Free Radical Graft Polymerization Research Articles

Effects of Modified Vermiculite on Water Absorbency and Swelling Behavior of Chitosan-g-Poly(Acrylic Acid)/Vermiculite Superabsorbent Composite

In this study, a series of chitosan-g-poly(acrylic acid)/vermiculite (CTS-g-PAA/VMT) superabsorbent composites containing VMT, acid-activated, ion-exchanged, and organic modified VMT were synthesized by free-radical graft polymerization in aqueous solution, using N,N’-methylenebisacrylamide as a crosslinker and ammonium persulfate as an initiator. The effects of VMT and modified VMT on equilibrium water absorbency, swelling rate, and swelling behavior in various cationic saline solutions (NaCl, CaCl2, and FeCl 3) and different pH value solution of superabsorbent composites were systematically investigated. FTIR spectroscopy confirmed that acrylic acid had been grafted onto CTS and the -OH groups of VMT participated reaction. The introduction of acid-activated, ion-exchanged, and organic modified VMT into chitosan-g-poly(acrylic acid) polymeric network could improve water absorbency and swelling rate compared with that of VMT. The water-absorbing capacity of superabsorbent composites decrease gradually with the increasing of the concentration of NaCl, CaCl2, and FeCl3 solutions from 0.01 to 10 mM. The all prepared samples have similar swelling behavior in different pH value solution and the equilibrium water absorbencies of samples keep roughly constant in the pH range from 4 to 12.

Reverse atom transfer radical graft polymerization of 4‐vinylpyridine onto inorganic oxide surfaces

AbstractThin poly(4‐vinylpyridine) films terminally anchored onto nonporous inorganic oxide substrates were synthesized by aqueous phase reverse atom transfer radical graft polymerization (ATRGP). Surface initiators were immobilized on the inorganic substrate surface by chemically attaching glycidoxytrimethoxy silane onto the substrates followed by conversion of the glycidoxy silane into azobis silane by a reaction with 4,4′‐azobis(4‐cyanovaleric acid). Reverse ATRGP of 4‐vinylpyridine onto the active surface azo sites was carried out in a 1‐methyl‐2‐pyrrolidone/water solvent mixture using CuCl2/2,2′‐bipyridine as the catalyst‐ligand complex with initial monomer concentration [M]0 = 2.32M at 90°C. Controlled radical polymerization was achieved at catalyst to initiator molar ratios of 2 : 1 and 3 : 1, with a catalyst to ligand molar ratio of 1 : 2. Controlled polymerization was indicated by a first‐order rate of polymerization kinetics, with respect to monomer conversion at the surface and in solution, the linear increase of the P4VP graft yield with time, and a low polydispersity index (PDI < 1.40). The highest graft yield of 8 mg/m2 was achieved at a 3 : 1 catalyst to initiator molar ratio which corresponded to a number‐average molecular weight of 11,500 g/mol, surface density of 0.69 μmol/m2, and a polydispersity index of 1.28. AFM surface analysis of the grafted polymer films, prepared by reverse ATRGP of 4VP, revealed a decrease in the RMS surface roughness (RRMS = 1.04 nm) and feature size (feature diameter = 20–45 nm), relative to uncontrolled free radical graft polymerization (RRMS = 1.42 nm; feature diameter = 60–145 nm), thereby providing an additional indication that a denser and controlled reaction was achieved via reverse ATRGP. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009

Controlled Nitroxide-Mediated Styrene Surface Graft Polymerization with Atmospheric Plasma Surface Activation

