Multiarm Star Research Articles

Amphiphilic multiarm star polylactide with hyperbranched polyethylenimine as core: A systematic reinvestigation

The Sn(Oct)2 catalyzed polymerization condition of l-lactide (LLA) and racemic lactide (r-LA) using hyperbranched polyethylenimine (PEI) as the macroinitiator was optimized. Multiarm star polymers bearing PEI core and well-controlled poly(l-lactide) (PLLA) or poly(racemic lactide) (PDLLA) arms were successfully prepared under the optimized condition, including: (1) high concentration of Sn(Oct)2 catalyst was required; (2) with respect to the polymerization of r-LA, the optimal temperature was in the range of 115–130 °C; (3) as for the polymerization of LLA, the optimal temperature was only around 130 °C. Model experiments demonstrated that secondary amine could also effectively initiate the Sn(Oct)2 catalyzed polymerization of LA under the optimized condition, however, its initiation efficiency was usually a little less than 100%, unlike the primary amine and hydroxyl initiators. The results of Gel Permeation Chromatography demonstrated that the obtained multiarm star polymers with PDLLA arms had narrower dispersities than those with PLLA arms. Thermal analyses demonstrated that raising the arm numbers of the stars impaired their thermal stability a little. Stars with PDLLA arm were amorphous. The glass transition temperature of all the PDLLA-based polymers was similar and had no obvious relationship with the arm length, arm number and the molecular architecture. The crystallizability of star polymers with PLLA arm was weaker than that of linear PLLA, and only star polymers with long PLLA arm showed obvious crystallization. The guest encapsulation and release properties of the obtained star polymers were also investigated. It was found that their guest encapsulation capacity had correlation with the PLA arm length, the PEI core size and the degree of quaternization of PEI core, but had no relationship with the type of the PLA arm (PDLLA or PLLA). Whereas the guest release rate was strongly affected by the arm type.

Coarse graining of star-polymer – colloid nanocomposites

We consider mixtures of self-avoiding multiarm star polymers with hard colloids that are smaller than the star polymer size. By employing computer simulations, and by extending previous theoretical approaches, developed for the opposite limit of small star polymers [A. Jusufi et al., J. Phys.: Condens. Matter 13, 6177 (2001)], we coarse-grain the mixture by deriving an effective cross-interaction between the unlike species. The excellent agreement between theory and simulation for all size ratios examined demonstrates that the theoretical approaches developed for the colloidal limit can be successfully modified to maintain their validity also for the present case of the protein limit, in contrast to the situation for mixtures of colloids and linear polymers. We further analyze, on the basis of the derived interactions, the non-additivity parameter of the mixture as a function of size ratio and star functionality and delineate the regions in which we expect mixing as opposed to demixing behavior. Our results are relevant for the study of star-colloid nanocomposites and pave the way for further investigations of the structure and thermodynamics of the same.

New Improved Thermosets Obtained From Diglycidylether of Bisphenol A and a Multiarm Star Copolymer Based on Hyperbranched Poly(glycidol) Core and Poly(methyl methacrylate) Arms

AbstractA well‐defined multiarm star copolymer, hyperbranched poly(glycidol)‐b‐poly(methyl methacrylate) (PGOH‐b‐PMMA), is used as a modifier in the curing of diglycidylether of bisphenol A (DGEBA) using 1‐methyl imidazole (1MI) as anionic initiator. The effect of the polymer topology on the curing and gelation processes is studied. The addition of the PGOH‐b‐PMMA to the resin leaves the complex viscosity unaltered. The addition of the modifier decreases the shrinkage after gelation compared to that measured in the curing of the neat resin. By DMTA a single relaxation process in the pure DGEBA and modified thermoset is found. The addition of the star‐like modifier led to an improvement on the mechanical characteristics such as the impact strength and microhardness in comparison to the neat material.

Cyclodextrin-based dendritic supramolecules; new multivalent nanocarriers

Multiarm star copolymers containing a β-cyclodextrin core and poly(2-ethyl-2-oxazoline) arms (CD-POX) were synthesized and characterized. Synthesized CD-POX star copolymers were able to form supramolecular dendritic polymers (SDPs) in aqueous solutions. The ability of SDPs to encapsulate and transfer small guest molecules and also metal nanoparticles was investigated.

