Self-association and microenvironment of random amphiphilic copolymers of sodium N-acryloyl-l-valinate and N-dodecylacrylamide in aqueous solution

The present paper reports on the syntheses and association behavior of two random copolymers of sodium N-acryloyl-l-valinate and N-dodecylacrylamide in buffered (pH 8.0) aqueous solution containing 0.1 M NaCl. Surface tension and viscosity results showed pronounced amphiphilic nature of the copolymers in aqueous solution at pH 8.0. Steady-state fluorescence studies using pyrene and N-phenyl-1-naphthylamine as probe molecules suggested microdomain formation through interpolymer association above a critical concentration called ‘critical aggregation concentration’ (CAC) as low as ca. 10⁻³ g L⁻¹. The local polarity of the hydrophobic domain formed in aqueous solution was estimated from steady-state fluorescence spectra of pyrene. The microviscosity of the domains was evaluated using 1,6-diphenyl-1,3,5-hexatriene as a fluorescent probe using steady-sate fluorescence depolarization and time-resolved fluorescence method. Dynamic light scattering technique was performed over a wide range of concentration to determine hydrodynamic size of the aggregates. It was observed that both copolymers retain rather open conformation in dilute solutions having polymer concentrations less than CAC. However, with increase in concentration the intermolecular association becomes favorable towards the formation of more compact aggregates. The transmission electron microscopic images of both copolymers at a concentration above CAC revealed spherical aggregates of uniform diameter (∼50 nm).     

N-1-alkylitaconamic acids-co-styrene copolymers. Surface characterization

A serie of six N-1-alkylitaconamic acids-co-styrene copolymers with alkyl groups varying in side chain length from 3 to 12 was used in this study. The surface behaviour of the copolymers has been studied as function of the side chain length. Contact angle data for two of these copolymer surfaces were obtained in water and several liquids. From this information the surface energy was determined. Differences in the wettability of N-1-alkylitaconamic acid-co-styrene are found. The results are discussed in terms of hydrophobic and polar effect of the copolymers. Results on spread monolayers characteristics of these copolymers on water surfaces are also reported. Surface pressure-area (π-A) isotherms on a pure water subphase exhibit a transition region depending on the length of the alkyl side chain of the itaconamic acid moiety. It was also analyzed the phase transition in the monolayer at the air/water interface by brewster angle microscopy (BAM). Molecular mechanics approach was used to obtain predictions about the local interaction energies between segments. It was possible to conclude that the local interaction energies of propyl and decyl derivatives are quite similar while the hexyl derivative has higher interaction energy. The analysis of the coulombic energies together with the dispersion van der Waals energies (VDW) would be indicative, in first approximation, that carbonyl groups are more exposed in the case of propyl than in the other copolymers studied.     

The preparation of copolymerized fluorescent microspheres of styrene using detergent-free emulsion polymerization

The polymerisable fluorescent monomer, 4-ethoxyl-N-allyl-1,8-naphthalimide, was synthesized from 4-bromo-1,8-naphthalic anhydride and characterized using FT-IR spectra, ¹H NMR and fluorescence spectra. A copolymer of styrene and 4-ethoxyl-N-allyl-1,8-naphthalimide was prepared by detergent-free emulsion polymerization and characterized using fluorescence spectra, SEM and fluorescence microscopy. The fluorescence spectrum of the copolymer displayed excitation at 365 nm and emission at 435 nm in acetone. SEM revealed that the copolymers were microspheres, with smooth surfaces and displayed intense blue fluorescence.     

