Synthesis of water-soluble homo- and block-copolymers by RAFT polymerization under γ-irradiation in aqueous media

The ambient temperature (20 °C) reversible addition fragmentation chain transfer (RAFT) polymerization of several water-soluble monomers conducted directly in aqueous media under γ-initiation (at dose rates of 30 Gy h⁻¹) proceeds in a controlled fashion. Using functional trithiocarbonates, i.e., S,S-bis(α,α′-dimethyl-α″-acetic acid) trithiocarbonate (TRITT), 3-benzylsulfanyl thiocarbonylsulfanyl propionic acid (BPATT), and dithioester, i.e., 4-cyanopentanoic acid dithiobenzoate (CPADB), as chain transfer agents, fully water-soluble polymers of monomers such as N,N-dimethylacrylamide, 2-hydroxyethyl acrylate, acrylamide or oligo(ethylene glycol) methacrylate and stimuli-responsive polymers of monomers such as acrylic acid, N-isopropylacrylamide, 2-(dimethylamino)ethyl methacrylate or 2-acrylamido-2-methylpropane sulfonic acid can be obtained over a wide range of degrees of polymerization up to 10,000 with low polydispersity (typically ) to near quantitative conversions. Well-defined block copolymers between these monomers, based on several asymmetric macro-RAFT agents, can be obtained, suggesting that the RAFT agents are stable throughout the polymerization process so that complex and well-defined architectures can be obtained.     

Recent advances in polymer shape memory

Traditional shape memory polymers (SMPs) are those capable of memorizing a temporary shape and recovering to the permanent shape upon heating. Although such a basic concept has been known for half a century, recent progresses have challenged the conventional understanding of the polymer shape memory effect and significantly expanded the practical potential of SMPs. In this article, notable recent advances in the field of SMPs are highlighted. Particular emphasis is placed on how the new developments have changed the conventional view of SMPs, what they mean for practical applications, and where the future opportunities are.     

NMR studies of thermo-responsive behavior of an amphiphilic poly(asparagine) derivative in water

The thermo-responsive behavior of a unique biocompatible polymer, poly(N-substituted α/β-asparagine) derivative (PAD), has been studied with several NMR methods. The ¹H and ¹³C solution NMR measurements of the PAD in DMSO-d₆ were used to investigate the isolated polymer and perform spectral assignments. By systematic addition of D₂O we have tracked structural changes due to aggregation and observed contraction of hydrophilic side chains. Solution and cross polarization/magic angle spinning (CP/MAS) ¹³C NMR approaches were implemented to investigate the aggregates of the PAD aqueous solution during the liquid to gel transition as the temperature was increased. At temperatures near 20 °C, all of the peaks from the PAD were observed in the ¹³C CP/MAS and ¹³C solution NMR spectra, indicating the presence of polymer chain nodes. Increasing the temperature to 40 °C resulted in a partial disentanglement of the nodes due to thermal agitation and further heating resulted in little to no additional structural changes. Deuterium T₁–T₂ and T₂–T₂ two-dimensional relaxation spectroscopies using an inverse Laplace transform, were also implemented to monitor the water–PAD interaction during the phase transition. At temperatures near 20 °C the dynamical characteristics of water were manifested into one peak in the deuterium T₁–T₂ map. Increasing the temperature to 40 °C resulted in several distinguishable reservoirs of water with different dynamical characteristics. The observation of several reservoirs of water at the temperature of gel formation at 40 °C is consistent with a physical picture of a gel involving a network of interconnected polymer chains trapping a fluid. Further increase in temperature to 70 °C resulted in two non-exchanging water reservoirs probed by deuterium T₂–T₂ measurements.     

pH- and temperature-responsive amphiphilic diblock copolymers of 4-vinylpyridine and oligoethyleneglycol methacrylate synthesized by RAFT polymerization

Diblock copolymers of 4-vinylpyridine (4VP) and oligoethyleneglycol methyl ether methacrylate (OEGMA) were synthesized for the first time using RAFT polymerization technique as potential drug delivery systems. Effects of the number of ethylene glycol units in OEGMA, chain length of hydrophobic P4VP block, pH, concentration and temperature on the solution behavior of the copolymers were investigated comprehensively. Copolymer chains formed micelles at pH values higher than 5 whereas unimeric polymers were observed to exist below pH 5, owing to the repulsion between positively charged P4VP blocks. The size of the micelles was dependent on the relative length of blocks, P4VP and POEGMA. Thermo-responsive properties of copolymers were investigated depending on the pH and length of P4VP block. The increase in the length of P4VP block decreased the LCST substantially at pH 7. At pH 3, LCST of copolymers shifted to higher temperatures due to the increased interaction of copolymers with water through positively charged P4VP block.     

