Preparation and Properties of Polystyrene–Poly(aryl ether‐sulfone)–Polystyrene ABA Triblock Copolymers

Controlled molecular weight poly(aryl ether‐sulfone) oligomers with amine groups at the chain termini were prepared by the reactions of 4,4′‐isopropylidene diphenol, 4,4′‐dichlorodiphenyl sulfone and 3‐aminophenol in dimethylacetamide. The oligomers were characterized and the terminal amino groups were converted to N,N‐diethyldithiocarbamates. These oligomers were used subsequently to initiate controlled radical polymerization of styrene in the presence of UV light to afford ABA triblock copolymers. Model compound studies were undertaken prior to the block copolymer synthesis. The block copolymers were characterized by standard techniques.     

Triblock Copolymers Based on Sucrose Methacrylate and Methyl Methacrylate: RAFT Polymerization and Self‐Assembly

ABA and BAB triblock amphiphilic copolymers based on sucrose methacrylate and methyl methacrylate are synthesized by sequential reversible addition–fragmentation chain transfer polymerization using S,S′‐bis(R,R′‐dimethyl‐R′′‐acetic acid)‐trithiocarbonate as a chain transfer agent. The copolymers present narrow molar mass dispersity, controlled molar mass and architecture as determined by gel permeation chromatography and ¹H and ¹³C nuclear magnetic resonance. The copolymers with molar and mass fractions of poly(sucrose methacrylate) block ranging from 1 to 22 mol% and 3 to 52 wt%, respectively, and different molar masses present characteristics of a surfactant such as self‐assembly. The self‐assembly of the triblock copolymers in water, N,N‐dimethylformamide (DMF), dichloromethane, tetrahydrofuran, or benzene results mostly in vesicles as confirmed by scanning electron microscopy images and small‐angle X‐ray of the dispersions. Moreover, the copolymers present the capability to stabilize aromatic molecules (Nile Red dye) and nonpolar solvents in an aqueous phase and polar ionic molecules (methylene blue) and water in a nonpolar medium, suggesting the potential for application in drug encapsulation, environmental remediation systems, and molecular extraction in liquid–liquid immiscible systems, for example. Films prepared by casting from copolymer solutions in DMF present a lamellar structure with the lamellar thickness varying according to the copolymer molar mass.     

Rod‐Length Dependent Aggregation in a Series of Oligo(p‐benzamide)‐Block‐Poly(ethylene glycol) Rod‐Coil Copolymers

Summary: The synthesis of a series of rod‐coil diblock copolymers with flexible poly(ethylene oxide) chains ( = 5 000 g · mol⁻¹) and rod blocks consisting of monodisperse oligo(p‐benzamide)s is described. The formation of defined supramolecular aggregates in solution as well as the solid state has been analyzed. The length of the oligo(p‐benzamide)s has been systematically varied from n = 1 to 7 units. The influence of n on aggregation in chloroform and aqueous solution was investigated by GPC as well as UV‐vis spectroscopy. A critical aggregation length was found for chloroform (n = 5) and water (n = 4), below which no aggregation is observed under otherwise identical experimental conditions. Aggregation of the polymers in chloroform solution can be chemically reversed by the addition of PCl₅, resulting in conversion of the aromatic amides into imidoyl chlorides. Such amide‐protected block copolymers show no aggregation in NMR and GPC experiments. Imidoyl chloride formation was shown to be reversible, i.e., addition of water regenerated the oligo(p‐benzamide) blocks.

Conversion of aramide block copolymer into molecularly dissolved form using PCl₅.     

Aqueous Gels of Triblock Copolymers of Ethylene Oxide and 1,2‐Butylene Oxide (Type BEB) Studied by Rheometry

Summary: A triblock copolymer of ethylene oxide and 1,2‐butylene oxide with hydrophilic central block and hydrophobic end blocks, denoted B₁₀E₂₂₇B₁₀, was prepared and characterised. The frequency dependence of dynamic modulus was determined for micellar solutions in the concentration range of 8–20 wt.‐% and the temperature range of 5–70 °C, and the fluid/gel diagram was established. The results were combined with those reported previously for copolymers B₁₂E₂₂₇B₁₂ and B₁₂E₁₁₄B₁₂ to obtain insight into the effect of B‐block and E‐block length on the rheological properties of systems of this type. The results are discussed in terms of micelles forming bridged networks and, at higher concentrations, packing in structured gels.

Temperature dependence of the logarithm of storage modulus for aqueous solutions of copolymer B₁₀E₂₂₇B₁₀ at the concentrations (wt.‐%) indicated. Conditions: f = 1 Hz, A ≈ 0.5%.     

