Synthesis of PAA-g-PNIPAM well-defined graft polymer by sequential RAFT and SET-LRP and its application in preparing size-controlled super-paramagnetic Fe3O4 nanoparticles as a stabilizer

A series of well-defined double hydrophilic graft copolymers, poly(acrylic acid)-g-poly(N-isopropylacrylamide) (PAA-g-PNIPAM), was employed as a novel water-soluble coating for constructing superparamagnetic iron oxide nanoparticles. The copolymer was synthesized via a three-step procedure: firstly, a well-defined hydrophobic PtBA-based backbone, poly(tert-butyl 2-((2-chloropropanoyloxy)-methyl)acrylate)-co-poly(tert-butyl acrylate), (PtBCPMA₁₉-co-PtBA₁₈), was prepared through RAFT copolymerization of a new trifunctional acrylic monomer, tert-butyl 2-((2-chloropropanoyloxy)methyl)acrylate and tert-butyl acrylate; secondly, taking this backbone as a macroinitiator to initiate SET-LRP of N-isopropylacrylamide resulted in well-defined (poly(tert-butyl 2-((2-chloropropanoyloxy)methyl)-acrylate)-co-poly(tert-butyl acrylate))-g-poly(N-isopropylacrylamide) ((PtBCPMA-co-PtBA)-g-PNIPAM) amphiphilic graft copolymers with relatively narrow polydispersities (M_w/M_n ≤ 1.31); thirdly, handling (PtBCPMA-co-PtBA)-g-PNIPAM in acidic conditions afforded PAA-g-PNIPAM graft copolymers. The resulting PAA-g-PNIPAM copolymers were directly utilized as a polymeric stabilizer in the preparation of superparamagnetic Fe₃O₄ nanoparticles. The particle size can be readily tuned in the range of 12.1–23.2 nm by varying the amount of PAA-g-PNIPAM copolymer or the length of PNIPAM side chain. Besides, the structure and properties of prepared Fe₃O₄/polymer nanocomposites were characterized by XRD, FT-IR, TGA, TEM, and magnetic measurement in detail.     

Grafting of end-functionalized poly(tert-butyl acrylate) to poly(ethylene-co-acrylic acid) film

Poly(tert-butyl acrylate) (PtBA) was grafted to the surface of poly(ethylene-co-acrylic acid) (EAA) film and the pendant groups of the tethered PtBA were modified to create chemically tailored surface modifying layers. The carboxylic acid groups in the copolymer film served as the grafting sites for the covalent tethering of end-functionalized PtBA. The progression of these reactions was monitored using attenuated total reflectance (ATR)-FTIR and X-ray photoelectron (XPS) spectroscopies along with static contact angle measurements. By controlling the reaction conditions, the chemical functionality of the grafted layer ranged from tert-butyl ester (EAA-g-PtBA) to carboxylic acid (EAA-g-PAA) and was demonstrated by corresponding changes in wettability. The choice of PtBA as the tethered polymer allows for the subsequent substitution of the tert-butyl ester groups. To demonstrate, a novel procedure was used to replace the tert-butyl ester with N,N-dimethylethylenediamine (DMEDA) to form EAA-g-PDMEDA. These reaction schemes can be used to create tunable surface-grafted layers with various pendant group chemistries.     

Janus gold nanoparticle with bicompartment polymer brushes templated by polymer single crystals