Dynamic-Covalent Macromolecular Stars with Boronic Ester Linkages

Macromolecular stars containing reversible boronic ester linkages were prepared by an arm-first approach by reacting well-defined boronic acid-containing block copolymers with multifunctional 1,2/1,3-diols. Homopolymers of 3-acrylamidophenylboronic acid (APBA) formed macroscopic dynamic-covalent networks when cross-linked with multifunctional diols. On the other hand, adding the diol cross-linkers to block copolymers of poly(N,N-dimethylacrylamide (PDMA))-b-poly(APBA) led to nanosized multiarm stars with boronic ester cores and PDMA coronas. The assembly of the stars under a variety of conditions was considered. The dynamic-covalent nature of the boronic ester cross-links allowed the stars to reconfigure their covalent structure in the presence of monofunctional diols that competed for bonding with the boronic acid component. Therefore, the stars could be induced to dissociate via competitive exchange reactions. The star formation-dissociation process was shown to be repeatable over multiple cycles.

Organobase‐Catalyzed Synthesis of Multiarm Star Polylactide With Hyperbranched Poly(ethylene glycol) as the Core

AbstractMultiarm star copolymers consisting of the polyether‐polyol hyperbranched poly(ethylene glycol) (hbPEG) as core and poly(L‐lactide) (PLLA) arms are synthesized via the organobase‐ catalyzed ring‐opening polymerization of lactide using hbPEG as a multifunctional macroinitiator. Star copolymers with high molecular weights up to 792 000 g mol−1 are prepared. Detailed 2D NMR analysis provides evidence for the attachment of the PLLA arms to the core and reveals that the adjustment of the monomer/initiator ratio enables control of the arm length. Size exclusion chromatography measurements show narrow molecular weight distributions. Thermal analysis reveals a lower glass transition temperature, melting point, and degree of crystallization for the star‐shaped polylactides compared to linear polylactide.

Covalently stabilized vesicles derived from amphiphilic multiarm star polymers: Preparation, characterization, and their capability of hosting different polarity of guests

The large amount of terminal hydroxyl groups of amphiphilic multiarm star copolymers with hyperbranched polyethylenimine (PEI) as the hydrophilic core and poly(ε-caprolactone) (PCL) arms as the hydrophobic shell were completely transformed into the radical-crosslinkable methacrylate (MA) groups. The resulting PEI-b-PCL-MA polymers could self-assemble into vesicles in water, which was verified by dynamic light scattering (DLS) and transmission electron microscopy (TEM). After crosslinking the intravesicular MA groups, covalently stabilized vesicles (CSVs) were generated. These CSVs were further characterized by DLS and TEM, and it was found that the corona of the vesicles was not the simple double-layer structure, but contained a certain amount of PEI-b-PCL unimolecular micellar units between the double-layer. These CSVs could accommodate both apolar and polar guests using their hydrophobic PCL zones and void cores, respectively. Moreover, these CSVs showed superior capacities for apolar guests to their noncrosslinked precursors and the corresponding traditional amphiphilic multishell star polymers. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012

Multiarm star poly(glycidol)- block-poly(styrene) as modifier of anionically cured diglycidylether of bisphenol A thermosetting coatings

Well-defined multiarm star copolymer poly(glycidol)- b-poly(styrene) (PGOH- b-PS) with an average number of PS arms per molecule of 85 has been prepared. The core first approach has been selected as the methodology using atom transfer radical polymerization (ATRP) of styrene to grow the arms from an activated hyperbranched poly(glycidol) as core. This activated hyperbranched macroinitiator was prepared by esterification of hyperbranched poly(glycidol) (PGOH) with 2-bromoisobutyryl bromide. PGOH- b-PS was used to modify diglycidylether of bisphenol A coatings cured by anionic ring-opening mechanism using 1-methyl imidazole as the initiator. The kinetics of the curing process, studied by dynamic scanning calorimetry (DSC), was not much affected when PGOH- b-PS was added to the formulation. By rheometry the effect of this new polymer topology on the complex viscosity ( η *) of the reactive mixture was analyzed. The phase-separation of the modified coatings was proved by dynamic thermomechanical analysis (DMTA) and electronic microscopy (SEM and TEM) showing nano- or microphase separation as a function of the modifier content. The addition of this star polymer led to increase in the rigidity in terms of Young's modulus and in the microhardness in comparison to neat DGEBA.