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.

     

Tunable Morphologies From Bulk Self‐Assemblies of Poly(acryloxypropyl triethoxysilane)‐b‐poly(styrene)‐b‐poly(acryloxypropyl triethoxysilane) Triblock Copolymers

Reactive poly(acryloxypropyl triethoxysilane)‐b‐poly(styrene)‐b‐poly(acryloxypropyl triethoxysilane) (PAPTES‐b‐PS‐b‐PAPTES) triblock copolymers are prepared through nitroxide‐mediated polymerization (NMP). The bulk morphologies formed by this class of copolymers cast into films are examined by small‐angle X‐ray scattering (SAXS) and transmission electron microscopy (TEM). The films morphology can be tuned from spherical structures to lamellar structures by increasing the volume fraction of PS in the copolymer. Thermal annealing at temperatures above 100 °C provides sufficient PS mobility to improve ordering.     

Self‐Assembly of ATRP‐Synthesized PCH‐b‐PtBA‐b‐PCH Triblock Copolymers Observed by Time‐Resolved SAXS

The phase behavior of PCH‐b‐PtBA‐b‐PCH triblock copolymers has been studied. Measurements in the wide‐angle region probed the existence of microphase segregation through variation of block mobility and thermal expansion coefficients. SAXS experiments pointed out that most copolymers present ordered nanostructures, mostly hexagonally packed cylinders, the morphology being confirmed by AFM. An unusual disorder‐to‐order transition is observed in one copolymer synthesized from a macroinitiator with intermediate length and the highest outer‐block molecular weight, whereas none of the copolymers shows an order‐to‐disorder transition upon heating over the temperature range analyzed.

     

Self‐Assembled Acrylic ABA Triblock Copolymer Hydrogels with Various Block Compositions: Water Absorbency,Rheology, and SAXS

A series of well‐defined symmetric poly(methyl methacrylate)‐b‐poly(sodium methacrylate)‐b‐poly(methyl methacrylate) (PMMA‐b‐PSMA‐b‐PMMA) triblock copolymers with various block compositions is synthesized. The amphiphilic ABA triblock copolymers form polyelectrolyte hydrogels in water by self‐assembly. The hydrophobic PMMA endblocks act as physical cross‐links in the form of frozen micelles, while the hydrophilic PSMA midblocks span the 3D network. The influence of various synthetic parameters on the self‐assembly and the macroscopic properties of these hydrogels is systematically investigated by water absorbency, oscillatory shear rheology, and small‐angle X‐ray scattering. The polymer concentration during the hydrogel formation affects the ratio between looping and bridging chains. The number of MMA units per endblock (n_A) determines the size and the relaxation rates of the physical cross‐links and thus, the mechanical stability of the hydrogels. More SMA units in the midblock (n_B) increase the water absorbency, while the mechanical moduli decrease. Even lower G‐moduli are achieved by partly exchanging the symmetric ABA triblock with AB diblock copolymers, which can only form non‐elastic dangling ends.     

A Convenient Method for the Synthesis of the Amphiphilic Triblock Copolymer Poly(L‐lactic acid)‐block‐Poly(L‐lysine)‐block‐Poly(ethylene glycol) Monomethyl Ether

The amphiphilic triblock copolymer PLA‐b‐PLL‐b‐MPEG is prepared in three steps through acylation coupling between the terminal amino groups of PLA‐b‐PZLL‐NH₂ and carboxyl‐terminal MPEG, followed by the deprotection of amines. The block copolymers are characterized via FT‐IR, ¹H NMR, DSC, GPC, and TEM. TEM analysis shows that the triblock polymers can form polymeric micelles in aqueous solution with a homogeneous spherical morphology. The cytotoxicity assay indicates that the final triblock polymer micelles after deprotection show low cytotoxicity against Bel7402 human hepatoma cells. MPEG and PLL were introduced into the main chain of PLA affording a kind of ideal bioabsorbable polymer materials, which is expected to be useful in drug and gene delivery.

     

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.

     

RAFT‐Based Synthesis and Characterization of ABC versus ACB Triblock Copolymers Containing tert‐Butyl Acrylate,Isoprene, and Styrene Blocks

The syntheses of the ABC and ACB triblock copolymer topological isomers poly(tert‐butyl acrylate)‐block‐polyisoprene‐block‐polystyrene (ABC) and poly(tert‐butyl acrylate)‐block‐polystyrene‐block‐polyisoprene (ACB) using the RAFT‐controlled radical polymerization method are described. The triblock copolymers were isolated with molecular weights on the order of 20 000 Da and molecular weight distributions of 1.3–1.5. The polymers described herein demonstrated thermal decompositions at 200 °C, characteristic of tert‐butyl esters, and their structure and composition were confirmed by a combination of infrared, ¹H, and ¹³C NMR spectroscopies.