Esterification of ethylene-vinyl alcohol copolymers in homogeneous phase using N,N′-dimethylpropyleneurea as solvent

Ethylene-vinyl alcohol copolymers (EVAL) were esterified with 3,5-dinitrobenzoyl chloride using the cycled urea N,N′-dimethylpropyleneurea (1,3-dimethyl-3,4,5,6-tetrahydro-2(1H)-pyrimidinone) (DMPU) as the solvent. Ethylene-vinyl alcohol-vinyl-3,5-dinitrobenzoate terpolymers (EVALVDNB) and ethylene-vinyl-3,5-dinitrobenzoate copolymers (EVDNB) were obtained. Both EVAL copolymers (6-73 mol% VAL) and esterified polymers, EVDNB, and EVALVDNB dissolve in DMPU. The substitution may become total under the experimental conditions. The degree of transformation was determined by ¹H NMR. EVDNB copolymers were characterised by IR spectroscopy and ¹H and ¹³C NMR. Thermal properties were studied by DSC. The glass transition temperature of the EVDNB copolymers having a low VDNB content (up to 14 mol%) is roughly constant, whereas above 50 mol% increases. Melting temperature decreases as the VDNB content is increased, owing to the fact that the VDNB groups are excluded from the polyethylene crystal lattice.     

Toward an understanding of thermoresponsive transition behavior of hydrophobically modified N-isopropylacrylamide copolymer solution

Poly(N-isopropylacrylamide-co-vinyl laurate)(PNIPAAm-co-VL) copolymers were prepared at various feed ratios via conventional radical random copolymerization. The formation, composition ratios and molecular weight of copolymers were examined. The thermoresponsive behaviors of PNIPAAm and PNIPAAm-co-VL solutions at low and high concentrations were intensively investigated by turbidity measurement, Micro-DSC, temperature-variable state fluorescence, ¹H NMR and dynamic light scattering (DLS). Several important results were obtained that (1) incorporation of PVL results in much lower and broader LCST regions of the copolymer solutions, and facilitates the formation of hydrophobic microdomains far below LCST, causing a pronounced aggregation in solutions (2) temperature-variable ¹H NMR spectra shows that during the phase transition, the ‘penetration’ of PNIPAAm into the hydrophobic core is a process accompanied with a transition of isopropyl from hydration to dehydration as well as a self-aggregation of hydrophobic chains at different temperature stages (3) according to the ¹H NMR spectra of polymer solutions obtained at varied temperatures, the microdomains from hydrophobic VL moieties have a different accessibility for isopropyl groups and the entire chains during phase transition (4) temperature-variable DLS demonstrates that the temperature-induced transition behavior of copolymers is supposedly divided into three stages: pre-LCST aggregation (<20 °C), coil-globule transition at LCST (20-25 °C) and post-LCST aggregation (>25 °C).     

Drug diffusion in hydrophobically modified N,N-dimethylacrylamide hydrogels

The use of hydrophobically modified hydrogels for drug release was investigated. Copolymers of N,N-dimethylacrylamide and 2-(N-ethyl-perfluorooctanesulfonamido) ethyl acrylate (FOSA) were prepared by free-radical polymerization. The drug release rates, dynamic swelling behavior, and pH sensitivities of copolymers ranging in composition from 0 to 30 mol% FOSA were studied. Pheniramine maleate, an ocular antihistamine, was used as the model drug substance. Hydrogels of DMA produced with increasing amounts of FOSA had a decreased equilibrium media content and exhibited a slower drug release rate. Early-time, late-time and Etters approximation drug diffusion coefficients ranged from 0.4×10⁻³ to 12.3×10⁻³ mm²/min. The diffusion of the drug model was less sensitive to pH of the buffered media over the range of pH 4-8, but increasing the media pH slowed the permeability slightly by decreasing the swellability of the hydrogel. The power law exponent (n≈0.5) and the swelling interface number (Sw?1) suggested that the drug release mechanism from these hydrogels was Fickian and not swelling controlled. These novel thermoprocessible hydrogels have potential to be used as controlled ocular drug delivery devices (e.g. contact lenses or ocular inserts).     

