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ABSTRACT In this work, four polymerization mechanisms with flow are proposed where sustained oscillation can be present: (1) Polyaddition initiated by radicals produced by initiator decomposition, which is catalyzed by the polymer, first-order termination, a polymer disappears by a reaction with a third substance and by leaving the system in the exit flow. (2) Polycondensation where the polymer has catalytic activity upon H to produce a monomer, first-order termination and a monomer disappears by a reaction with a third substance, and there is exit flow for all the species. (3) Polycondensation with catalytic activity of the polymer upon A to produce a monomer, first-order termination, conformational transitions of the polymers, one of the polymer conformations disappears by a reaction with a monomer and exit flow for all the species. (4) Polycondensation combined with polyaddition where the polycondensation polymer product has catalytic activity on the monomer, first-order termination and exit flow of all the species. [ABSTRACT FROM AUTHOR]

ABSTRACT Two new kinetic mechanisms of polymerization are proposed (0Re-1,2 and 0Re-1) to study the instability of the steady states by the Lyapounov method. Both mechanisms take into account a reinitiation step in the polymerization process, but they are different because 0Re-1,2 considers two kinds of termination steps: a zero- and a second-order terminations. However, in the second mechanism, 0Re-1, only one of those termination steps (the zero-order one) is presented. In both proposed mechanisms, the nondiffusive and diffusive cases are studied. The results show that they are able to display arrangements in time (limit cycle oscillations) and space (order form fluctuations). [ABSTRACT FROM AUTHOR]

Abstract: For two variable chemical oscillators, a simple strategy is to combine autocatalysis with a term including a binomial Michaelian denominator (constant+concentration). This strategy is applied in this paper to the context of polymerization reactions. In order to achieve autocatalysis a kinetic scheme of the form is applied, where the dimeric form D is included. The Michaelian term can be obtained in different ways, for example: (i) a third compound Z flows through the system and eliminates the monomer, (ii) compound Z can be radical initiators, and (iii) compound Z, which eliminates monomer, can be a special conformation (adsorbent) of the polymer. Three mechanisms are developed using these Michaelian terms. After decreasing the number of variables to two, by assuming a non-significant variation of the others, and normalizing the kinetic equations with respect to a certain steady state, phase plane techniques are applied to prove the existence of a limit cycle in the three models. Both monomer concentration and propagator concentration in the three models (cases 2.1 and 2.2) or the concentration of the adsorbent polymers are taken as oscillating variables. [Copyright &y& Elsevier]

Abstract: A simple polymerization scheme has been studied introducing small modifications leading to a stable focus type steady state (with damped oscillations) or unstable focus type (which combined with a no return enclosure for phase trajectories will show cycle limit sustained oscillations). Two variables have been employed in this analysis: X∝ monomer, . Limit cycle oscillations requires the addition of autocatalysis with respect to the monomer, J+X→2X, and so does an “enzymatic” block assuming that . The combination of both collateral additions makes the steady state an unstable focus and allows a simple Poincaré–Bendixson proof for the existence of the limit cycle. [ABSTRACT FROM AUTHOR]

A simple radical polymerization is proposed in this paper, with step-by-step chain growth (Ri + M → Ri+1), and termination by transfer to a third body (Ri + S → polymer) such as the solvent. It is assumed that, for a certain critical degree of polymerization n, the propagator Rn reacts with substrate H to produce a deactivator (V) of the first propagator (H + Rn → Rn + V; V + R1 → P1) R1. Assuming that monomer, M, and precursor concentrations are constant, and assuming that the deactivator reaches fast a steady state, the resulting kinetic equations are formally linear, but they admit, perturbations rj(t) of the steady-state concentrations of the propagators R1, R2, ..., Rn, which are periodic functions of time. Even more, they can be purely sinusoidal functions (which have been called “harmonic,” in analogy to the solutions of the well-known classical harmonic oscillator) with phase shift between perturbations rj(t) = Rj- (Rj)0 and rj+1(t) = Rj+1 - (Rj+1)0. Based on these periodic solutions and aiming to a model as simple as possible, a theoretical analysis is made, resulting in that the minimum value for n would be n = 3. Of course, these harmonic oscillations “driven by trimer” are equally found in the group of all the remaining propagators with polymerization degree higher than 3 (variable Y = ∑∞j=4 Rj). © 2009 Wiley Periodicals, Inc. Int J Chem Kinet 41: 507–511, 2009 [ABSTRACT FROM AUTHOR]