     

Thermo‐Responsive Copolymers Based on Poly(N‐isopropylacrylamide) and Poly[2‐(methacryloyloxy)ethyl phosphorylcholine]: Light Scattering and Microscopy Experiments

Aqueous self‐assembly of thermosensitive triblock copolymers based on poly(N‐isopropylacrylamide) and poly[2‐(methacryloyloxy)ethyl phosphorylcholine] (PNIPAM_m–PMPC_n–PNIPAM_m and PNIPAM_m–PMPC_n–S–S–PMPC_n–PNIPAM_m) were studied using light scattering (SLS and DLS), TEM and fluorescence experiments. These techniques were used to investigate the morphological transition as a function of the temperature, below and above the LCST of the PNIPAM, at various triblock copolymer concentrations ranging from dilute to semi‐dilute regimes. Below the LCST and at low concentrations, aqueous solutions show micellar behavior, while above the LCST self‐assembly leading to large nanoparticles stabilized with PMPC chains. Such behavior is the onset of a gel‐like phase transition observed at higher concentrations.

     

Synthesis and Self‐Assembly Behavior of Organic–Inorganic Poly(ethylene oxide)‐block‐Poly(MA POSS)‐block‐Poly(N‐isopropylacrylamide) Triblock Copolymers

In this work, the synthesis of 3‐methacryloxypropylheptaphenyl POSS, a new POSS macromer (denoted MA‐POSS) is reported. The POSS macromer is used to synthesize PEO‐b‐P(MA‐POSS)‐b‐PNIPAAm triblock copolymers via sequential atom transfer radical polymerization (ATRP). The organic‐inorganic, amphiphilic and thermoresponsive ABC triblock copolymers are characterized by means of nuclear magnetic resonance spectroscopy (NMR) and gel permeation chromatography (GPC). Differential scanning calorimetry (DSC) and atomic force microscopy (AFM) show that the hybrid ABC triblock copolymers are microphase‐separated in bulk. Cloud point measurements show that the effect of the hydrophiphilic block (i.e. PEO) on the LCSTs is more pronounced than the hydrophobic block (i.e. P(MA‐POSS)). Both transmission electron microscopy (TEM) and dynamic light scattering (DLS) show that all the triblock copolymers can be self‐organized into micellar aggregates in aqueous solutions. The sizes of the micellar aggregates can be modulated by changing the temperature. The temperature‐tunable self‐assembly behavior is interpreted using a combination of the highly hydrophobicity of P(MA‐POSS), the water‐solubility of PEO and the thermoresponsive property of PNIPAAm in the triblock copolymers.     

Water‐Soluble Stoichiometric Polyelectrolyte Complexes Based on Cationic Comb‐Type Copolymers

Summary: The interaction between the cationic comb‐type copolymer poly(acrylamide‐co‐MAPTAC)‐graft‐polyacrylamide, synthesized by free‐radical polymerization, and the anionic homopolymer NaPA has been investigated. Water‐soluble PECs are formed through a charge neutralization process. They adopt a compact structure, and form nanoparticles consisting of a hydrophobic PEC core and a protective hydrophilic PAM corona. By increasing the composition of the graft copolymers in neutral PAM side chains, the aggregation number and the size of the nanoparticles decrease. Moreover, increasing the ionic strength of the solution favours the dissociation of the complexes.

A schematic representation of the PEC nanoparticles formed at N_NaPA = 0.5.     

Initiating Mechanism of the Anionic Polymerization of Methacrylates with t‐BuOK and the Synthesis of ABA Type Triblock Copolymers

Using potassium tert‐butoxide (t‐BuOK) as initiator, anionic polymerization and copolymerization of the methacrylate monomers are carried out in tetrahydrofuran at 0 °C or above. The polymers are characterized by gel permeation chromatography (GPC), proton nuclear magnetic resonance (¹H‐NMR), Fourier transform infrared (FTIR), and dynamic mechanical analyzer (DMA). There is a critical concentration of the active species in t‐BuOK, above which no matter how much more t‐BuOK is added, it has no initiation activity. Among the active species of t‐BuOK, the main part possesses large average vibration distance between anion–cation pairs, which can initiate all methacrylate monomers in this study. While another part of the active species possesses the small average vibration distance, which can only initiate the polymerization of methyl methacrylate at slow rate. It provides an supporting evidence for the “channel idea” proposed previously. Finally, poly(methyl methacrylate)‐b‐poly(lauryl methacrylate)‐b‐poly(methyl methacrylate) (PMMA‐b‐PLMA‐b‐PMMA) three‐block copolymer is synthesized. This study paves the way for the synthesis of thermoplastic elastomers with full polar monomers and full saturated chains by anionic polymerization.     

