Stimulus-Responsive Polymeric Nanogels As Smart Drug Delivery Systems

Nanogels are three-dimensional nanoscale networks formed by physically or chemically cross-linking polymers. Nanogels have been explored as drug delivery systems due to their advantageous properties, such as biocompatibility, high stability, tunable particle size, drug loading capacity, and possible modification of the surface for active targeting by attaching ligands that recognize cognate receptors on the target cells or tissues. Nanogels can be designed to be stimulus responsive, and react to internal or external stimuli such as pH, temperature, light and redox, thus resulting in the controlled release of loaded drugs. This “smart” targeting ability prevents drug accumulation in non-target tissues and minimizes the side effects of the drug. This review aims to provide an introduction to nanogels, their preparation methods, and to discuss the design of various stimulus-responsive nanogels that are able to provide controlled drug release in response to particular stimuli

Grafting of Polystyrene Chains at the Edge of Graphene Nanolayers by "Grafting Through" Approach Using Reversible Addition-Fragmentation Chain Transfer Polymerization

Edge-functionalized graphene nanolayers with polystyrene chains were prepared by a “grafting through” reversible addition-fragmentation chain transfer (RAFT) polymerization. For this purpose, double-bond containing modifier (MD) was prepared. After edge-functionalization of graphene oxide (GO) by two different amounts of MD and preparation of modified graphenes (LFG and HFG), RAFT polymerization of styrene was applied for preparation of functionalized GO with different densities of polystyrene chains. Fourier transform infrared spectroscopy showed that MD and polystyrene chains were grafted at the edge of GO. Gas chromatography showed that conversion decreased by the addition of modified GO content and also grafting density of MD. Number-average molecular weight and polydispersity index of polystyrene chains were derived from gel permeation chromatography. Increase of modified graphene content results in a decrease in molecular weight of attached polystyrene chains and also an increase in their PDI value. Increase of grafting density of MD results in decrease of molecular weight of polystyrene chains with no considerable variation in PDI value. Thermogravimetric analysis results showed that char residue is about 45.1 and 46.8% for LFG and HFG, respectively. The content of degradation ascribed to polystyrene increased with increase of grafting density of MD and decreased with increase of modified graphene content. X-ray diffraction results were used for evaluation of interlayer spacing of graphene layers after functionalization process and also study of nanocomposites structure. The results of scanning electron microscopy and transmission electron microscopy show that graphene layers with high clarity turned to opaque layers with lots of creases by oxidation and attachment of polystyrene chains

Synthesis of Polystyrene/MCM–41 Nanocomposites through AGET ATRP and ARGET ATRP

Polystyrene/MCM–41 nanocomposites were synthesized by atom transfer radical polymerization (ATRP) at 110°C. Activators generated by electron transfer (AGET) and activators regenerated by electron transfer (ARGET), as two novel initiation techniques, for ATRP were used. Specific structure, surface area, particles size and their distribution and spongy and porous structure of the synthesized MCM–41 nanoparticles were evaluated using X–ray diffraction, nitrogen adsorption/desorption isotherm analysis, scanning and transmission electron microscopy images, respectively. The final monomer conversion was determined using gas chromatography. Number and weight average molecular weights (Mn and Mw) and polydispersity index (PDI) were also evaluated by gel permeationchromatography. According to the results, addition of 3 wt% MCM–41 nanoparticles into the polymerization media resulted in lowering conversion from 81 to 58% in the AGET ATRP system. Moreover, a reduction in the molecular weight of the products from 17116 to 12798 g/mol was also occurred, although, the polydispersity index increased from 1.24 to 1.58. The similar results were also obtained by ARGET ATRP system; lowering conversion from 69 to 43% and molecular weight from 14892 to 9297 g/mol, and an increase of PDI from 1.14 to 1.41. The improvement in thermal stability of the nanocomposites, as a result of higher MCM–41 nanoparticles loading, was confirmed by thermogravimetric analysis. In addition, according to the analytical results of differential scanning calorimetry, a decrease in glass transition temperature, due to the addition of 3 wt% of MCM–41 nanoparticles (from 100.1 to 91.5°C in AGET ATRP system and from 100.3 to 85.8°C in ARGET ATRP), was achieved

Atom Transfer Radical Polymerization of Styrene in Presence of Mesoporous Silica Nanoparticles: Application of Reverse, Simultaneous Reverse and Normal Initiation Techniques