Synthesis of a new multiarm star polymer based on hyperbranched poly(styrene) core and poly(ε‐caprolactone) arms and its use as reactive modifier of epoxy thermosets

AbstractA well‐defined multiarm star copolymer poly(styrene)‐b‐poly(ε‐caprolactone) (PSOH‐b‐PCL) with an average number of PCL arms per molecule of 60 has been prepared. 4‐Chloromethyl styrene (4‐CMS) was polymerized by means of atom transfer radical polymerization (ATRP) to obtain a hyperbranched poly(styrene) with chlorines as terminal groups. Subsequently, chlorines were substituted by reaction with diisopropanolamine (DIPA) to give the hydroxyl‐ended derivative. Finally, the hydroxyl‐ended hyperbranched poly(styrene) has been used as a macroinitiator core to polymerize ε‐caprolactone by means of cationic ring‐opening polymerization so as to obtain the star copolymer. In a second step, PSOH‐b‐PCL was used as reactive modifier of diglycidylether of bisphenol A formulations cured by 1‐methyl imidazole (1‐MI) obtaining nanostructured thermosets. The curing process was studied by dynamic scanning calorimetry and Fourier transform infrared spectroscopy (FTIR). By rheometry, the effect of this new polymer topology on the complex viscosity (η*) of the reactive mixture and on the gelation process was also analyzed. The thermomechanical characteristics of the modified materials were determined. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011

Effect of polymer topology on the curing process and mechanical characteristics of epoxy thermosets modified with linear or multiarm star poly(ε-caprolactone)

A well-defined multiarm star copolymer, hyperbranched poly(glycidol)-b-poly(ε-caprolactone), with an average of 100–110 arms per molecule and a molecular weight of arms of 1000 g/mol (s-PCL) and a linear PCL analog (l-PCL) were used as modifiers in the curing of diglycidylether of bisphenol A (DGEBA) using ytterbium triflate as cationic initiator. The effect of the polymer topology on the curing and gelation was studied by dynamic scanning calorimetry (DSC) and rheometry. The addition of s-PCL to the resin left the complex viscosity (η∗) practically unaltered. In contrast the addition of l-PCL incremented substantially the viscosity. The addition of star-shaped modifiers decreased the shrinkage after gelation in a higher extent than the linear analog. The homogeneity of pure DGEBA and modified thermosets was proved by dynamic thermomechanical analysis (DMTA) and electronic microscopy (SEM). The addition of star-like structures led to a higher impact energy fracture in comparison to pure DGEBA and l-PCL modified thermosets and to a lower effect on the microhardness than the linear analog.

Amphiphilic multiarm star block copolymer-based multifunctional unimolecular micelles for cancer targeted drug delivery and MR imaging

We report on the fabrication of multifunctional polymeric unimolecular micelles as an integrated platform for cancer targeted drug delivery and magnetic resonance imaging (MRI) contrast enhancement under in vitro and in vivo conditions. Starting from a fractionated fourth-generation hyperbranched polyester (Boltorn H40), the ring-opening polymerization of ɛ-caprolactone (CL) from the periphery of H40 and subsequent terminal group esterification with 2-bromoisobutyryl bromide afforded star copolymer-based atom transfer radical polymerization (ATRP) macroinitiator, H40-PCL-Br. Well-defined multiarm star block copolymers, H40-PCL-b-P(OEGMA-co-AzPMA), were then synthesized by the ATRP of oligo(ethylene glycol) monomethyl ether methacrylate (OEGMA) and 3-azidopropyl methacrylate (AzPMA). This was followed by the click reaction of H40-PCL-b-P(OEGMA-co-AzPMA) with alkynyl-functionalized cancer cell-targeting moieties, alkynyl-folate, and T1-type MRI contrast agents, alkynyl-DOTA–Gd (DOTA is 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetrakisacetic acid), affording H40-PCL-b-P(OEGMA-Gd-FA). In aqueous solution, the amphiphilic multiarm star block copolymer exists as structurally stable unimolecular micelles possessing a hyperbranched polyester core, a hydrophobic PCL inner layer, and a hydrophilic P(OEGMA-Gd-FA) outer corona. H40-PCL-b-P(OEGMA-Gd-FA) unimolecular micelles are capable of encapsulating paclitaxel, a well-known hydrophobic anticancer drug, with a loading content of 6.67 w/w% and exhibiting controlled release of up to 80% loaded drug over a time period of ∼120 h. In vitro MRI experiments demonstrated considerably enhanced T1 relaxivity (18.14 s−1 mm−1) for unimolecular micelles compared to 3.12 s−1 mm−1 for that of the small molecule counterpart, alkynyl-DOTA–Gd. Further experiments of in vivo MR imaging in rats revealed good accumulation of unimolecular micelles within rat liver and kidney, prominent positive contrast enhancement, and relatively long duration of blood circulation. The reported unimolecular micelles-based structurally stable nanocarriers synergistically integrated with cancer targeted drug delivery and controlled release and MR imaging functions augur well for their potential applications as theranostic systems.