In this paper, the possibility for autocatalysis in polymerization reactions is explored by introducing part of a polymerization mechanism in a model known as Brusselator. It is assumed that monomer concentration is practically constant. Four possibilities are examined: (1) a first radical propagator X, which has an isomer of position of the free electron, Y, dimerizes reversibly and this dimer catalyzes the isomerization of Y to X; (2) the radical propagator X is a polymer with a critical degree of polymerization and has an isomer of position of the free electron Y. This critical radical propagator catalyzes the conversion of Y to X; (3) any radical propagator has an isomer of position of the free electron, Y, and any polymer obtained by recombination of the radicals can catalyze the conversion of Y into its corresponding isomer X; and (4) any radical propagator with a critical degree of polymerization can catalyze the conversion of Y into its corresponding isomer. Isomorphism equations are obtained in all mechanisms, which implies the possibility of limit cycle oscillations (Brusselator model). © 2008 Wiley Periodicals, Inc. Int J Chem Kinet 40: 617–623, 2008 [ABSTRACT FROM AUTHOR]

Abstract: The polymerization of n-hexyl methacrylate (n-HMA) in three-component microemulsion stabilized with dodecyltrimethylammonium bromide (DTAB) is reported as a function of monomer and initiator concentrations and temperature. The obtained latices were bluish, transparent, and translucent. Particle sizes and molar masses were on the order of 20 nm and , respectively. In all cases, high reaction rates and final conversions of 98% were obtained. Polymerization temperature has a strong effect on reaction rate and conversion. [Copyright &y& Elsevier]

Hydrogels of acrylic acid and itaconic acid has been synthesized with different monomers ratios. The swelling process of the different xerogels immersed in water and salt solutions has been studied. The swelling of hydrogels loaded with metal cations (Cu2+, Zn2+) was also investigated. The swelling process was monitored by the increase in the weight of hydrogel as a function of time. The absorption properties of metal ions were studied by using the hydrogel, and different concentrations of copper and zinc solutions (prepared from sulphate salts). The influence of pH on the absorption process was studied. For the absorption of metal ions, the amount of ions absorbed within the hydrogel can be calculated from the initial and equilibrium concentrations of the metal ions in aqueous phase, the weight of the hydrogel, and the volume of solution used. Metal absorption increased when pH, salt concentration in external solution and itaconic acid content is levelled. The swelling isotherms which consisted of an initial fast increase levelled off asymptotically to the equilibrium swelling limit. The experimental data clearly suggest that our hydrogels follow a second-order kinetics for both cases (unload and metal loaded). The kinetics rate constant and the equilibrium water content, K, have been calculated for every monomer ratio from the experimental data according to the kinetics equation. Both magnitudes decreased as the itaconic acid content decreases in the xerogels. [ABSTRACT FROM AUTHOR]

The mechanical and thermodynamic properties of poly(acrylic acid-co-methyl methacrylate) hydrogels with varying ratios of AA/MMA are reported. Swelling capacity increases but elasticity decreases as the content of AA in the hydrogels increases. Thermodynamic analysis indicates that swelling is an unfavorable and endothermic process. Both thermodynamic quantities increase with increasing hydrophobic nature of the hydrogel. The amount of freezing and non-freezing water in the hydrogels was determined by DSC. The ratio of non freezing to total water content increases with the increasing hydrophobic nature of the hydrogels. [ABSTRACT FROM AUTHOR]