Polymer layer growth by free radical graft polymerization (FRGP) and controlled nitroxide-mediated graft polymerization (NMGP) of polystyrene was achieved by atmospheric pressure hydrogen plasma surface activation of silicon. Kinetic polystyrene layer growth by atmospheric pressure plasma-induced FRGP (APPI-FRGP) exhibited a maximum surface-grafted layer thickness (125 A after 20 h) at an initial monomer concentration of [M] 0 = 2.62 M at 85 degrees C. Increasing both the reaction temperature ( T = 100 degrees C) and initial monomer concentration ([M] 0 = 4.36 M) led to an increased initial film growth rate but a reduced polymer layer thickness, due to uncontrolled thermal initiation and polymer grafting from solution. Controlled atmospheric pressure plasma-induced NMGP (APPI-NMGP), using 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO), exhibited a linear increase in grafted polystyrene layer growth with time due to controlled surface graft polymerization as well as reduced uncontrolled solution polymerization and polymer grafting, resulting in a polymer layer thickness of 285 A after 60 h at [TEMPO] = 10 mM, [M] 0 = 4.36 M, and T = 120 degrees C. Atomic force microscopy (AFM) surface analysis demonstrated that polystyrene-grafted surfaces created by APPI-NMGP exhibited a high surface density of spatially homogeneous polymer features with a low root-mean-square (RMS) surface roughness ( R rms = 0.36 nm), similar to that of the native silicon surface ( R rms = 0.21 nm). In contrast, polymer films created by APPI-FRGP at [M] 0 = 2.62 M demonstrated an increase in polymer film surface roughness observed at reaction temperatures of 85 degrees C ( R rms = 0.55 nm) and 100 degrees C ( R rms = 1.70 nm). The present study concluded that the current approach to APPI controlled radical polymerization may be used to achieve a grafted polymer layer with a lower surface roughness and a higher fractional coverage of surface-grafted polymers compared to both conventional FRGP and APPI-FRGP.

Post‐Synthesis Functionalization of (Meth)acrylate Based Monoliths via Electron Beam Triggered Graft Polymerization

AbstractPorous polymeric monoliths were prepared via electron beam triggered free radical polymerization of (meth)acrylates. Post‐synthesis functionalization of these supports was accomplished via electron beam initiated free radical graft polymerization of methacryloyl‐substituted NHC precursors. The grafted precursors were converted into the corresponding copper complexes. Cu‐loadings were between 1.3 mg · g−1 and 1.5 mg · g−1. These supported catalysts were used in selected CO hydrosilylation and cyanosilylation reactions using a continuous flow setup.magnified image

A novel hydrophilic polymer-ceramic composite membrane 1: Acrylic acid grafting membrane

A novel hydrophilic ceramic/polymeric pervaporation membrane was prepared by free-radical graft polymerization of acrylic acid (AA) onto the ceramic membrane which was pre-coated with a SiO 2 active layer by in situ hydrolysis-deposit method. The influence of reaction conditions on the SiO 2 layer was studied. The surface property and the morphology of the membrane were analyzed by means of SEM, FT-IR and TGA. Only the AA-chain grafting on the support coated with an active silica layer can lead to high performance membranes for solvent dehydration. The grafted chains imparted a high selectivity to the membrane, but their confinement in rigid ceramic pores led to a singular decrease in the permeation flux when the temperature increases. The membrane performances for the separation of ethanol/water mixtures by pervaporation for different monomer concentrations in the grafting solution were measured. The membrane grafted in 6 wt.% of AA had good separation performances: for the dehydration of 95 wt.% ethanol at 30 °C, the permeation flux was 540 g m −2 h −1 and the water content in permeate was higher than 98.7 wt.%.