Janus particles have attracted increasing attention from the communities of materials science, chemistry, physics and biology. While large size Janus particles are readily achieved, synthesizing Janus nanoparticles (JNP) with diameters smaller than ∼20 nm remains a challenging task. In this article, we report a systematic study on growing polymer brushes on polymer-single-crystal-immobilized 6 and 15 nm diameter gold nanoparticles (AuNPs) using atom transfer radical polymerization. JNPs with bicompartment polymer brushes, such as poly(ethylene oxide) (PEO)/poly(methyl methacrylate), PEO/poly(tert-butyl acrylate), and PEO/poly(acrylic acid), were synthesized. The grafting densities can be carefully controlled. The Janus feature of these particles was confirmed using both platinum nanoparticle decoration and UV/Vis spectroscopy analysis. The surface plasmon resonance absorbance of Janus particles exhibited a blue shift compared with that of symmetric AuNPs with either homopolymer or mixed polymer brushes. This work demonstrated that using polymer single crystal as the templates, small size (<20 nm diameter) JNPs having bicompartment polymer brushes can be readily obtained. The ability to tune grafting density and molecular weight of polymer brushes can lead to controlled particle amphiphilicity.     

Block, blocky gradient and random copolymers of 2-ethylhexyl acrylate and acrylic acid by atom transfer radical polymerization

The controlled synthesis of low-T_g poly(2-ethylhexyl acrylate) (P2EHA) and derived random, block and blocky gradient copolymers via atom transfer radical polymerization (ATRP) is described. After optimizing the reaction conditions for the homopolymerization of 2EHA via ATRP, the synthesis of a variety of copolymers with poly(t-butyl acrylate) (PtBuA) was investigated. First, AB-block copolymers were targeted, starting from P2EHA and PtBuA as macroinitiators. Second, random copolymers of tBuA and 2EHA with different monomer ratios were synthesized. Finally, the synthesis of “blocky” gradient copolymers via a one-pot procedure was investigated, starting with the homopolymerization of tBuA, followed by the addition of 2EHA. The hydrolysis of the PtBuA-segments to poly(acrylic acid) (PAA), which was carried out with methanesulfonic acid, resulted in block, blocky gradient and random copolymers consisting of PAA and P2EHA. Solubility testing of the copolymers in slightly basic water (pH ∼ 9) demonstrated that the gradient structure significantly enhances solubility compared to the block copolymer structures with equal composition. The polymers have been characterized by MALDI-TOF MS, GPC and ¹H NMR.     

Synthesis of amphiphilic copolymers by ATRP initiated with a bifunctional initiator containing trichloromethyl groups

Bifunctional polystyrene macroinitiators, having various molecular weights, were prepared by atom transfer radical polymerization (ATRP), initiated with bifunctional initiator 1,3-bis{1-methyl-1[(2,2,2-trichloroethoxy) carbonylamino]ethyl}benzene in conjunction with CuCl catalyst and polyamine ligands. These macroinitiators were subsequently used for ATRP of tert-butyl acrylate (t-BuA), giving BAB triblocks poly[(t-BuA)-b-(Sty)-b-(t-BuA)] as precursors of amphiphilic copolymers. Both the polymerization steps proceeded as controlled processes with linear semi-logarithmic conversion plots and lengths of the blocks following theoretical predictions. Hydrolysis of outer poly(t-BuA) blocks led to triblock copolymers with the central polystyrene block and outer blocks of poly(acrylic acid), the molecular weights of which ranged from ca. 5 × 10³ to almost 1 × 10⁵ Da.     

Rheological properties of ABA telechelic polyelectrolyte and ABA polyampholyte reversible hydrogels: A comparative study

Two types of reversible hydrogels formed by poly(t-butyl acrylate)-poly(2-vinyl pyridine)-poly(t-butyl acrylate) (PtBA-P2VP-PtBA) and poly(acrylic acid)-poly(2-vinyl pyridine)-poly(acrylic acid) (PAA-P2VP-PAA), named telechelic polyelectrolyte and block polyampholyte, respectively, of the same degree of polymerization were studied in aqueous solutions at pH 3.7 in terms of their rheological properties. The different structural characteristics of the formed 3D networks that arise from hydrophobic interactions of the telechelic polyelectrolyte and electrostatic interactions of the polyampholyte, lead to significant different rheological properties. The results tend to show that a thermo-sensitive weak hydrogel is formed by the polyampholyte while a stiff, but fragile, hydrogel is formed by the telechelic polyelectrolyte.     

