spectroscopy were used to obtain information about the swelling behavior of hydrogel films of the photo-cross-linked polymers, giving a measurement of film thickness and refractive index. The transition temperatures of the cross-linked polymer gels showed similar trends to those of the corresponding linear polymers in solution, and the gels were shown to be both temperature- and pH-responsive, with the transition temperature ranging from 25.3 to 44.9 °C for films having a 200 nm dry film thickness. However, the swelling behavior of the cross-linked gels was found to vary as a function of dry film thickness, and three samples were selected for a more detailed study of how film thickness affects the transition temperature and swelling ratio of hydrogel films. Dry film thicknesses ranged from 9 nm to 2.3 μm, and the swelling behavior of the films fell into two distinct regimes separated by a critical thickness, which ranged from 280 to 500 nm. In the thin-film regime, the transition temperature of the films was independent of film thickness, but the refractive index of the films in the collapsed state decreased as film thickness decreased, indicating that these films are not able to fully collapse. In the thick-film regime, the swelling ratio of the films was independent of film thickness, but the transition temperature decreased as much as 2.6 °C as the film thickness increased. This was explained by the constraint imposed on the film by the presence of a fixed substrate, with the length scale of this constraint related to the critical thickness. In these films, the ionizable comonomers were found to have little effect on the swelling ratio, which is determined primarily by cross-linking density in the swollen state and by film thickness in the collapsed state.

Photo-Cross-Linkable PNIPAAm Copolymers. 1. Synthesis and Characterization of Constrained Temperature-Responsive Hydrogel Layers

Photo-cross-linkable co- and terpolymers of N-isopropylacrylamide (NIPAAm), 2-(dimethylmaleimido)-N-ethylacrylamide (DMIAAm) as the chromophore, and N,N-dimethylacrylamide (DMAAm) were prepared by free radical polymerization. Aqueous solutions of the co- and terpolymers showed lower critical solution temperature (LCST) behavior, and the corresponding phase transition temperature (Tc) was detected by differential scanning calorimetry (DSC). Tc decreased with increasing amount of DMIAAm, as low as 24.7 °C for 9.2 mol % DMIAAm, and increased with increasing DMAAm content, as high as 59.5 °C for 52.6 mol % DMAAm. The resulting polymers were shown to be photo-cross-linkable, and the sensitivity of the polymers toward UV light was studied by monitoring the photo-cross-linking reaction with ATR−FTIR. With 2 wt % thioxanthone as the photosensitizer, nearly full conversion could be achieved with 10 min of irradiation even though the photo-cross-linking was performed in the glassy state. Surface plasmon resonance (SPR) spectroscopy and optical microscopy were used to obtain information about the swelling behavior of thin hydrogel films. In the SPR scans the plasmon resonance minimum and the first waveguide mode were fit to Fresnel calculations to determine the refractive index (n) and the layer thickness (d) of the hydrogel. The volume degree of swelling was calculated from the refractive index, and the swelling ratio was calculated from the layer thickness. Changes in the degree of swelling, Tc, and the width of the transition (ΔTc) were observed by changing the chromophore content and, as a result, the gel cross-linking density. For a hydrogel film with a dry thickness of 200 nm, the collapsed film thickness above Tc was around 220 nm and only weakly dependent on chromophore content. However, at temperatures below Tc, the swollen film thickness was strongly dependent on the chromophore content and ranged from 1200 nm for 2.4 mol % DMIAAm to 800 nm for 9.2 mol % DMIAAm. The reverse is true for the refractive index, which increases as the film thickness decreases. A comparison of volume degree of swelling and swelling ratio was utilized to demonstrate the high anisotropy of swelling in these hydrogel layers that were physisorbed to the substrate and therefore constrained from expanding or contracting laterally. The swollen film expanded 9.5% laterally as compared to the dry film, and this value appeared to be independent of temperature. The swelling perpendicular to the substrate ranged from 6.4% for 9.2 mol % DMIAAm at temperatures above Tc to 630% for 2.4 mol % DMIAAm at temperatures below Tc.

Polytetrahydrofuran Amphiphilic Networks, 5. Synthesis and Swelling Behavior of Thermosensitive Poly(N-isopropylacrylamide)-l-polytetrahydrofuran Networks

A series of thermosensitive and thermoreversible poly(N-isopropylacrylamide)-l-polytetrahydrofuran (PNIPAAm-l-PTHF) amphiphilic networks were synthesized by free-radical copolymerization of N-isopropylacrylamide with the macromolecular crosslinker polytetrahydrofuran diacrylate (PTHFDA). The resulting PNIPAAm-l-PTHF networks show not only the properties of PNIAPPm and PTHF segments, but also the combined properties of the two segments. These networks are amphiphilic and their swelling is composition-dependent both in water and in organic solvents. The relationship of their equilibrium swelling ration (SR e ) and the content of network (W PTHF ) in organic solvents can be defined as: SR e = A* W PTHF + B. They present four solubility parameters that correspond to the hydrophobic and hydrophilic segments of networks. The swelling dynamics of the networks in water in a non-Fickian process, while in cyclohexane they approaches a Fickian process when the PTHF content is about 50-60%. Due to the presence of PTHF segments, the temperature effect on the swelling behavior of the PNIPAAm-l-PTHF networks is significantly different from other reported PNIPAAm copolymers or networks .The thermosensitivity and thermoreversibility of these networks is composition-dependent.