Three-arm Star Block Copolymers Research Articles

Fast and efficient preparation of three-arm star block copolymers via tetrazine ligation

The inverse Electron Demand Diels-Alder (IEDDA) reaction between tetrazine and trans-cyclooctenol (TCO) is a fast and effective method employed to connect two or more molecules. In this study, an efficient and fast methodology based on tetrazine/trans-cyclooctene IEDDA reaction was developed for preparation of star-shaped block copolymers. Three-arm star polylactide-b-poly(ethylene glycol) (PLA-b-PEG) was chosen as a model system to show efficiency of the approach. First, three-arm PLAs with two different molecular masses were synthesized in a typical ring opening polymerization and functionalized with tetrazine end functionalities. while TCO end functionalized PEGs in two different molecular masses were perpared. Exact stoichiometric IEDDA click reaction between tetrazine moieties on three ends of the star polymer (s-3PLA-Tz) in the core and trans-cyclooctene (TCO) moieties of the periphery polymer (PEG-TCO) was performed in high click efficiency (up to 95.8%) in a short period (after 50–75 min). Ligation of PEG was confirmed by UV–vis, NMR and FT-IR spectroscopies, and size exclusion chromatography (SEC) and differential scanning calorimetry (DSC) analyses. In this way, various copolymers of desired architectures can be prepared in a short time and in high efficiency.

Structural Studies of Three-Arm Star Block Copolymers Exposed to Extreme Stretch Suggests a Persistent Polymer Tube.

We present structural small-angle neutron scattering studies of a three-armed polystyrene star polymer with short deuterated segments at the end of each arm. We show that the form factor of the three-armed star molecules in the relaxed state agrees with that of the random phase approximation of Gaussian chains. Upon exposure to large extensional flow conditions, the star polymers change conformation resulting in a highly stretched structure that mimics a fully extended three-armed tube model. All three arms are parallel to the flow, one arm being either in positive or negative stretching direction, while the two other arms are oriented parallel, right next to each other in the direction opposite to the first arm.

Thermoresponsive (star) block copolymers from one-pot sequential RAFT polymerizations and their self-assembly in aqueous solution

A series of hydrophobically end-capped linear triblock copolymers as well as of three-arm and four-arm star block copolymers was synthesized in a one-pot procedure from N,N-dimethylacrylamide (DMA) and N,N-diethylacrylamide (DEA). The sequential reversible addition-fragmentation chain transfer (RAFT) polymerization of these monomers via the R-approach using bi-, tri- and tetrafunctional chain transfer agents (CTAs) bearing hydrophobic dodecyl moieties proceeded in a well-controlled manner up to almost quantitative conversion. Polymers with molar masses up to 150 kDa, narrow molar mass distribution (PDI ≤ 1.3) and high end group functionality were obtained, which are thermoresponsive in aqueous solution showing a LCST (lower critical solution temperature) transition. The temperature-dependent associative behavior of the polymers was examined using turbidimetry, static and dynamic light scattering (SLS, DLS), and small angle neutron scattering (SANS) for structural analysis. At 25 °C, the polymers form weak transient networks, and rather small hydrophobic domains are already present for polymer concentrations of 5 wt%. However, when heating above the LCST transition (35–40 °C) of the PDEA blocks, the enhanced formation of hydrophobic domains is observed by means of light and neutron scattering. These domains have a size of about 12–15 nm and must be effectively physically cross-linked as they induce high viscosity for the more concentrated samples. SANS shows that these domains are ordered as evidenced by the appearance of a correlation peak. The copolymer architecture affects in particular the extent of ordering as the four-arm star block copolymer shows much more repulsive interactions compared to the analogous copolymers with a lower number of arms.

Three-arm star block copolymers of ε-caprolactone and acrylic acid: Synthesis and micellization behavior

A series of well-defined three-arm star poly(ε-caprolactone)-b-poly(acrylic acid) copolymers having different block lengths were synthesized via the combination of ring-opening polymerization (ROP) and atom transfer radical polymerization (ATRP). First, three-arm star poly(ε-caprolactone) (PCL) (Mn = 2490–7830 g mol−1; Mw/Mn = 1.19–1.24) were synthesized via ROP of ε-caprolactone (ε-CL) using tris(2-hydroxyethyl)cynuric acid as three-arm initiator and stannous octoate (Sn(Oct)2) as a catalyst. Subsequently, the three-arm macroinitiator transformed from such PCL in high conversion initiated ATRPs of tert-butyl acrylate (tBuA) to construct three-arm star PCL-b-PtBuA copolymers (Mn = 10,900–19,570 g mol−1; Mw/Mn = 1.14–1.23). Finally, the three-arm star PCL-b-PAA copolymer was obtained via the hydrolysis of the PtBuA segment in three-arm star PCL-b-PtBuA copolymers. The chain structures of all the polymers were characterized by gel permeation chromatography, proton nuclear magnetic resonance (1H NMR), and Fourier transform infrared spectroscopy. The aggregates of three-arm star PCL-b-PAA copolymer were studied by the determination of critical micelles concentration and transmission electron microscope. © 2013 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013

AFM Study of Micelle Chaining in Surface Films of Polystyrene-block-Poly(ethylene oxide) Stars at the Air/Water Interface