Synthesis of thermoresponsive poly(N-isopropylmethacrylamide) and poly(acrylic acid) block copolymers via post-functionalization of poly(N-methacryloxysuccinimide)

Polymers exhibiting a thermoresponsive, lower critical solution temperature (LCST) phase transition have proven to be useful for many applications as “smart” or “intelligent” materials. A series of poly(N-isopropylmethacrylamide) (PNIPMAM) polymer, poly(N-isopropylmethacrylamide)-b-poly(acrylic acid) (PNIPMAM-b-PAA) diblock, and poly(acrylic acid)-b-poly(N-isopropylmethacrylamide)-b-poly(acrylic acid) (PAA-b-PNIPMAM-b-AA) triblock copolymer samples were synthesized via ATRP. A facile post-functionalization route was developed that uses an activated ester functionality to convert poly(N-methacryloxysuccinimide) (PMASI) blocks to LCST capable polyacrylamide, while poly(t-butyl acrylate) (PtBA) blocks were converted to water-soluble poly(acrylic acid) (PAA). The post-functionalization was monitored via ¹H NMR and ATR-FTIR. The aqueous solution properties were explored and the PNIPMAM polymers were shown to have a LCST phase transition varying from 35 to 60 °C. The ability to synthesize block copolymers that are thermoresponsive and water-soluble will be of great benefit for broader applications in drug delivery, bioengineering, and nanotechnology.     

Molecular dynamics of poly(butylene tert-butyl isophthalate) and its copolymers with poly(butylene terephthalate) as revealed by broadband dielectric spectroscopy

The dynamic behaviour of a set of copolyesters made of 1,4-butanediol, terephthalic acid and 5-tert-butyl isophthalic acid, as well as the two parent homopolymers was studied. Although the insertion of the tert-butyl isophthalate units in the main chain of poly(butylene terephthalate) modified the glass transition, the β relaxation did not change dramatically. By using broadband dielectric spectroscopy we have obtained a detailed picture of the dynamic behaviour of these systems. In all cases, a strong correlation between the α relaxation and the β relaxation above T_g was observed in accordance with recent proposals based on the coupling model.     

Effect of carboxylic acid monomer and butadiene on particle growth in the emulsifier-free emulsion copolymerization of styrene-butadiene-carboxylic acid monomer

Carboxylated styrene-butadiene rubber (XSBR) latexes were prepared by emulsifier-free batch emulsion copolymerization of styrene and butadiene with different types of carboxylic acid monomers (AA, MAA, IA). It was found that the particle growth is dependent on the hydrophilic nature of carboxylic acid monomers. SEM studies showed that N_p is almost constant in the particle growth stage (conversion above 10%). Through some calculations by data obtained from DLS technique, average diameter of monomer swollen polymer particles of all the XSBR latexes at the same conversion of 0.4 was obtained to be 368.91, 174.17 and 437.15 nm for AA, MAA and IA, respectively. Several kinetic parameters related to the particle growth stage such as the average number of growing chain per particle were calculated to be 0.474, 0.370 and 1.685 for AA, MAA and IA, respectively. It was observed that these kinetic parameters increase with increasing average diameter of monomer swollen polymer particles, which is consistent with the emulsion polymerization kinetics. Moreover, results indicated that the polymerization rate per particle or equivalently the average number of the growing chain per particle (particle growth stage) decreases by replacing a part of styrene with butadiene in the emulsion copolymerization recipe of styrene-carboxylic acid monomer.     

