Block Copolymers of Vinyl Ethers as Thermo‐Responsive Colloidal Stabilizers of Organic Pigments in Aqueous Media

Summary: The colloidal stability of the aqueous dispersions of hydrophobic organic pigments, CuPc and carbon black, stabilized by a wide range of polymer structures based on alkyl vinyl ethers was studied. It was shown that, unlike the homopolymers and the random copolymers, the amphiphilic AB, ABA and BAB block copolymers of MVE with IBVE or ODVE show stabilizing activities that depend on their hydrophilic/hydrophobic balance and polymer architecture. After optimization, the colloidal stabilization is competitive with commercial PEO‐PPO block copolymers (Pluronic®). It was found that the sedimentation of the dispersions was much faster at a higher temperature, above the LCST of the PMVE‐blocks. The loss of the stabilizing activity of the block copolymers correlates with an increase of the hydrophobicity of the treated pigment surface. These properties enable the creation of colloidal dispersions with stabilities that can be tuned as a function of temperature.

Poly(methyl vinyl ether) ABA and BAB block copolymers as colloidal stabilizers of organic pigments.     

Diblock Copolymers with Miscible Blocks via One‐Pot Sequential Cationic Polymerization and Their Block‐Length‐Dependent Vesicular Aggregation

A series of well‐defined poly(α‐methylstyrene)‐block‐poly(isobutyl vinyl ether) (PMS‐b‐PIBVE) diblock copolymers of varying compositions is synthesized via sequential cationic polymerization using an FeCl₃‐based initiating system at 0 °C. The copolymers exhibit a single glass transition temperature (T_g). The T_g vs. composition relationships for the copolymers show positive deviation from a linear relationship, which is explained by the Kwei equation. The copolymer (PMS₃₅‐b‐PIBVE₁₄₅ and PMS₆₀‐b‐PIBVE₁₂₈) molecules self‐aggregate to form vesicles in organic solvents. However, the copolymers (PMS₈₃‐b‐PIBVE₉₄, PMS₁₀₇‐b‐PIBVE₅₂, and PMS₁₂₉‐b‐PIBVE₃₂) with higher rigid PMS contents do not form vesicles. PMS₃₅‐b‐PIBVE₁₄₅ vesicles can encapsulate hydrophobic and hydrophilic fluorescent macromolecules.

     

Thermoresponsive Glycopolymers via Controlled Radical Polymerization

New glycomonomers 3′‐(1′,2′:5′,6′‐di‐O‐isopropylidene‐α‐D ‐glucofuranosyl)‐6‐methacrylamido hexanoate (MAIpGlcC₅) and 3′‐(1′,2′:5′,6′‐di‐O‐isopropylidene‐α‐D ‐glucofuranosyl)‐6‐methacrylamido undecanoate (MAIpGlcC₁₀) with hydrophobic spacer units were synthesized and their homopolymers, as well as random copolymers with N‐isopropylacrylamide (NiPAAm) were prepared in varying compositions. The acidolysis of the isopropylidene protection groups of the polymers gave well‐defined sugar‐containing water‐soluble homopolymers (PMAGlcC_n, n = 5, 10) and copolymers. By using the reversible addition–fragmentation chain transfer (RAFT) process, it was possible to afford these copolymers with a polydispersity index (PDI) of 1.1–1.5. Furthermore, NiPAAm homopolymers with an active chain transfer unit at the chain end could be prepared by RAFT, which were used as macro‐chain transfer agents (macro‐CTAs) to prepare a variety of sugar containing responsive block copolymers from new glycomonomers by the monomer addition concept. The cloud points of the aqueous solutions of the copolymers were strongly affected by the comonomer content, spacer chain length of the glycomonomer, and the chain architecture of the copolymers. Especially by the block copolymer concept, glycopolymers with lower critical solution temperatures (LCSTs) in the physiologically interesting range could be realized.