A series of three-arm star block copolymers were examined using atomic force microscopy (AFM). These stars consisted of a polystyrene core composed of ca. 111 styrene units/branch with poly(ethylene oxide) (PEO) chains at the star periphery. Each star contained different amounts of PEO, varying from 107 to 415 ethylene oxide units/branch. The stars were spread as thin films at the air/water interface on a Langmuir trough and transferred onto mica at various surface pressures. Circular domains representing 2D micelle-like aggregated molecules were observed at low pressures. Upon further compression, these domains underwent additional aggregation in a systematic manner, including micellar chaining. At this point, domain area and the number of molecules/domain increased with increasing pressure. In addition, it was found that longer PEO chains led to greater intermolecular separation and less aggregation. These AFM results correspond to attributes seen in the surface pressure-area isotherms of the stars. In addition, they demonstrate the viability of AFM as a quantitative characterization technique.

The effect of hard and soft segment composition and molecular architecture on the morphology and mechanical properties of polystyrene–polyisobutylene thermoplastic elastomeric block copolymers

This paper reports the effects of hard (polystyrene, PS) and soft (polyisobutylene, PIB) segment composition and the molecular architecture (linear versus star, PS and PIB block length) on the morphology and mechanical properties of polystyrene/polyisobutylene (SIBS) block copolymers synthesized by living carbocationic polymerization. Atomic force microscopy, dynamic mechanical thermal analysis and tensile testing verified the phase-separated nature of the block copolymers, which behaved as thermoplastic elastomers (TPEs). The morphology of these TPEs is similar to polydiene-based TPEs, and is defined by the soft/hard segment composition. Interestingly, topology (linear vs star) did not have a major influence on morphology. Tensile testing showed that for both linear and three-arm star block copolymers, the modulus and tensile strength increased while elongation at break decreased with higher PS content. However, three-arm star block copolymers showed larger moduli than their linear homologues with similar PS content and PIB arm length, indicating the influence of molecular architecture on mechanical properties. These results might serve as a foundation for macromolecular engineering design for optimizing properties.

Synthesis of three-arm star block copolymers

We describe the synthesis and characterization of three-arm star block copolymers based on polystyrene, poly(ethylene oxide), poly(ϵ-caprolactone), poly(methyl methacrylate), poly(tert-butyl methacrylate) and poly(L-lactide) blocks. The copolymers were obtained by a route consisting of two successive initiation steps on functional macroinitiator. Some results on micellization and crystallization are given. They indicate an increase in the miscibility of different incompatible blocks.

Cationic Macromolecular Design and Synthesis Using Furan Derivatives

General methodologies for the synthesis of block copolymers and functional polymers using furan derivatives are reported on the basis of a study of addition reactions of 2-substituted furans to living polyisobutylene (PIB+). Rapid and quantitative monoaddition of 2-methylfuran (2-MeFu) and 2-tert-butylfuran (2-tBuFu) to PIB+ has been observed in conjunction with TiCl4 as Lewis acid in hexanes (Hex)/CH2Cl2 or CH3Cl 60/40 v/v at −80 °C and with BCl3 in CH3Cl at −40 °C, in the presence of proton trap. Upon monoaddition, the formation of the stable allylic cation was confirmed by trapping the resulting cation with tributyltin hydride, which yielded PIB with dihydrofuran functionality. Quenching with methanol resulted in the quantitative formation of 2-alkyl-5-PIB-furan. The stable allylic cation was found to be an efficient initiating species for the polymerization of methyl vinyl ether (MeVE), yielding P(IB-b-MeVE) block copolymers by sequential monomer addition with up to 75% crossover efficiency. 2-PIB-Fu, a polymeric capping agent, was obtained in quantitative yields in a reaction of PIB+ with 2-Bu3SnFu in Hex/CH3Cl 60/40 v/v in the presence of TiCl4 at −80 °C. In a subsequent reaction of 2-PIB-Fu with living PIB+, under conditions similar to capping PIB+ with 2-tBuFu, close to quantitative coupling of the two PIB chains was achieved. The concept of coupling a living cationic polymer with an ω-furan functional polymer to obtain AB-type linear block copolymers and for the synthesis of ABC- and AA‘B-type three-arm star-block copolymers is presented.

A modified glass reactor for the preparation of linear and star-branched block copolymers via living anionic polymerization

A modular high-vacuum system for large-scale anionic polymerization reactions was designed that utilizes mechanical agitation in place of magnetic stirring, Teflon Rotoflo stopcocks in place of glass breakseals, and spherical o-ring joints in place of direct glass seals. A standard reactor body was used, and depending upon reactor design, it was fitted with appropriate reactant ampules, volumetric charging cylinders, etc., to facilitate polymerization, blocking reactions, and linking reactions with minimal effort. To demonstrate the efficacy of the system a number of linear and three-arm star-block copolymers comprised of oligostyrene outer blocks and polybutadiene inner blocks were synthesized. The block copolymers were characterized by narrow molecular weight dispersity, and the star polymers showed greater than 95% linking efficiency. The polybutadiene blocks contained approximately 40% 1,2-enchainment and were exhaustively hydrogenated using H2 and a nickel octoate/triethyl aluminum catalyst. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 66: 151–159, 1997