Atom transfer radical polymerization (ATRP) of styrene in presence of mesoporous silica nanoparticles was carried out at 110 °C. Reverse atom transfer radical polymerization (RATRP) and simultaneous reverse and normal initiation for atom transfer radical polymerization (SR&NI ATRP) techniques were used as two appropriate introduced techniques for circumventing oxidation problems. Usage of metal catalyst in its higher oxidation state was the main feature of these initiation techniques in which deficiencies of normal ATRP were circumvented. Structure, surface area and pore diameter of synthesized mesoporous silica nanoparticles were evaluated using X–ray diffraction and nitrogen adsorption/desorption isotherm analysis. Average particle size was estimated around 600 nm by electron microscopy images. In addition, according to these images, nanoparticles revealed an appropriate size distribution. Particles size and their distribution were examined using scanning. Final monomer conversion was determined by using gas chromatography. The number and weight average molecular weights (Mn and Mw) and polydispersity indexes (PDI) were also evaluated by gel permeation chromatography. According to the results obtained, addition of mesoporous silica nanoparticles in both RATRP and SR&NI ATRP systems revealed similar effects: decrement of conversion and Mn and also increment of PDI values observed by increasing of mesoporous silica nanoparticles content. Improvement in thermal stability of the nanocomposites in comparison with neat polystyrene was demonstrated by thermogravimetric analysis (TGA). Moreover, in case of nanocomposites, thermal stability was obtained by higher loading of nanoparticles. A decrease in glass transition temperature by higher content of mesoporous silica nanoparticles has been demonstrated by differential scanning calorimetry analysis

Stimuli-responsive covalent adaptable hydrogels based on homolytic bond dissociation and chain transfer reactions

Covalent adaptable hydrogels containing dynamic covalent bonds are gaining significant interest based on their ability of stimulicontrolled reversible bond dissociation, structural reorganization, color change, and self-healing through covalent bond exchange or dissociation. Potential applications of such hydrogels have been explored in coatings, sealants, tissue adhesives, soft robotics, tissue engineering, and stimuliresponsive lithography. Stimuli-induced homolytic bond cleavage leads to the formation of radicals with the ability of recombination or transfer to induce bond exchange and color variation. The incorporation of such stimuliresponsive homolytically cleavable bonds in hydrogels can lead to stimulimechanochromism, and mechanoluminescence. The resultant smart materials are interesting for different applications, ranging from patterning and shapeshifting, cell encapsulation and culturing, and protein binding to strain sensing and damage reporting. The recent progress in the preparation of covalently adaptable hydrogels based on stimuli-responsive homolytical bond dissociation and recombination or chain transfer will be discussed in this review. More specifically, the different types of chemistry that can be used for development of covalent adaptable hydrogels based on light-induced, temperature-induced, and mechanically induced homolytic bond dissociation will be discussed. Moreover, the applications of the resultant covalent adaptable hydrogels will be highlighted, focusing on stress sensing and damage reporting, tissue engineering, and self-healable hydrogels, as well as stimuli-controlled patterning and shapeshifting

Synthesis of Poly(styrene-co-butyl acrylate)/Clay Nanocomposite via In Situ AGET ATRP

Poly(styrene-co-butyl acrylate)/clay nanocomposites were synthesized via in situ atom transfer radical polymerization using activators generated by electron transfer in the presence of a montmorillonite ion-exchanged with mixed surfactants of dodecyl trimethyl ammonium bromide and vinyl trimethyl ammonium chloride. The living nature of polymerization is confrmed by occurrence of narrow molecular weight distribution of the nanocomposites in which copolymers with polydispersity index of about 1.13-1.15 were obtained. Partial exfoliation of clay layers in the copolymer matrix was demonstrated by XRD patterns and further studies of TEM images. Thermogravimetric analysis (TGA) results demonstrated a slight increase in the thermal stability of nanocomposites in comparison with the neat copolymer. DSC results indicated a decrease in the glass transition temperature (Tg) of nanocomposites by the addition of clay content which are attributed to low molecular weights of the copolymers and weaker interactions between polymer chains. The chemical structure and composition of copolymers was identifed by 1H NMR analysis

Effect of molecular weight and polymer concentration on the triple temperature/pH/ionic strength-sensitive behavior of poly(2-(dimethylamino)ethyl methacrylate)

<p>Poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) samples were synthesized <i>via</i> aqueous atom transfer radical polymerization with DP_<i>n</i> of 35, 151, 390, and 546 and dispersity of 1.13, 1.17, 1.20, and 1.18, respectively. All samples were exposed to temperature and pH variations at different concentration of polymer and salt (NaCl). Results indicated that cloud point (<i>T</i>_cl) can be controlled by changing DP_<i>n</i>, polymer concentration, and ionic strength of solution. According to results, <i>T</i>_cl of the PDMAEMA chains shifted to lower temperatures with increasing solution pH at all molecular weight ranges due to deprotonation of tertiary amine groups in polymer structure. However, higher molecular weight polymers were more sensitive to pH variation especially in alkaline media. Also, higher polymer concentration acted as driving force to decrease cloud point of samples and formation of aggregates that was more predominant for higher molecular weights at alkaline media. <i>T</i>_cl of PDMAEMA chains decreased with increasing ionic strength even at low pH values for low molecular weight polymers.</p