     

Synthesis of Symmetrical Triblock Copolymers of Styrene and N‐isopropylacrylamide Using Bifunctional Bis(trithiocarbonate)s as RAFT Agents

Six new bifunctional bis(trithiocarbonate)s were explored as RAFT agents for synthesizing amphiphilic triblock copolymers ABA and BAB, with hydrophilic “A” blocks made from N‐isopropylacrylamide and hydrophobic “B” blocks made from styrene. Whereas the extension of poly(N‐isopropylacrylamide) by styrene was not effective, polystyrene macroRAFT agents provided the block copolymers efficiently. End group analysis by ¹H NMR spectroscopy supported molar mass analysis and revealed an unexpected side reaction for certain bis(trithiocarbonate)s, namely a fragmentation to simple trithiocarbonates while extruding ethylenetrithiocarbonate. The amphiphilic block copolymers with short polystyrene blocks are directly soluble in water and self‐organize into thermoresponsive micellar aggregates.

     

Amphiphilic Multi‐Arm Star‐Block Copolymers for Encapsulation of Fragrance Molecules

Novel amphiphilic multi‐arm star‐block copolymers with a hyperbranched core, a hydrophobic inner shell, and a hydrophilic outer shell have been prepared from a commercial hyperbranched polyester macroinitiator by ring‐opening polymerization of ε‐caprolactone, followed by atom transfer radical polymerization of tert‐butyl acrylate (tBuA). Hydrolysis of the tert‐butyl groups was then used to convert the poly(tBuA) blocks to poly(acrylic acid), resulting in stable amphiphilic core‐shell structures with significantly higher degrees of functionality than reported so far in the literature. A strong correlation between the maximum concentration of selected hydrophobic guest molecules and the concentration of amphiphilic star‐block copolymer in aqueous solution was observed by ¹H NMR, demonstrating the capacity of these copolymers to encapsulate and disperse significant loadings (up to about 27 wt.‐%) of volatile hydrophobic molecules such as fragrances in water.

     

Modification of Natural Rubber by Grafting with Hydrophilic Vinyl Monomers

Summary: The grafting of two hydrophilic vinyl monomers, dimethylaminoethyl acrylate (DMAEA) and dimethylaminoethyl methacrylate (DMAEMA), onto natural rubber latex was carried out by emulsion polymerization using redox initiation. The effects of the redox initiator concentration and type (cumene hydroperoxide/tetraethylenepentamine, CHP/TEPA; tert‐butyl hydroperoxide/tetraethylenepentamine, t‐BHP/TEPA; potassium persulfate/potassium metabisulfite, K₂S₂O₈/K₂S₂O₅), the monomer concentration and the reaction temperature on the conversion, the grafting efficiency, the water absorption and the contact angles of the grafted copolymers films, and the colloidal stability of the latexes at low pH, were investigated. Infrared spectroscopic analysis confirmed that DMAEA and DMAEMA were grafted onto the natural rubber particles. The hairy layer structure of NR‐g‐poly(DMAEMA) latex particles was investigated by transmission electron microscopy using positive and negative stainings with OsO₄ and phosphotungstic acid, respectively.

TEM micrograph of NR‐g‐poly(DMAEMA) latex particles stained with 2% OsO₄.     

pH‐Responsive Micelles Based on Amphiphilic Block Copolymers Bearing Ortho Ester Pendants as Potential Drug Carriers

Amphiphilic block copolymers (PEG‐b‐PEYM) bearing acid‐labile ortho ester pendant chains were synthesized by atom transfer radical polymerization, and the effect of chain‐length of the ortho‐ester‐bearing PEYM block on micelle properties was investigated. The length of the PEYM block affected the critical micelle concentration and pH‐dependent average particle size but not the kinetics of side‐chain hydrolysis. A hydrophobic dye, Nile red, was loaded in the micelles. Both the loading content and the size of the loaded micelles were influenced by the length of the PEYM block, whereas the release kinetics of Nile red was not. Provided that these acid‐labile micelles are completely cell compatible, they are potentially useful bio‐responsive nanocarriers for enhancing the efficacy of hydrophobic drugs.