Highlights: [•] New gels based on PEG and PVA of different degrees of hydrolysis were synthesized. [•] PVA degree of hydrolysis and PEG content of the network control the swelling capacity. [•] The pH influences the degree of swelling and the rate of hydrolysis of the hydrogel. [•] The degree of hydration decreases with temperature because their entropy is negative. [Copyright &y& Elsevier]

Poly( N-isopropylacrylamide) (PNIPA) and Poly( N-isopropylacrylamide-co-acrylic acid) (P(NIPA-co-AA)) microgels loaded with 5-aminolevulinic acid (ALA) were prepared by the spray-drying method. The amount of drug loaded was 290 µg ALA/mg microgel for PNIPA and 244 µg ALA/mg microgel for P(NIPA-co-AA) microgels. Maximum in vitro drug release took place within 15-30 min for PNIPA and 1-1.5 h for P(NIPA-co-AA) microgels as a function of pH, at 37°C. Transdermal delivery from microgels showed permeation fluxes 10 times higher than the passive diffusion flux. The cytotoxicity of microgels synthesized in HeLa cells after the application of photodynamic therapy (PDT) was superior compared with the administration of ALA in solution alone. Finally, the use of these microgels as a delivery vehicle for ALA constitutes a system capable of enhancing its topical administration and PDT effectiveness. [ABSTRACT FROM AUTHOR]

Abstract: Networks of β-cyclodextrin have been prepared by reaction with acylated poly(ethylene glycol) with a molar mass of 600g/mol. Samples with different β-cyclodextrin/poly(ethylene glycol) ratios: 1/4, 1/6, 1/8 and 1/10 have been prepared. Both components are bonded by ester groups, resulting in a network that can be degraded by hydrolysis in basic and acidic media. The maximum stability of the hydrogels is reached at pH 4. The hydrogel percentage water content depends on β-cyclodextrin content ranging from 82 to 98, and the swelling data obtained for these hydrogels fit well with a second order kinetics. The sorption behavior of these hydrogels has been tested by employing 1-naphthol as model molecule. The sorption capacity is close to other cyclodextrin networks previously reported and depends on the hydrogel composition and the concentration of 1-naphthol. [Copyright &y& Elsevier]

The inverse emulsion copolymerization of acrylamide (AM) with dimethylaminoethylacrylate methyl chloride (Q9) using redox initiation with low surfactant concentration in an isoparaffinic solvent has been studied. The kinetics of conversions of inverse-emulsion polymerizations were investigated by HPLC method. It was an interesting way to know rapidly the conversion of each monomer with high reproducibility. In all experiments the monomer concentration was 25% respect the total weight or higher and the experiments were carried out at 50∘C. The effects of initiator concentration, composition of the monomer mixture and monomer concentration on the polymerization conversion and viscosity of copolymers solutions have been examined. For copolymer characterization, the results of viscosity were compared with commercial copolymer viscosities. The formulations with 3,000 ppm of initiator were the most interesting of all copolymer compositions since they presented very high conversions and their viscosities were higher than those of commercial copolymer solutions. The analysis of monomer concentration was carried out with 3,000 ppm and the conversion and viscosity were higher when the monomer concentration was increased. [ABSTRACT FROM AUTHOR]

The thermoreversible gelation of blends of polystyrene-block-poly(ethylene/butylene)-block-polystyrene (SEBS) and polystyrene-block-poly(ethylene/propylene) (SEP) copolymers in n -octane was studied. The solvent is selective for the polyolefine blocks of the copolymers. The influence of the composition of the hybrid gels on the sol–gel transition and on the mechanical properties of the gels was analyzed. The sol–gel transition temperature increased with the concentration of both type of copolymers and did not depend on the hybrid gel composition for SEBS2 proportions higher than 50% at a total copolymer concentration higher than 6 wt%. The mechanical properties of the different gels were examined through oscillatory shear and compressive stress relaxation measurements. The elastic storage modulus increased with the triblock copolymer concentration but kept almost constant with the diblock copolymer concentration for SEBS concentrations higher than 5.0%. The stress relaxation rate was not dependent on the concentration of triblock and diblock copolymers, but the hybrid gels show lower stress relaxation rates than the pure SEBS2 gels. In the hybrid SEBS/SEP gels the SEP chains impart stability to the micelles or nodes of the network whereas the SEBS chains are responsible for the bridges that keep the gel as one-phase system. [ABSTRACT FROM AUTHOR]