Poly(N-phenylmaleimides) bearing chiral oxazolinyl pendant: Supramolecular aggregation and enantioselectivity in fluorescence response

N-Phenylmaleimide derivatives bearing a chiral oxazoline substituent at the benzene ring (N-[o-(4,5-dihydro-1,3-oxazol-2-yl)phenyl]maleimides, OPMIs) were polymerized using a binary initiating system composed of Et₂Zn and n-BuLi to in situ produce the zinc complexes of optically active poly(OPMI)s. The significant changes in chiroptical and fluorescent properties were observed for these polymers upon complexation with metal ions due to the formation of supramolecular structure, evidenced by circular dichroism, fluorescence spectrum and atomic force microscopy. The fluorescence spectra of poly(OPMI)/Zn^II complexes in the presence of (R)/(S)-1,1′-bi-2-naphthol (BINOL) and (R)/(S)-2-amino-1-propanol were studied in THF media. It was found that the fluorescence intensity of the complexes responded differently to both enantiomers of BINOL and the amino alcohol in a quenching and enhancement fashion, respectively. Furthermore, the enantioselective fluorescence response was strongly dependent on the amount of incorporated zinc in polymer matrices.     

Controllable synthesis of poly(N-vinylpyrrolidone) and its block copolymers by atom transfer radical polymerization

At room temperature atom transfer radical polymerization (ATRP) of N-vinylpyrrolidone (NVP) was carried out using 5,5,7,12,12,14-hexamethyl-1,4,8,11-tetra-azacyclo-tetradecane (Me₆Cyclam) as ligand in 1,4-dioxane/isopropanol mixture. Methyl 2-chloropropionate (MCP) and copper(I) chloride were used as initiator and catalyst, respectively. The polymerization of NVP via ATRP could be mediated by the addition of CuCl₂. The resultant poly(N-vinylpyrrolidone) (PNVP) has high conversion of up to 65% in 3 h, a controlled molecular weight close to the theoretical values and narrow molecular weight distribution between 1.2 and 1.3. The living nature of the ATRP for NVP was confirmed by the experiments of PNVP chain extension. With PNVP-Cl as macroinitiator and N-methacryloyl-N′-(α-naphthyl)thiourea (MANTU) as a hydrophobic monomer, novel fluorescent amphiphilic copolymers poly(N-vinylpyrrolidone)-b-poly(N-methacryloyl-N′-(α-naphthyl)thiourea) (PNVP-b-PMANTU) were synthesized by ATRP. PNVP-b-PMANTU copolymers were characterized by ¹H NMR, GPC-MALLS and fluorescence measurements. The results revealed that PNVP-b-PMANTU presented a blocky architecture.     

Synthesis and characterization of highly functionalized polymers based on N,N,N′,N′-tetraalkyl-4,4′-diaminostilbene and maleic anhydride

Novel, highly functionalized rod-like copolymers have been synthesized by alternating copolymerization of N,N,N′,N′-tetraalkyl-4,4′-diaminostilbenes (TDASs) with maleic anhydride. These unique copolymers have been characterized by SEC, DSC and TGA. The solubility of these copolymers in organic solvents is strongly dependent on the length of the alkyl chains on the amino groups and the solubility in aqueous media is pH dependent. Light scattering studies indicate that the conformation of the polymer backbone does not change upon increasing temperature or introducing charges to the amino groups. High chain rigidity is further corroborated by the high T_g of the polymers. There is no observed glass transition below 280 °C. The light scattering and thermal results are indicative of a rod-like backbone structure.     

Synthesis and characterization of dual stimuli-responsive block copolymers based on poly(N-isopropylacrylamide)-b-poly(pseudoamino acid)

The current study synthesized amphiphilic thermal/pH-sensitive block copolymers PNiPAAm-b-PHpr by condensation polymerization of trans-4-hydroxy-l-proline (Hpr) initiated from hydroxy-terminated poly(N-isopropylacrylamide) (PNiPAAm) as the macroinitiator in the presence of the catalyst, SnOct₂. ¹H NMR, FTIR, and gel permeation chromatography (GPC) characterized these copolymers. Their solutions showed reversible changes in optical properties: transparent below a lower critical solution temperature (LCST) and opaque above the LCST. The LCST values depended on the polymer composition and the media. With critical micelle concentrations (CMCs) in the range of 1.23-3.73 mg L⁻¹, the block copolymers formed micelles in the aqueous phase owing to their amphiphilic characteristics. Increased hydrophobic segment length or decreased hydrophilic segment length in an amphiphilic diblock copolymer produced lower CMC values. The current work proved the core-shell structure of micelles by ¹H NMR analyses of the micelles in D₂O. Transmission electron microscopy analyzed micelle morphology, showing a spherical core-shell structure. The micelles had an average size in the range of 170˜210 nm (blank), and 195˜280 nm (with drug). Observations showed high drug entrapment efficiency and drug-loading content for the drug micelles.     