     

Synthesis and Characterization of Amphiphilic Poly(ethylene oxide)‐block‐poly(hexyl methacrylate) Copolymers

We describe the preparation of amphiphilic diblock copolymers made of poly(ethylene oxide) (PEO) and poly(hexyl methacrylate) (PHMA) synthesized by anionic polymerization of ethylene oxide and subsequent atom transfer radical polymerization (ATRP) of hexyl methacrylate (HMA). The first block, PEO, is prepared by anionic polymerization of ethylene oxide in tetrahydrofuran. End capping is achieved by treatment of living PEO chain ends with 2‐bromoisobutyryl bromide to yield a macroinitiator for ATRP. The second block is added by polymerization of HMA, using the PEO macroinitiator in the presence of dibromobis(triphenylphosphine) nickel(II), NiBr₂(PPh₃)₂, as the catalyst. Kinetics studies reveal absence of termination consistent with controlled polymerization of HMA. GPC data show low polydispersities of the corresponding diblock copolymers. The microdomain structure of selected PEO‐block‐PHMA block copolymers is investigated by small angle X‐ray scattering experiments, revealing behavior expected from known diblock copolymer phase diagrams.

SAXS diffractograms of PEO‐block‐PHMA diblock copolymers with 16, 44, 68 wt.‐% PEO showing spherical (A), cylindrical (B), and lamellae (C) morphologies, respectively.     

Direct Access to Mesoporous Crystalline TiO2/Carbon Composites with Large and Uniform Pores for Use as Anode Materials in Lithium Ion Batteries

Mesoporous and highly crystalline TiO₂ (anatase)/carbon composites with large (>5 nm) and uniform pores were synthesized using PI‐b‐PEO block copolymers as structure directing agents. Pore sizes could be tuned by utilizing block copolymers with different molecular weights. The resulting mesoporous TiO₂/carbon was successfully used as an anode material for Li ion batteries. Without addition of conducting aid (Super P), the electrode showed high capacity during the first insertion/desertion cycle due to carbon wiring inside the walls of mesoporous TiO₂/carbon. The electrode further showed stable cycle performance up to 50 cycles and the specific charge capacity at 30 C was 38 mA h (g of TiO₂)^?1, which indicates CCM‐TiO₂/carbon can be used as a material for high rate use.

     

Structure of Adsorption Layers of Amphiphilic Copolymers on Inorganic or Organic Particle Surfaces

The structure of adsorption layers of amphiphilic block and block‐like copolymers of poly(isobornyl acrylate) and poly(acrylic acid) on the surface of hydrophilic titanium dioxide and hydrophobic copper phthalocyanine (CuPc) pigments in an aqueous dispersion has been studied by the electrokinetic sonic amplitude (ESA) method. The electroacoustic behaviour of the polyelectrolyte block copolymer‐coated particles could be described in the context of the polymer gel layer theory. The polymer layer around the particles was found to be much thinner for CuPc as compared to the TiO₂ substrate. This is attributed to differences in the adsorption mechanism and the composition of the adsorption layer normal to the substrate surface. Adsorption models were established that consider effects of the copolymer structure.

     

Facile One‐Pot Approach for Preparing Functionalized Polymeric Nanoparticles via ROMP

A facile approach to the preparation of functionalized polymeric nanoparticles with UV‐crosslinkable core and bromine‐bearing shell has been developed from the polymerization‐induced self‐assembly of the block copolymers PBNBE‐b‐PCONBI and PBNBE‐b‐PCONBI‐b‐PONBDM. The block copolymers were characterized by means of ¹H NMR and GPC. The polymeric nanoparticles with a fixed shell length and varied core lengths were obtained through the control of the molar ratio of monomers in the feed. The micelles were crosslinked using UV‐irradiation, and the characteristics of the polymeric nanoparticles in toluene, THF, or CHCl₃ before and after UV irradiation were investigated by means of DLS, UV‐Vis, GPC, AFM, and TEM.