Chitosan nanogels functionalized with folic acid and crosslinked with PEG diacid or tartaric acid have been prepared by inverse microemulsion method for targeted delivery into cancer cells. Nanogels with varying degrees of crosslinking (4.5–10 mol% of crosslinker) have been obtained. The particle sizes determined by TEM are below 50 nm with narrow size distributions. Nanogels show a clear improvement in solubility compared with pure chitosan. Their swelling ability is pH sensitive, due to the protonation of the free amino groups when the pH is varied from 10 to 4. The nanogels have been loaded with 5-fluorouracil, which is retained at pHs greater than 7.4 and released at pH = 5.5. These nanoparticles are degraded by lysozyme at physiological pH in a period of 20 days and are easily labeled with fluorescein isothiocyanate. All these features make them promising materials for the controlled release of anticancer drugs. [ABSTRACT FROM AUTHOR]

Temperature-sensitive poly(N-isopropylacrylamide) (PNIPA) nanohydrogels were synthesized by nanoemulsion polymerization in water-in-oil systems. Several cross-linking degrees and the incorporation of acrylic acid as comonomer at different concentrations were tested to produce nanohydrogels with a wide range of properties. The physicochemical properties of PNIPA nanohydrogels, and their relationship with the swelling-collapse behaviour, were studied to evaluate the suitability of PNIPA nanoparticles as smart delivery systems (for active packaging). The swelling-collapse transition was analyzed by the change in the optical properties of PNIPA nanohydrogels using ultraviolet-visible spectroscopy. The thermodynamic parameters associated with the nanohydrogels collapse were calculated using a mathematical approach based on the van't Hoff analysis, assuming a two-state equilibrium (swollen to collapsed). A mathematical model is proposed to predict both the thermally induced collapse, and the collapse induced by the simultaneous action of two factors (temperature and pH, or temperature and organic solvent concentration). Finally, van't Hoff analysis was compared with differential scanning calorimetry. The results obtained allow us to solve the problem of determining the molecular weight of the structural repeating unit in cross-linked NIPA polymers, which, as we show, can be estimated from the ratio of the molar heat capacity (obtained from the van't Hoff analysis) to the specific heat capacity (obtained from calorimetric measurements). [ABSTRACT FROM AUTHOR]

Highlights: [•] Folate-chitosan nanogels were crosslinked with genipin in microemulsion medium. [•] Nanogels are water soluble at neutral and basic pHs. [•] Degradation rate was slightly higher at endosomic pH 5.5 than at physiological 7.4. [•] Nanogels displayed a pH-responsive release of 5-fluorouracil. [ABSTRACT FROM AUTHOR]

A correction to the article "Solvent effect on the complex formation of polyvinylpyrrolidone and polymonomethy itaconate" that was published in the March 1, 2013 issue is presented.

Abstract: Chitosan nanoparticles crosslinked with genipin were prepared by reverse microemulsion that allowed to obtain highly monodisperse (3–20nm by TEM) nanogels. The incorporation of genipin into chitosan was confirmed and quantitatively evaluated by UV–vis and 1H NMR. Loosely crosslinked chitosan networks showed higher water solubility at neutral pHs than pure chitosan. The hydrodynamic diameter of the genipin–chitosan nanogels ranged from 270 to 390nm and no remarkable differences were found when the crosslinking degree was varied. The hydrodynamic diameters of the nanoparticles increased slightly at acidic pH and the protonation of ionizable amino groups with the pH was confirmed by the zeta potential measurements. The biocompatible and biodegradable nature, as well as the colloidal and monodisperse particle size of the prepared nanogels, make them attractive candidates for a large variety of biomedical applications. [Copyright &y& Elsevier]