Poly(ethylene isophthalate)s: effect of the tert-butyl substituent on structure and properties

A comparative study of the two isophthalic acid deriving homopolyesters poly(ethylene isophthalate) (PEI) and poly(ethylene 5-tert-butyl isophthalate) (PE^tBI), including synthesis, crystal structure, and thermal and permeability properties, was carried out. The two polyesters were prepared by condensation polymerization in the melt. In both cases, minor amounts of cyclic dimers were observed to form, which were characterized by nuclear magnetic resonance and mass spectroscopy. PEI and PE^tBI were thermally stable up to 400 °C and they appeared to be semicrystalline polyesters, having their melting temperatures between 130 and 135 °C. Their glass-transition temperatures were 62 and 94 °C, respectively. The crystal structure adopted by the two polyesters seemed to consist of a regularly folded conformation, clearly different from the almost extended conformation characteristic of poly(ethylene terephthalate). Gas permeability measurements for N₂, O₂, and CO₂ revealed that PE^tBI is more permeable to these gases than PEI, in spite of having a higher T_g. Furthermore, water vapor diffusion was found to be increased by the insertion of the tert-butyl group, whereas water absorption diminished. The differences in gas and water vapor transport properties observed for these two polyesters were discussed on the basis of their respective molecular structures.     

Chemical and Colloidal Stability of Carboxylated Core-Shell Magnetite Nanoparticles Designed for Biomedical Applications

Despite the large efforts to prepare super paramagnetic iron oxide nanoparticles (MNPs) for biomedical applications, the number of FDA or EMA approved formulations is few. It is not known commonly that the approved formulations in many instances have already been withdrawn or discontinued by the producers; at present, hardly any approved formulations are produced and marketed. Literature survey reveals that there is a lack for a commonly accepted physicochemical practice in designing and qualifying formulations before they enter in vitro and in vivo biological testing. Such a standard procedure would exclude inadequate formulations from clinical trials thus improving their outcome. Here we present a straightforward route to assess eligibility of carboxylated MNPs for biomedical tests applied for a series of our core-shell products, i.e., citric acid, gallic acid, poly(acrylic acid) and poly(acrylic acid-co-maleic acid) coated MNPs. The discussion is based on physicochemical studies (carboxylate adsorption/desorption, FTIR-ATR, iron dissolution, zeta potential, particle size, coagulation kinetics and magnetization measurements) and involves in vitro and in vivo tests. Our procedure can serve as an example to construct adequate physico-chemical selection strategies for preparation of other types of core-shell nanoparticles as well.     

Growth of silver nanoparticles on poly(acrylic acid) brushes and their properties

A simple procedure is employed for the growth of silver nanoparticles (Ag NPs) onto the silicon substrate modified by poly(acrylic acid) (PAA) brushes, via: (1) surface-initiated ATRP of tert-butyl acrylate on Si surface to the preparation of poly(tert-butyl acrylate) brushes, (2) acid hydrolysis of PBA to the formation of PAA, and (3) in situ synthesis of Ag NPs via chemical reduction of AgNO₃ in the presence of PAA brushes. The polymer brushes are thoroughly characterized. Moreover, Ag nanoparticles are homogeneously immobilized into the brush layer and have been used to fabricate a sensor platform of surface-enhance Raman scattering for the detection of organic molecules and effectively catalyze the reduction of methylene blue by NaBH₄.     

Highly isotactic poly(vinyl alcohol). III: Heterogeneous cationic polymerization of tert-butyl vinyl ether

We studied the polymerization of tert-butyl vinyl ether (tBVE) and benzyl vinyl ether with heterogeneous catalysts, that is, modified Ziegler type (Vandenberg type) catalysts and metal sulfate-sulfuric acid complexes.Vandenberg type catalysts gave high molecular weight and highly isotactic poly(tBVE)s with relatively narrow molecular weight distribution at high temperature, and then the resultant poly(tBVE)s were converted into the stereoregular poly(vinyl alcohol)s (PVAs). With titanium based Vandenberg type catalyst, a relative high isotactic PVA, which has 52% triad isotacticity, was obtained from the poly(tBVE) polymerized at 30 °C. It was found from NMR study that the content of the triad tacticity of PVAs derived from poly(tBVE) catalyzed by titanium based catalysts agreed with the value calculated from the chain-end control model (Bovey's model). This fact suggests that the steric structure of the adding monomer in this system is determined by same mechanism to homogeneous BF₃ complexes catalysts system. On contrast to that, the metal sulfate-sulfuric acid complexes show significantly low activity to tBVE polymerization.     