     

Association Behavior between End‐Functionalized Block Copolymers PEO‐PPO‐PEO and Poly(acrylic acid)

Summary: The end groups of ABA‐triblock copolymers HO–PEO–PPO–PEO–OH, (PEO – poly(ethylene oxide), PPO – poly(propylene oxide)), have been modified with ammonia, ethylene diamine and linear polyethylenimine (LPEI) by substitution of the α,ω‐ditosyl ester of the triblock copolymer (TsO–PEO–PPO–PEO–OTs) with amines, or by the hydrolysis of the corresponding poly(2‐methyl‐2‐oxazoline) (PMeOx) containing ABCBA block copolymers. The latter block copolymer structures have been obtained by the polymerization of MeOx using TsO–PEO–PPO–PEO–OTs as a macro‐initiator. Adding poly(acrylic acid) (PAA) to these (poly)amine terminated block copolymers leads to the formation of networks through a combination of PAA–PEO hydrogen bonding and PAA–(poly)amine acid‐base reaction. Depending on the number of amino groups at both chain ends of the block copolymer, the corresponding complexes behave as liquids, gels or precipitates. Introduction of as little as 1–5 wt.‐% block copolymers H₂N–PEO–PPO–PEO–NH₂ or H₂NCH₂CH₂NH–PEO–PPO–PEO–NHCH₂CH₂NH₂ to the system of HO–PEO–PPO–PEO–OH/PAA leads to viscous liquids with strong shear‐thickening behavior.

Reversible gel formation via the ternary PAA/HO–PEO–PPO–PEO–OH/H₂N–PEO–PPO–PEO–NH₂ system under shear conditions.     

Assembled Structures and Chiroptical Properties of Amphiphilic Block Copolymers Synthesized by RAFT Polymerization of N‐Acryloyl‐L‐alanine

Amino acid‐based amphiphilic block copolymers involving poly(N‐acryloyl‐L ‐alanine), poly(A‐Ala‐OH), which exhibits a characteristic chiroptical property and pH‐dependent solubility, have been synthesized by reversible addition‐fragmentation chain transfer (RAFT) polymerization. The direct polymerization of A‐Ala‐OH without any protecting chemistry using the dithiocarbamate‐terminated polystyrene or poly(N‐acryloyl‐L ‐phenylalanine methyl ester) as a macrochain transfer agent produced well‐defined amphiphilic block copolymers. The self‐assembly behaviors and chiroptical properties of these amphiphilic block copolymers in selective solvents were investigated by dynamic light scattering, circular dichroism, and UV–Vis spectroscopic methods.

     

EPR Study into Cross‐Termination and Fragmentation with the Phenylethyl–Phenylethyl Dithiobenzoate RAFT Model System

The reversible addition–fragmentation chain transfer (RAFT) equilibrium constant of the phenylethyl–1‐phenylethyl dithiobenzoate model system is deduced via EPR spectroscopy to be 31 ± 4 L mol^?1 at 110 °C. The difference in activation energies of addition and fragmentation, E_a(k_ad) – E_a(k_β) ≈ ?45.6 kJ mol^?1, is close to that of the polystyryl–polystyryl dithiobenzoate system. Significant amounts of products from cross‐termination and from “missing step” reaction of the cross‐termination product with the phenylethyl radical demonstrate that intermediate radical termination, rather than slow fragmentation of the RAFT intermediate radical, is responsible for the rate retardation in the dithiobenzoate‐mediated styrene polymerization.

     

Amorphous‐Crystalline PSnPEOn Heteroarm Star Copolymers: Crystallinity and Crystallization Kinetics

Summary: The crystallization behavior and kinetics of poly(ethylene oxide) in polystyrene/poly(ethylene oxide) heteroarm star copolymers were studied by differential scanning calorimetry and optical microscopy. A comparison between star and linear amorphous‐crystalline block copolymers showed that the macromolecular architecture is an important factor affecting crystallinity. The following points were observed: the equilibrium melting point is higher in the star copolymers, the crystallinity reduces as the number of arms increases, leading to smaller and ill‐defined spherulites, and crystallization proceeds faster in linear copolymers at low supercooling.

Half crystallization times, t_1/2, calculated from the Avrami analysis of the latent heats, obtained during the isothermal crystallization experiments as a function of supercooling, ΔT, for all copolymers.     