Effect of Solvent Type on High‐Temperature Thermal Gradient Interaction Chromatography of Polyethylene and Ethylene–1‐Octene Copolymers

The effects of the solvent type and operation conditions on the high‐temperature thermal gradient interaction chromatography (HT‐TGIC) of ethylene homopolymers, ethylene–1‐octene copolymers, and their blends are investigated. While the HT‐TGIC profiles of single polymers measured with 1,2,4‐trichlorobenzene (TCB) and chloronaphthalene (CN) are similar, they are always narrower when o‐dichlorobenzene (ODCB) is used, particularly for samples with lower 1‐octene fractions. Significant differences between the experimental and the calculated profiles of binary blends are observed with all three solvents, but better peak separation is seen when the ODCB is used. Having higher fractions of a 1‐octene‐poor component in the blend causes a more significant distortion of the shape expected for the peak from the component with the higher 1‐octene fraction. The effect of the molecular weight on HT‐TGIC profiles is also studied using samples with the same comonomer content and different molecular weights. Samples with low molecular weight have broader distributions and significant lower‐temperature tails, particularly when TCB is used. Chain crystallization after adsorption effects may also play a minor role for low‐comonomer samples. Finally, HT‐TGIC profiles are compared with their equivalent crystallization elution fractionation (CEF) profiles. The HT‐TGIC curves are broader than the equivalent CEF profiles, but these differences decrease as the comonomer content increases.

     

ABA and BAB Triblock Copolymers Based on 2‐Methyl‐2‐oxazoline and 2‐n‐Propyl‐2‐oxazoline: Synthesis and Thermoresponsive Behavior in Water

Inspired by the well‐known amphiphilic block copolymer platform known as Pluronics or poloxamers, a small library of ABA and BAB triblock copolymers comprising hydrophilic 2‐methyl‐2‐oxazoline (A) and thermoresponsive 2‐n‐propyl‐2‐oxazoline (B) is synthesized. These novel copolymers exhibit temperature‐induced self‐assembly in aqueous solution. The formation and size of aggregates depend on the polymer structure, temperature, and concentration. The BAB copolymers tend to agglomerate in water, with the cloud point temperature depending on the length of poly(2‐n‐propyl‐2‐oxazoline) chain. On the other hand, ABA copolymers form smaller aggregates with hydrodynamic radius from 25 to 150 nm. The dependence of viscosity and viscoelastic properties on the temperature is also studied. While several Pluronic block copolymers are known to form thermoreversible hydrogels in the concentration range 20–30 wt%, thermogelation is not observed for any of the investigated poly(2‐oxazoline)s at the investigated temperature range from 10 to 50 °C.

     

Star‐Like Poly(N‐isopropylacrylamide) and Poly(ethylene glycol) Copolymers Self‐Arranged in Newfound Single Crystals and Associated Novel Class of Polymer Brush Regimes with V‐Type Tethers

Linear poly(ethylene glycol) (PEG)‐block‐poly(N‐isopropylacrylamide) (PNIPAM) and star‐like three‐arm PEG‐star‐(PNIPAM)₂ copolymers having one PEG and two PNIPAM blocks are synthesized by atom transfer radical polymerization (ATRP). Single crystals of these block copolymers are grown from amyl acetate and toluene dilute solutions. To recognize PNIPAM and PEG thicknesses, small angle X‐ray scattering (SAXS) is applied. V‐type brushes behave differently from linear brushes because doubly grafted PNIPAM blocks from a common point onto PEG substrate exert a higher osmotic pressure, leading to a thinner crystal. In addition to three ordinary regimes, a fourth or extremely stretched regime is detected for V‐type PNIPAM brushes. Although in PEG₅₀₀₀‐star‐(PNIPAM)₂ single crystals with overall PNIPAM molecular weight of 37 000 g/mol, each PNIPAM arm is shorter than PNIPAM grafts in linear PEG₅₀₀₀‐block‐PNIPAM₂₆₀₀₀ single crystals, their switching point from second to third regime is significantly lower (22 vs 33 °C). The V‐type configuration of PNIPAM brushes cause them to be entered into the extremely stretched or fourth regime, which has not been previously detected for coily brushes. The boundary between third and fourth regimes for PEG₅₀₀₀‐star‐(PNIPAM)₂ single crystals is verified at 31.5 and 23.5 °C in amyl acetate and toluene, respectively.