Electrochemical composite formation of thiophene and N-methylpyrrole polymers on carbon fiber microelectrodes: Morphology,characterization by surface spectroscopy,and electrochemical impedance spectroscopy

Electrochemical composite thin film formation (∼0.6–0.7 μm) of thiophene and N-methylpyrrole on carbon fiber microelectrodes (diameter ∼7 μm) was carried out by cyclic voltammetry in order to understand and improve the surface properties and capacitance behaviour of carbon fibers. Carbon fiber microelectrodes were coated with polythiophene and N-methylpyrrole was electrografted onto the thiophene electrode. The electrocoated carbon fiber surface mophology was characterized by scanning electron microscopy and atomic force microscopy and by FTIR-reflectance spectroscopy for their composition. The effect of monomer concentration and scan number on electropolymerization has also been investigated. The impedance behaviour of composite electrodes was characterized by electrochemical impedance spectroscopy. The composite of polythiophene and poly-N-methylpyrrole exhibits better charge storage properties than polythiophene coated carbon fiber microelectrodes.     

Sulfonic acid-functionalized ordered nanoporous Na-montmorillonite (SANM): A novel,efficient and recyclable catalyst for the chemoselective N-Boc protection of amines in solventless media

Sulfonic acid-functionalized ordered nanoporous Na⁺-montmorillonite (SANM) was used as an efficient catalyst for N-tert-butoxycarbonylation of amines with di-tert-butyl dicarbonate under solvent-free conditions at room temperature. Various aliphatic, aromatic, heterocyclic amines and aminols were protected as their corresponding mono-carbamates in excellent yields and short reaction times. No competitive side reactions such as isocyanate, urea, and N,N-di-Boc formation were observed. The reported method is mild, chemoselective and has the advantages such as heterogeneous catalysis, low cost and the recyclability of the catalyst.     

Effect of composition and stereoregularity on phase-transition behavior of aqueous N-ethylacrylamide/N-n-propylacrylamide copolymer solutions

Radical copolymerizations of N-ethylacrylamide and N-n-propylacrylamide (NNPAAm) at various ratios were carried out at −40 °C, in toluene in the presence of 3-methyl-3-pentanol, or in N-ethylacetamide. Syndiotactic-rich copolymers with racemo diad contents of 67.1–70.2%, and isotactic-rich copolymers with meso diad contents of 60.9–64.5% were prepared. Syndiotactic-rich copolymers with NNPAAm compositions of ≥92.9 mol% exhibited large hystereses in the phase-transition temperatures of their aqueous solutions. Isotactic-rich copolymers with NNPAAm compositions of 39.2–67.6 mol% exhibited large hystereses in the phase-transition temperatures of their aqueous solutions. Those of composition >67.6 mol% were insoluble in water. Stereosequence analysis suggested that isotactic sequences favored intramolecular hydrogen bonding between contiguous NNPAAm units, more than syndiotactic sequences. Enhanced intramolecular hydrogen bonding in isotactic sequences was responsible for the large hystereses and insolubility of isotactic-rich copolymers with high NNPAAm compositions.     