Amphiphilic Block Copolymers Containing Regioregular Poly(3‐hexylthiophene) and Poly(2‐ethyl‐2‐oxazoline)

Poly(3‐hexylthiophene)‐block‐poly(2‐ethyl‐2‐oxazoline) amphiphilic rod–coil diblock copolymers have been synthesized by a combination of Grignard metathesis (GRIM) and ring‐opening cationic polymerization. Diblock copolymers containing 5, 15, and 30 mol‐% poly(2‐ethyl‐2‐oxazoline) have been synthesized and characterized. The synthesized rod–coil block copolymers display nanofibrillar morphology where the density of the nanofibrills is dependent on the concentration of the poly(2‐ethyl‐2‐oxazoline) coil segment. The conductivity of the diblock copolymers was lowered from 200 to 35 S · cm^?1 with an increase in the content of the insulating poly(2‐ethyl‐2‐oxazoline) block. By contrast, the field‐effect mobility decreased by 2–3 orders of magnitude upon the incorporation of the poly(2‐ethyl‐2‐oxazoline) insulating segment.

     

Self‐Assembly and Photoresponsivity Property of Amphiphilic ABA Triblock Copolymers Containing Azobenzene Moieties in Dilute Solution

The self‐assembly and photoresponsivity of amphiphilic azobenzene‐containing ABA triblock copolymers PA6C_m‐b‐PEG_n‐b‐PA6C_m synthesized by atom transfer radical polymerization (ATRP) were reported. Different self‐assembly morphologies formed by the gradual addition of water to the copolymer solutions in THF. The formation process and aggregate morphology were characterized by UV–Visible spectroscopy and transmission electron microscope (TEM). The triblock copolymers start to form aggregates at the critical water content (CWC). With the addition of water, the aggregates show different morphologies, such as spherical micelles, vesicles, network‐like aggregates, and colloidal spheres, which involves the transformation between primary and secondary aggregates and the association/disassociation of aggregates. Photoresponsive property and aggregation behavior of these copolymers in solution under UV–Visible light irradiation were also investigated.

     

Peculiar Behavior of Degenerative Chain Transfer Polymerization of a Phosphonated Methacrylate

Living/controlled radical polymerization of dimethyl(methacryloyloxy)methyl phosphonate (MAPC₁) has been attempted using degenerative transfer to produce block copolymers. RAFT polymerization of this monomer is sensitive to very low level of oxygen and in any case limited to low monomer conversion. Reverse iodine transfer polymerization (RITP) leads to higher monomer conversion with a limited amount of living polymer (55% by ¹H NMR), precluding an efficient synthesis of block copolymers. A PMMA‐b‐PMAPC₁ diblock copolymer was therefore synthesized by iodine transfer polymerization (ITP) of MAPC₁ from a PMMA‐I macro‐chain transfer agent prepared by RITP. The diblock copolymer, purified by selective precipitation, contains an average of five MAPC₁ units and has potential an adhesive and as a corrosion inhibitor.

     

Synthesis of Novel Linear PEO‐b‐PS‐b‐PCL Triblock Copolymers by the Combination of ATRP,ROP, and a Click Reaction

A new strategy to synthesize a series of well‐defined amphiphilic PEO‐b‐PS‐b‐PCL block copolymers is presented. First, bromine‐terminated diblock copolymers PEO‐b‐PS‐Br are prepared by ATRP of styrene, and converted into azido‐terminated PEO‐b‐PS‐N₃ diblock copolymers. Then propargyl‐terminated PCL is prepared by ROP of ε‐caprolactone. The PEO‐b‐PS‐b‐PCL triblock copolymers with from 1.62 × 10⁴ to 1.96 × 10⁴ and a narrow PDI from 1.09 to 1.19 are finally synthesized from these precursors. The structures of these triblock copolymers and their precursors have been characterized by NMR, IR, and GPC analysis.

     

A Facile Approach for Transferring Hydrophobic Magnetic Nanoparticles into Water‐Soluble Particles

A novel, easy and high‐efficient method is described for transferring hydrophobic magnetic Fe₃O₄ nanoparticles from organic to aqueous solution by wrapping a thermo‐responsive and photocrosslinkable poly(N‐isopropylacrylamide) (PNIPAm) terpolymer around the particles. The wrapping procedure is introduced by the co‐nonsolvent transition of PNIPAm in the mixing solvent and the polymer can dissolve in water carrying Fe₃O₄ nanoparticles by noncovalent interaction. The temperature‐dependant and magnetic properties of the water‐soluble particles are characterized in this paper.