UV-Patterned Poly(ethylene glycol) Matrix for Microarray Applications

A versatile method to fabricate polymeric matrixes for microarray applications is demonstrated. Several different design strategies are presented where a variety of organic films, such as plastic polymers and self-assembled monolayers (SAMs) on planar silica and gold substrates, act as supports for the graft polymerization procedure. An ensemble of poly(ethylene glycol) methacrylate monomers are combined to obtain a matrix with desired properties: low nonspecific binding and easily accessible groups for postimmobilization of ligands. The free radical graft polymerization process occurs under irradiation with UV light in the 254-266 nm range, which offers the possibility to introduce patterns by means of a photomask. The arrays are created on inert and homogeneous coatings prepared either by graft polymerization of a methoxy-terminated PEG-methacrylate or self-assembly of a methoxy-terminated oligo(ethylene glycol) thiol. Carboxylic acid groups, introduced in the array spots either during graft polymerization or upon wet chemical conversion of hydroxyls, grant the capability to immobilize proteins and other molecules via free amine groups. Immobilization of fluorescent species as well as biotin followed by exposure to a fluorescently labeled antibody directed toward biotin display both excellent integrity of the spots and low nonspecific binding to the surrounding framework. Beside patterns of uniform height and size, an array of spots with varying thickness (a sort of gradient) is demonstrated. Such gradient samples enable us to address critical issues regarding the mechanism(s) behind spatially resolved free radical polymerization of methacrylates. It also offers a convenient route to optimize the matrix properties with respect to thickness, loading capacity, protein diffusion/penetration, and nonspecific binding.

Radical graft functional modification of cellulose with allyl monomers: Chemistry and structure characterization

Cotton cellulose was successfully functionalized via a free radical graft polymerization process. Potassium persulfate served as an effective water soluble radical initiator to generate cellulosic radicals. The polymeric radicals could react with allyl monomers such as allyl-dimethylhydantion (ADMH) to form surface grafted cellulose. The reaction sites generated by potassium persulfate were probably at carbon 3 and 4 in glucose ring via oxidative hydrogen abstraction. The cellulosic radicals can initiate grafting polymerization of ADMH with a maximum polymerization degree of about 12 based on LC–MS results. The radical graft polymerization mechanisms were proposed based on LC–ESI/MS analysis. The ideal covalent bonding between cellulose and poly (allyl-dimethylhydantion) (PADMH) ensured permanent graft of the monomers on cotton and durability of the expected functions on the treated cotton.

Investigations of Membrane Electrolytes for PEM Fuel Cell Applications

The work concerns new polymeric materials for electrolytes synthesized at the Electrotechnical Institute. Poly(styrene-4-sulfonate)-grafted poly(vinylidene fluoride) (PVDF-g- PSSA) membranes proton-exchange membranes were synthesised by free radical graft polymerization of polystyrene into PVDF films (50 *m thick) followed by sulfonation. Physical and electrochemical properties of that membranes were investigated. The membranes were tested as electrolytes in a single cell configuration system. The influence of working condition, as gas humidity and cell temperature on fuel cell performance were measured. Obtained performances were lower in comparison with commonly used electrolyte materials. At this stage of development the synthesized materials are not suitable for fuel cell application. The results gave authors information for selection membrane with the best parameters and chemical process configuration.

A novel method to immobilize collagen on polypropylene film as substrate for hepatocyte culture

To improve the biocompatibility of polypropylene membrane with hepatocytes, film was firstly allowed to generate peroxide groups by ammonium peroxydisulfate (APS) thermal decomposition and was then carboxylated by free radical grafting polymerization using acrylic acid as the monomer and ferrous(II) ions as redox initiator. Collagen was then covalently immobilized through amide bonds between the residue amine groups and carboxylic ones in the presence of water-soluble carbodiimide. The film was characterized by Fourier transformed infrared spectroscopy in attenuated total reflection mode (ATR–FTIR), X-photoelectron spectroscopy (XPS) and confocal laser scanning microscopy (CLSM), etc. The experimental results show that there exist hydroperoxide groups after APS aqueous thermal decomposition, which are subsequently converted into the polymeric free radicals initiating the vinyl monomers to subject grafting polymerization. The maximum collagen immobilization content is about 10.5 μg/cm 2 by ninhydrin assay. Hepatocytes cultured on collagen immobilized film show stable phenotype and normal liver-specific functions more than 20 days as observed by scanning electronic microscopy (SEM) and biochemical parameters determination.