Clickable poly(ionic liquid)s for modification of glass and silicon surfaces

Covalent attachment of poly(ionic liquid)s (PILs) by click chemistry on glass or silicon (Si) surfaces was performed. Poly[1-(4-vinylbenzyl)-3-butylimidazolium bis(trifluoromethylsulfonyl)imide] (polyVBBI⁺Tf₂N⁻), and copolymers of polyVBBI⁺Tf₂N⁻ with fluorescein O-methacrylate were synthesized by conducting an atom transfer radical polymerization (ATRP) from initiators containing azide or thioacetate groups. The azide- and thiol-terminated PILs were then successfully grafted onto alkyne and alkene modified glass/Si wafers by thermal azide–alkyne cycloaddition and photoinitiated thiol-ene click reactions, respectively. The modified surfaces were characterized by contact angle measurements and ellipsometry. The fluorescent PIL functionalized surfaces showed strong fluorescence under UV irradiation. This procedure of tethering PILs to substrates also provides an easy way to change the surface hydrophilicity by replacing the anions in the grafted PILs. The present approach could be readily applied for surface modifications with other types of PILs or their copolymers to achieve different functionalities on various surfaces.     

H NMR study of temperature-induced phase transitions in D2O solutions of poly(N-isopropylmethacrylamide)/poly(N-isopropylacrylamide) mixtures and random copolymers

¹H NMR spectroscopy was used to investigate temperature-induced phase transitions in D₂O solutions of poly(N-isopropylmethacrylamide) (PIPMAm)/poly(N-isopropylacrylamide) (PIPAAm) mixtures and P(IPMAm/IPAAm) random copolymers of various composition on molecular level. While two phase transitions were detected for PIPMAm/PIPAAm mixtures, only single phase transition was found for P(IPMAm/IPAAm) copolymers. The phase transition temperatures of PIPAAm component (appears at lower temperatures) are not affected by the presence of PIPMAm in the mixtures; on the other hand, the temperatures of the phase transition of PIPMAm component (appears at higher temperatures) are affected by the phase separation of the PIPAAm component and depend on concentration of the solution. For P(IPMAm/IPAAm) random copolymers, a departure from the linear dependence of the transition temperatures on the copolymer composition was found for a sample with 75 mol% of IPMAm monomeric units.     

Main-chain sulphur containing water soluble poly(N-isopropylacrylamide-co-N,N′-dimethylacrylamide sulphide) copolymers via interfacial polycondensation

Here, we report the first synthesis of water soluble poly(N-isopropylacrylamide-co-N,N′-dimethylacrylamide sulfide) copolymers via conventional interfacial polycondensation method using phase transfer catalyst (PTC). The effect of various kinds of PTC having different aliphatic chain length and counter ion were employed to examine the kinetics of polysulfide polymer formation. The reactivity ratios, determined employing extended Kelene–Tüdös method using feed composition obtained from ¹H NMR analysis, suggest that N-isopropylacrylamide (NIPAM) is more reactive than N,N′-dimethylacrylamide (DMA) in both mono- and di-sulfide polymers. Thermal transition behavior investigated by differential scanning calorimetry (DSC) demonstrated that as the sulphur rank of the sulfide main chain linkages increased, the flexibility of the polymers increased reflected by lower glass transition temperature (T_g) values. The thermal degradation behavior and the major degradation products have been characterized by thermogravimetric analysis (TGA) and electron-impact mass spectroscopy (EI-MS), respectively. Both the studies reveal that the degradation takes place due to weak-link scission of the polymeric main chain. Solubility in water and in most of the common organic solvents even after the sulphur rank increased from 1 to 2 in the main chain, is expected to render potential applications in biological field as well as in industry for these interesting new class of polymers.     

A comparative study on corrosion-resistant performance of halogenated copolymers

Methacrylate-based copolymers are frequently used as anticorrosive organic coatings. Polymeric coating on metallic surfaces provides protection by a barrier action. Thermal properties have a significant influence on corrosion-resistance. This paper deals with the effect of thermal properties of 2,4,6-tribromophenyl methacrylate-co-glycidyl methacrylate and N-(p-bromophenyl)-2-methacrylamide-co-glycidyl methacrylate copolymers in corrosion-resistant behavior on low nickel stainless steel (LNSS). Hence attempts have been made to synthesize a set of copolymers by free radical polymerization and compare their corrosion-resistance properties. The copolymers were structurally characterized by Fourier transform-infrared, and ¹H-nuclear magnetic resonance spectroscopic techniques. The molecular weight of the copolymers was determined by gel permeation chromatography. Thermal studies were carried out using thermogravimetric analysis and differential scanning calorimetry. Corrosion performances of LNSS coated with two different copolymers was investigated in 1 M H₂SO₄ using potentiodynamic polarization and electrochemical impedance spectroscopic methods. The corrosion study reveals that poly(TBPMA-co-GMA) showed better corrosion-resistance than poly(PBPMA-co-GMA).     