Nitroxide-mediated polymerization to form symmetrical ABA triblock copolymers from a bidirectional alkoxyamine initiator

Bidirectional alkoxyamine 2 was synthesized and used as the initiator in the polymerization of styrene (S), n-butyl acrylate (nBA), t-butyl acrylate (tBA), isoprene (I), and dimethylacrylamide (DMA). A variety of symmetrical ABA triblock copolymers were prepared, ranging in size from 5 to 59 kDa. Kinetics studies and gel permeation chromatography (GPC) confirmed the “living” nature of these polymerizations. Trifluoroacetic acid was used to convert the PtBA blocks of these polymers into poly(acrylic acid) (PAA) blocks, forming ABA amphiphilic triblock copolymers. AFM images of PAA-b-PnBA-b-PAA and PAA-b-PS-b-PAA triblock copolymers ionized by the addition of 2,2′-(ethylenedioxy)bis(ethylamine) show evidence of self-assembly.     

Synthesis of disubstituted amine-functionalized diene-based polymers

Diene-based polymers with two amine groups within each repeat unit were successfully synthesized by bulk and solution free radical polymerization techniques. All polymers have exclusive 1,4-microstructure. The number average molecular weights of the materials obtained were in the range of 30–52 × 10³ g/mol using 2,2′-azobisisobutyronitrile (AIBN), t-butyl peracetate (t-BPA), or t-butyl hydroperoxide (t-BHP) as the initiators. The highest molecular weight achieved was 72 × 10³ g/mol when t-butyl peroxide (t-BPO) was used as the initiator. Quantitative quaternization was achieved yielding hydrophilic water soluble polymers. Prior to quaternization, the polymers are hydrophobic and dissolve in most organic solvents.     

Synthesis of pH-sensitive micelles from linseed oil using atom transfer radical polymerisation (ATRP)

This paper reports the synthesis of an amphiphilic copolymer from linseed oils and its successive auto-association in water into pH-sensitive micelles. An original ATRP lipoinitiator is first designed from linseed oil in two steps. tert-butyl acrylate (tBA) polymerization is consequently initiated from this original initiator and amphiphilic copolymers are obtained after subsequent acidolysis of the PtBA block into poly(acrylic acid) (PAA). The ability of a lipid-b-PAA copolymer to auto-associate in water is finally investigated through different techniques (Fluorescence, Surface Tension, QELS). This copolymer forms well-defined micelles in acidic media with a low critical micellar concentration (cmc) of 7.6 mg L^?1 and dissociates when the pH is raised above 7.     

Polypyrrole nanocoatings of poly(styrene-co-methacrylic acid) particles

In this study the properties of polypyrrole (PPy) nanocoating over poly(styrene-co-methacrylic acid) (PS-MAA) particles were investigated. Monodisperse PS-MAA templates were obtained by free surfactant emulsion polymerization. The addition of methacrylic acid into the monomer feed mixture reduced particle size, ionic charge and hydrophobicity of the template surface. Correlations between template sizes and compositions, PPy confinement (granularity, shell size, etc.) and electrical conductivity, σ, are discussed. After dissolving the PPy/PS-MAA composites in tetrahydrofurane, PPy void spheres are obtained proving the core-shell nature of the coated particles. Bare styrene templates show densely packed PPy coatings and electrical conductivities near 7 S cm⁻¹ at high PPy loadings; on the contrary, at the same PPy mass percentage, the richer methacrylic acid particles show low packed PPy grains and conductivities that fall to 0.8 S cm⁻¹. In core-shell particles the PPy mass per unit area, Γ, is the key parameter which determines the insulator-conductor transition for any particle size. The conductivity values of PPy/PS-MAA composites follow a percolation law: σ∝t(Γ−Γc), with a critical threshold Γ_c=(0.262 ± 0.002)×10⁻⁶ g cm⁻². The critical exponent obtained t = 1.98 ± 0.07 agrees with the predicted value t = 2.0 for three-dimensional lattices of random resistors.     