Rapid Polymer Brush Growth by TEMPO-Mediated Controlled Free-Radical Polymerization from Swollen Plasma Deposited Poly(maleic anhydride) Initiator Surfaces

Pulsed plasma-chemical deposition of poly(maleic anhydride) is shown to be a substrate-independent method for functionalizing solid surfaces with initiator sites for nitroxide-mediated controlled free-radical graft polymerization. Swelling of the initiator film via aminolysis can lead to grafted polymer brushes that are 1 order of magnitude thicker than those obtained by existing methods on solid surfaces.

Controlled free radical polymerization of styrene initiated from the alkoxyamine-functionalized silicon surface

We describe a new method of fabricating a brush-like polystyrene layer anchored on the surface of the silicon substrate, which involves three steps, namely (i) the attachment of 3-methacryloxypropyltrimethoxysilane onto the silicon surface; (ii) the reaction of vinyl moiety at another extremity of the anchored 3-methacryloxypropyltrimethoxysilane to 4-Hydroxyl-2, 2, 6, 6-tetramethyl-1-piperidinyloxy (HTEMPO·) catalyzed by azobisisobutyronitrile (AIBN); (iii) living free radical grafting polymerization of styrene in the presence of HTEMPO·. The controllable living free radical polymerization permits accurate control of both the molecular weight and the polydispersity. X-photoelectron spectroscopy measurement proved that the alkoxyamine initiator layer forms on the silicon surface. XPS and Ellipsometry measurements showed that the polystyrene chains were covalently anchored onto the silicon surface. The thickness of the grafted polymer layer can be accurately manipulated by altering the polymerization time. The new method allows synthesizing random copolymer and block copolymers by the sequential growth of monomers from the substrate surface.

Versatile Synthetic Route to Tailor-Made Proton Exchange Membranes for Fuel Cell Applications by Combination of Radiation Chemistry of Polymers with Nitroxide-Mediated Living Free Radical Graft Polymerization

A versatile method for the preparation of proton exchange membranes (PEMs) by the combination of radiation chemistry of polymers with nitroxide-mediated living free radical graft polymerization with subsequent sulfonation is presented. Thus, poly(vinylidene fluoride) (PVDF) membranes were first irradiated with electron beam and then the free radicals formed were immediately quenched with 2,2,6,6-tetramethylpiperidinyl-1-oxy (TEMPO). In the second step, the produced TEMPO-capped macroinitiator sites were utilized in nitroxide-mediated living free radical graft polymerization of styrene or for the controlled graft copolymerization of styrene and N-phenylmaleimide onto the PVDF membrane. In the final step, the membranes were either directly sulfonated, or in the former case, alternatively the alkoxyamine moieties at the polymer chain ends were substituted by a maleimide derivative prior to sulfonation. The introduction of the N-phenyl maleimide moieties into the grafted chains significantly increased the the...

Removal of methyl tert-butyl ether from water by pervaporation using ceramic-supported polymer membranes

Pervaporation removal of methyl tert-butyl ether (MTBE) from dilute aqueous solutions was demonstrated using ceramic-supported polymer (CSP) membranes. These membranes were created by the free radical graft polymerization of vinyl acetate onto a vinyl trimethoxysilane-activated porous silica substrate having a native average pore size of 500 Å. The resulting membranes consisted of poly(vinyl acetate) chains terminally and covalently anchored to the membrane surface. These pervaporation membranes performed with separation factors ranging from about 68 to 577 at total permeate fluxes of 0.31–0.70 kg/m 2 h over feed concentrations of 74–7400 ppm (0.01–1%, v/v). MTBE flux was controlled by the feed-side concentration boundary layer, while water permeation was controlled by the membrane resistance. The primary effect of the grafted PVAc in these CSP membranes was to increase separation by rejecting water, as opposed to increasing the flux of MTBE.