Optically active copolymers of N-(oxazolinyl)phenylmaleimides with methyl methacrylate: Synthesis and chiral recognition ability

The free radical copolymerization of three N-phenylmaleimides bearing a chiral oxazoline residue at ortho-position of the phenyl group (OPMI) with methyl methacrylate (MMA) was carried out in tetrahydrofuran by varying the molar ratio of the comonomers. The monomer reactivity ratios and Alfrey-Price Q-e values were determined. The data revealed that the monomer OPMI has a distinct tendency to cross-propagate and MMA is inclined to block formation. Meanwhile, the MMA co-units affected significantly the chiroptical property of resulted copolymers besides endowing them with a moderate film-forming ability. Poly(OPMI-co-MMA)s coated on macroporous silica were used as a high-performance liquid chromatography chiral selector, and the optical resolution performance was briefly evaluated toward some racemic compounds including amino- and hydroxy-acids as well as 1,1′-bi-2-naphthol. The observation indicated that the column combining MeOPMI/MMA copolymers with higher chiral component exhibits an extent of enantioselectivity in both normal and reversed-phase modes.     

A unique phase behavior of random copolymer of N-isopropylacrylamide and N,N-diethylacrylamide in water

Thermosensitive phase separation of aqueous solutions of the random copolymers of N-isopropylacrylamide (iPA) and N,N-diethylacrylamide (dEA) (PiPA-dEA) and of iPA and N-isopropylmethacrylamide(iPMA) (PiPA-iPMA) with different compositions has been investigated by using calorimetry, turbidimetry and infrared spectroscopy. Though the phase transition temperature (T_p) of PiPA-iPMA is a linear function of its composition, a deviation from additivity is observed for that of PiPA-dEA, that is, it has a minimum value at iPA/dEA = 1 (mol/mol). IR spectrum at the amide II mode of the copolymer suggests that part of N-H groups of iPA units form a hydrogen bond with CO groups of dEA units at T > T_p as well as with those of the iPA units. Effects of methanol on T_p of these copolymers have also been studied.     

Electropolymerization and electrochemical properties of (N-hydroxyalkyl)pyrrole/pyrrole copolymers

In this study, the N-hydroxyalkyl derivatives of pyrrole (Py), N-(2-hydroxyethyl)pyrrole (HE) and N-(3-hydroxypropyl)pyrrole (HP), were synthesized. The corresponding homopolymers, PHE and PHP, together with the copolymers of Py/HE and those of Py/HP were prepared by galvanostatic polymerization. These monomers and polymers were characterized by FTIR spectroscopy, elemental analysis, SEM and electrochemical techniques. The result of potential-time profiles showed that a higher potential was required for HE and HP than Py for the polymerization. This was ascribed to the steric hindrance of high concentration of the N-hydroxyalkyl groups. However, a similar potential was observed for the copolymerization of Py/HE and Py/HP systems as that of Py due to the reduction of the steric effect by lower content of the substituent. The SEM micrographs showed a rougher morphology for the films synthesized from the solutions with higher Py/derivatives ratio. The cyclic voltammograms indicated that all the copolymers were larger, while the homopolymers had smaller anodic/cathodic currents and specific charges than PPy. This implied that the existence of the proper amount of the N-hydroxyalkyl pendant groups enhanced the ionic mobility of the pyrrole polymers. The results of charge/discharge measurements showed that the copolymer PYHP82 has the highest discharge capacity among the pyrrole polymers prepared.