New water soluble azobenzene-containing diblock copolymers: synthesis and aggregation behavior

Homopolymers of azobenzene (azo) methacrylates with different substituents and their diblock copolymers with poly(2-(dimethylamino)ethyl methacrylate p(DMAEMA) were synthesized via atom transfer radical polymerization (ATRP). Controlled/‘living’ ATRP of azo methacrylates were achieved up to ∼50% conversion, after which deviation occurred. It was found that the copolymerization rate of 6-[4-phenylazo]phenoxy]hexylmethacrylate (PPHM) from p(DMAEMA) macroinitiator was almost identical to that for the homopolymerization of PPHM monomer, with k_app∼0.0078 min⁻¹. For the copolymerizations, almost complete incorporation of the azo methacrylate monomers could be obtained with low molecular weight macroinitiator (PDMAEMA)-Cl, whereas macroinitiators of long chain length did not give full conversion, most likely due to chain floding and steric hindrance caused by the bulk azo monomers. Because azo monomers are highly hydrophobic, only the diblock copolymers with short azo segment were soluble in water which self-assembled into micellar particles. The effect of photo-induced trans-cis isomerization on lower critical solution temperature (LCST) and surface tension were studied. The LCST of the diblock copolymers increased upon irradiation by UV light due to the cis conformers being more hydrophilic. However, the trans-cis isomerization had only small effect on the critical micelle concentration (cmc) and γ_cmc of azo methacrylate block copolymers, due to the formation of compact core of the micelles. The formations of core-shell micelles were established from LLS and TEM studies. All the three azo methacrylate amphiphilic block copolymers formed hard core-shell micelles with relatively small R_h values of 31 nm for p(DMAEMA₁₇₂-b-BPHM₇), 26 nm for p(DMAEMA₁₇₂-b-CPHM₇) and 32 nm for p(DMAEMA₁₇₂-b-PPHM₉). Whereas for the azo acrylate copolymer, p(DMAEMA₁₇₂-b-BPHA₆), large micelles with R_h∼78 nm with loose core was formed.     

Surface characterization of nanoparticles carrying pH-responsive polymer hair

The surface of near-monodisperse colloidal polystyrene (PS) latex particles carrying pH-responsive poly[2-(diethylamino)ethyl methacrylate] (PDEA) polymer chains as a steric stabilizer has been extensively characterized in dry and wet states. X-ray photoelectron spectroscopy and contact angle measurement studies confirm that PDEA chains are located on the PS latex particle surface. Transmission electron microscopy (TEM) studies reveal that the sterically stabilized PS particles have a core-shell morphology and wet atomic force microscopy data confirm that the pH-responsive PDEA component covers the PS latex core and the thickness of the protonated steric polymer is estimated to be approximately 15 nm. The PS latex particles can be easily dispersed in acidic aqueous media (pH 3.0) whilst they flocculate in basic aqueous media (pH 10.0) due to the pH-responsive PDEA hair. This dispersion-flocculation cycle of the PS latex is fully reversible.     

Synthesis and characterization of poly(styrene-co-fluorescein O-methacrylate)/poly(N-isopropylacrylamide)-Fe3O4 core/shell composite particles

We demonstrate the synthesis and characteristics of multifunctional poly(styrene-co-fluorescein O-methacrylate)/poly(N-isopropylacrylamide)-Fe₃O₄ [P(St/FMA)/PNIPAAm-Fe₃O₄] core/shell composite particles, in which the core consists of fluorescent materials and the shell consists of magnetic and thermo-responsive components. First, core/shell particles consisting of a fluorescent P(St/FMA) core and thermo-responsive PNIPAAm-rich shell were prepared by two-stage shot-growth emulsion polymerization. Next, Fe₃O₄ nanoparticles were immobilized via electrostatic interactions and then covalently linked to the shell via surface coordinated Aphen by a coupling reaction in order to obtain magnetic properties. The morphology of P(St/FMA)/PNIPAAm-Fe₃O₄ composite particles, confirmed by transmission electron microscopy (TEM), reveals that Fe₃O₄ nanoparticles are located in the PNIPAAm shell. The thermo-sensitivity of composite particles to hydrodynamic diameter was confirmed by using dynamic light scattering (DLS). Photoluminescence (PL) spectra indicate that the fluorescence emission intensity of core/shell particles is highly sensitive to the pH of an aqueous medium. The core/shell composite particles exhibited a combination of fluorescent, magnetic, pH and thermo-responsive behavior.     

Redispersible dried hydroxyapatite particles with grafted pH-sensitivity polymer brushes of poly(styrene-co-4-vinylpyridine)

Reversible aggregation-redispersion transition of colloidal particles is of considerable importance in colloidal science and various industrial fields. In the present study, stimuli-responsive hybrid hydroxyapatite (HA) particles were prepared with redispersible ability in aqueous solutions even after separation from the “as synthesized” state and subsequent dry heat treatment. We firstly modified HA particles with sodium oleate as chelating agent, then introduced bromoalkyl functional groups through an addition reaction of unsaturated oleate molecule with bromine, and finally grafted polymer brushes of polystyrene-co-4-vinylpyridine onto HA. It was the pH-sensitive polymer shell that had rendered HA particles the redispersible ability, which was attributed to the protonation of the pyridine blocks in acidic solutions. The reversible dispersion-aggregation transition of HA could be controlled by alternating the pH value of aqueous solution. Protonated pyridine blocks had provided enough electrostatic and steric repulsions for colloidal HA to avoid effective collision.