PH-dependent self-assembly: micellization and micelle-hollow-sphere transition of cellulose-based copolymers.

pH-Sensitive micelles are usually made from double-hydrophilic block copolymers which can dissolve molecularly in water in a certain pH range and aggregate spontaneously upon an appropriate change in the pH value. Significant advances in this field have been achieved in recent years.[1] Polymeric hollow spheres have great potential for the encapsulation of large quantities of guest molecules. Different approaches, such as by the use of block copolymers as precursors,[2] as well as colloidal particles[3a,b] or liposomes[3c,d] as templates, have been developed to obtain such nanocapsules. Our research group has produced micelles in which no chemical bonds connect the core and the shell,[4] thus enabling hollow spheres to be obtained by simple dissolution of the inner component of the micelles.[4a–c] However, these approaches are mostly irreversible, that is, once the hollow structure forms, the core–shell micelles cannot be reformed. Hydroxyethylcellulose (HEC) has been extensively studied as a nonionic and water-soluble cellulose ether (Scheme 1).[5] However, studies on the self-assembly of HEC are limited, despite its biocompatibility and biodegradability. Here we report the self-assembly of HEC-graftpoly(acrylic acid) (HEC-g-PAA) in water, which was prepared by free-radical graft polymerization of acrylic acid from HEC backbones (Table 1). The results demonstrate its micellization and the transition between micelles and hollow spheres; both processes were found to be pH-dependent and reversible. Figure 1 shows the relative scattering intensity I/I0 of the solutions of two HEC-g-PAA copolymers, CAA-1 and CAA2, as a function of the pH value (I0 is the intensity of the starting solution at pH 13.5). The two solutions exhibit a

Surface Graft Polymerization of Styrene onto Nano-Sized Silica with a One-Pot Method

A facile method for free radical graft polymerization of styrene onto the surface of nano-sized silica was developed by using a one-pot method. The initiator (2,2′-azobisisobutyronitrile, AIBN), the coupling agent (vinyl triethoxylsilane, VTEOS), styrene and the nano-sized silica were mixed and reacted in toluene medium. The two reactions, the copolymerization of VTEOS and styrene and the coupling reaction of the ethoxyl groups in the side chains of the copolymer and the silanol groups on the surface of the nano-sized silica, were occurred synchronously in the preparing procedure.The effects of the amount of AIBN and VTEOS, the polymerizing temperature and the polymerizing time on the conversion of styrene (C), the percentage of grafting (PG), the grafting efficiency (GE) and the average relative molecular masses (Mn and Mw) of the grafted polystryrene (PSg) and the non-grafted polystyrene (PSn) were discussed. IR and XPS showed that the surface of the nano-sized silica had been covered partially by PS and TEM showed that the dispersibility of PS grafted nano-sized silica in organic solvent was improved.

Synthesis of Poly(IB‐co‐VBDC)‐g‐PMMA via Photo‐initiated Free Radical Graft Polymerization

Photoinitiated free radical graft polymerization of methyl methacrylate (MMA) with poly[isobutene‐co‐(4‐vinyl benzyl N,N‐diethyldithiocarbamate)] [poly(IB‐co‐VBDC)] as macromolecular iniferter was investigated. The polymerization proceeds to give a high yield graft copolymer, however it was observed that even in the early stage of the polymerization there formed an insoluble polymer. In the presence of tetraethylthiuram disulfide (TETD) the gel fraction of the yield graft copolymer was drastically reduced and the polymerization was retarded as well. When the [TETD]/[VBDC] increased from 0 to 1.0, the gel fraction of the graft copolymer decreased from 33.2% to 1.6% (wt) while the fraction of the homopolymer of the MMA increased from 4.5% to 10.5% (wt). With the increasing of the UV irradiation time, both the MMA conversion and the molecular weight of the graft copolymer increased readily.