Abstract: Chitosan nanoparticles were obtained by crosslinking with two biocompatible dicarboxylic acids: polyethylene glycol dicarboxylic acid and tartaric acid. The water-in-oil (W/O) microemulsion method yielded particle sizes around 10–15 nm in the dried state (TEM) and 200–700 nm in the swollen state (QELS) from commercial chitosan. All the synthesized nanogels showed improved water solubility and most of them were stable at physiological pH. QELS studies revealed the influence of the hydrophilic character and flexibility of the crosslinker on the swelling behaviour of the nanogels. The nanoparticles showed a pH-sensitive volume transition that was consistent with the pKa of chitosan. The collected zeta potential data corroborated the electrostatic repulsion mechanism responsible for the pH-responsive behaviour. [Copyright &y& Elsevier]

Folate-targeted poly[(p-nitrophenyl acrylate)-co-(N-isopropylacrylamide)] nanohydrogel (F-SubMG) was loaded with 5-fluorouracil (5-FU) to obtain low (16.3 ± 1.9 μg 5-FU/mg F-SubMG) and high (46.8 ± 3.8 μg 5-FU/mg F-SubMG) load 5-FU-loaded F-SubMGs. The complete in vitro drug release took place in 8 h. The cytotoxicity of unloaded F-SubMGs in MCF7 and HeLa cells was low; although it increased for high F-SubMG concentration. The administration of 10 μM 5-FU by 5-FU-loaded F-SubMGs was effective on both cellular types. Cell uptake of F-SubMGs took place in both cell types, but it was higher in HeLa cells because they are folate receptor positive. After subcutaneous administration (28 mg 5-FU/kg b.w.) in Wistar rats, F-SubMGs were detected at the site of injection under the skin. Histological studies indicated that the F-SubMGs were surrounded by connective tissue, without any signs of rejections, even 60 days after injection. Pharmacokinetic study showed an increase in MRT (mean residence time) of 5-FU when the drug was administered by drug-loaded F-SubMGs. [ABSTRACT FROM AUTHOR]

Here, we present the oil/water (O/W) microemulsion polymerization in three-component microemulsions of n-butyl acrylate, ethyl acrylate, and methyl acrylate, monomers with similar chemical structures but different water solubilities using the cationic surfactant dodecyl trimethyl ammonium bromide. The effects of monomer water solubility, initiator type and initial monomer concentration on the polymerization kinetics were studied. Reaction rates were high with final conversions between 70 and 98% depending on the monomer and reaction conditions. The final latexes were bluish, with a particle size ranging between 20 and 50 nm and polymer with molar masses in the order of 106 g mol−1. Increasing monomer water solubility resulted in a slower reaction rate, larger particles and a lower number density of particles. A higher reaction rate, larger average particle size and higher particle number density were obtained by increasing the monomer concentration. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011 [ABSTRACT FROM AUTHOR]

In order to evaluate the influence of some body fluids on the hydrogels (HG) properties, in this work was studied the absorption properties of semi-interpenetrating (IPN) HG obtained from acrylamide (AAm) and chitosan (Q). To achieve this objective we employment a Hanks balanced salt solution, which is a physiological fluid that simulates the blood plasma. The semi-IPN hydrogels synthesized from AAm and Q, were subjected to absorption studies by gravimetric analysis, both in deionized water as in Hanks solution. The results of kinetic parameters indicated that the addition of Q to polyacrylamide (PAAm) hydrogel slightly increases the absorption capacity of the fluid and the Q confers greater affinity with Hanks solution. The X-rays diffraction analysis showed interaction between HG and the constituent elements of the Hanks solution. The results obtained confirm that the HG exhibit good affinity with the simulated body fluid, and therefore have great potential for medical applications. [ABSTRACT FROM AUTHOR]