Synthesis and gas permeability of nitrated and aminated Poly(diphenylacetylene)s

Poly(diphenylacetylene) [PDPA] derivatives were nitrated using a mixture of nitric acid and sulfuric acid. Nitration of poly[1-(p-trimethylsilyl)phenyl-2-phenylacetylene] (1a) proceeded, and the nitro groups were substituted to trimethylsilyl groups on benzene rings. The degree of nitration (DN) increased with reaction time, and it reached 0.41 for 3.0 h. Fluorine-containing PDPA (1b and 1c) showed less reactivity for nitration, and the DNs were only 0.18 and 0.06, respectively, under the same conditions. Nitration of PDPAs having n-butyl and t-butyl groups (1d and 1e) afforded the nitrated PDPAs, whose DN were 0.20 and 0.14, respectively. Reduction of nitro groups on PDPAs proceeded quantitatively to produce the corresponding PDPAs possessing amino groups (3a-e). Aminated PDPAs exhibited high CO₂ permselectivity compared to the corresponding non-aminated PDPAs. The CO₂ permeability coefficients of aminated PDPAs (3a-d) were lower than those of the corresponding polymers (1a-d) because the polymer chain packing was strengthened by the intermolecular interaction between polar amino groups, but t-butyl group-containing aminated PDPA (3e) showed high CO₂ permeability than 1e because the chain packing was weakened by the steric repulsion of bulky t-butyl groups.     

Core-shell polyacrylate and polystyrene-block-polyacrylate stars

The polymerization of p-(iodomethyl)styrene (PIMS) yields well-defined branched polymers with reactive iodomethyl groups. The branched poly[p-(iodomethyl)styrene] was used as the transfer agent in the iodine mediated radical polymerization of vinyl monomers. The polymerization proceeds in a controlled way and yields polystyrene and poly(t-butyl acrylate) star polymers with reactive groups at the end of their arms. Polymers so obtained were also used to prepare stars with block copolymer arms: polystyrene-block-poly(t-butyl acrylate). The characterization of star structures was performed by NMR and gel permeation chromatography with absolute molar mass detection (MALLS). Preliminary characterization of the thermal properties of these novel materials is reported.     

Double-responsive core-shell-corona micelles from self-assembly of diblock copolymer of poly(t-butyl acrylate-co-acrylic acid)-b-poly(N-isopropylacrylamide)

Self-assembly of poly(t-butyl acrylate-co-acrylic acid)-b-poly(N-isopropylacrylamide) [P(tBA-co-AA)-b-PNIPAM], which was obtained from part hydrolysis of PtBA-b-PNIPAM synthesized by sequential atom transfer radical polymerization (ATRP) was studied. Thermo- and pH-responsive core-shell-corona (CSC) micelles with different structures were formed from (PtBA-co-PAA)-b-PNIPAM in aqueous solution. At pH 5.8 and 25 °C, the block copolymer self-assembled into spherical core-shell micelles with hydrophobic PtBA segments as the core, hydrophilic PAA/PNIPAM segments as the mixed shell. Increasing temperatures, core-shell micelles converted into CSC micelles with PtBA as the core, collapsed PNIPAM as the shell and soluble PAA as the corona. Moreover, decreasing pH at 25 °C, PAA chains collapsed onto the core resulting in CSC micelles with PtBA as the core, PAA as the shell and PNIPAM as the corona.