Ceramic-supported polymer membranes for pervaporation of binary organic/organic mixtures

Asymmetric tubular alumina-supported poly(vinyl acetate) (PVAc) and poly(vinyl pyrrolidone) (PVP) membranes were created and characterized by pervaporation separation of binary mixtures of methanol and methyl tert-butyl ether (MTBE). The active separation layer was created by free-radical graft polymerization of PVAc and PVP onto a vinylsilane-modified alumina substrate with an average native pore diameter of 50 Å. The separation layer consisted of a surface phase of terminally anchored polymer chains with estimated radius of gyration about a factor of 4.5–6.8 larger than the membrane pore radius. The methanol separation factors for the PVP and PVAc-grafted pervaporation membranes reached values of 26 and 100, respectively, at the lower range of methanol concentrations (1–5% v/v) tested in the study. The separation impact of the grafted polymer chains was apparent given that the native (unmodified) and vinylsilylated alumina membranes lacked selectivity for the MTBE/methanol system. Total permeate flux attained with the PVAc and PVP-based membranes ranged from 0.055 to 1.26 and 0.55 to 6.19 kg/m 2 h, respectively, over the respective feed methanol concentration ranges of 1–90 and 5–90% (v/v). A tradeoff between separation and permeate flux was apparent from the decrease in permeation flux with increasing separation factor.

Contact Angle Study on Polymer-Grafted Silicon Wafers

Surface wettability and hydrophilicity of terminally grafted PVP and PVAc on smooth silicon wafers were investigated by advancing and receding contact angle measurements. Surface wetting trends correlated with polymer graft yield. Contact angle hysteresis results did not reveal a clear dependence of surface wetting on surface roughness (determined by atomic force microscopy) at the nanoscale level. Surface roughness appeared decrease with increasing chain length for the present surfaces produced by free radical graft polymerization. Contact angle analysis in terms of surface tension components demonstrated that the degree of hydrophilicity of polymer-grafted smooth inorganic surfaces depends on both polymer functionality and polymer graft yield. It is postulated that reorientation of surface-grafted polymer chains could account for changes in surface wettability, similar to those observed for solid polymer films.

Tentacle-type zwitterionic stationary phase prepared by surface-initiated graft polymerization of 3-[N,N-dimethyl-N-(methacryloyloxyethyl)-ammonium] propanesulfonate through peroxide groups tethered on porous silica.

A novel stationary phase with tentacle-type zwitterionic interaction layer was synthesized by free radical graft polymerization of 3-[N,N-dimethyl-N-(methacryloyloxyethyl)ammonium]propanesulfonate (SPE) from the surface of Kromasil porous silica particles. The polymerization was initiated by thermal cleavage of tert-butylperoxy groups covalently attached to the particle surface, and the material therefore carries a tentacle-type polymeric interaction layer with 3-sulfopropylbetaine functional moieties. The composition of the surface graft was determined by elemental analysis, and the surface charge was measured using photon correlation spectroscopy. The measured zeta-potentials were close to 0 and nearly independent of pH, and the tentacle character of the interactive layers were evident from the lack of colloidal stability in the absence of salt (antipolyelectrolytic behavior) and a marked increase in column back-pressure when the concentration of perchloric acid or perchlorate salt was increased. The chromatographic properties were evaluated on columns packed with the functionalized material, and it was shown that this zwitterionic stationary phase could simultaneously and independently separate inorganic anions and cations using aqueous solutions of perchloric acid or perchlorate salts as eluents. The material was also capable of separating two acidic and three basic proteins in a single run, using gradient salt elution at constant pH.

Kinetics of free‐radical graft polymerization of 1‐vinyl‐2‐pyrrolidone onto silica

AbstractThe kinetics of aqueous free‐radical graft polymerization of 1‐vinyl‐2‐pyrrolidone onto silica activated with vinyltrimethoxysilane was studied with a mechanistic polymerization model and experimental data for a temperature range of 70–90 °C. The polymerization was initiated with hydrogen peroxide with initial monomer concentrations ranging from 10 to 40 vol %. The kinetic model, which incorporates the hybrid cage–complex initiation mechanism, describes the experimental polymerization data for which the kinetic order, with respect to the monomer concentration, varies from 1 to . Surface chain growth occurs by both monomer addition and homopolymer grafting, although the latter contribution to the total polymer graft yield is less significant. Increasing the initial monomer concentration enhances both surface polymer density and average grafted chain length. Increasing reaction temperature, however, produces a denser surface layer of shorter polymer chains. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 40: 26–42, 2002