Unique slow dynamics and aging phenomena in soft glassy suspensions of multiarm star polymers

We use time-resolved rheology to elucidate the slow dynamics and aging in highly concentrated suspensions of multiarm star polymers. The linear and nonlinear rheological properties exhibit a terminal regime corresponding to a well-defined maximal relaxation time. Terminal relaxation is driven by arm relaxation which speeds up the escape of stars from their cages. The fact that the system fully relaxes and flows at long times has important consequences. The yield stress only exists in the limited range of frequencies or shear rates where solid-like behavior is observed. Aging is controlled by the total time elapsed after flow cessation and not by the time elapsed from flow cessation to the beginning of the measurement as in other glassy materials. Our results, which demonstrate the importance of particle architecture with respect to glassy dynamics, should be generic for long hairy particles.

Synthesis, characterization, and rheological properties of multiarm stars with poly(glycidol) core and poly(methyl methacrylate) arms by AGET ATRP

Well-defined multiarm star block copolymers poly(glycidol)-b-poly(methyl methacrylate) (PGOHBr-b-PMMAx) with an average number of PMMA arms of 85, 55, and 45 have been prepared. The core-first approach has been selected as the methodology using atom transfer radical polymerization (ATRP) of MMA from an activated hyperbranched poly(glycidol) as the core. These activated hyperbranched macroinitiators were prepared by esterification of hyperbranched poly(glycidol) (PGOH) with 2-bromoisobutyryl bromide. The effect of monomer/initiator ratio, catalyst concentration, time, temperature, and solvent on the growing of the arms has been studied in detail in order to optimize the process and to diminish the radical-radical coupling. The final products and intermediates were characterized by means of size exclusion chromatography (SEC), nuclear magnetic resonance (NMR) and Fourier transform-infrared (FTIR) spectroscopy. The length of PMMA arms was determined by SEC after cleavage of ester bond linked to PGOH core. Glass transition temperature (Tg), thermal stability and rheological properties of the multiarm star copolymers were also studied. Finally, tapping mode atomic force microscopy (TMAFM) allowed the clear visualization of nano-sized particles (80–200 nm) corresponding to individual star molecules. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011

“Arm-First” Synthesis of Core-Cross-Linked Multiarm Star Polyethylenes by Coupling Palladium-Catalyzed Ethylene “Living” Polymerization with Atom-Transfer Radical Polymerization

We report in this article the “arm-first” synthesis of core–shell-structured multiarm polyethylene (PE) star polymers having multiple linear-but-branched PE arms joined at a cross-linked polydivinylbenzene (polyDVB) core. This synthesis is achieved in two steps by coupling two mechanistically incompatible polymerization techniques, Pd-catalyzed coordinative ethylene “living” polymerization and atom-transfer radical polymerization (ATRP). Ethylene “living” polymerization was first carried out using a functionalized Pd–diimine catalyst, [(ArN═C(Me)–(Me)C═NAr)Pd(CH2)3C(O)O(CH2)2OC(O)C(CH3)2Br]+SbF6– (Ar = 2,6-(iPr)2C6H3) (1), to directly synthesize narrow-distributed PE macroinitiators (MIs) containing an end-capping 2-bromoisobutyryl group active for initiating ATRP. Featured with controllable molecular weights at low polydispersity, the PE MIs are employed subsequently in the second step to initiate cross-linking polymerization of divinylbenzene (DVB) via ATRP to obtain the core-cross-linked star polymers. As a demonstration, three PE MIs at different lengths (number-average molecular weight of 7.3, 10.3, and 13.7 kg/mol, respectively) were specifically synthesized and employed for star construction. In the ATRP step, the effects of the various polymerization parameters, including MI concentration, the molar ratio of DVB to MI, and MI length, on the polymerization kinetics, star yield, average arm number, and average molecular weights of the star polymers were investigated systematically. By controlling the polymerization parameters, a range of PE star polymers having narrow-distributed arm lengths (7.3–13.7 kg/mol) and controllable average arm numbers (ca. 5–43 per star) have thus been successfully synthesized. A study on dilution solution properties of these star polymers having various structural parameters reveals their spherical chain conformation and resemblance of rigid spheres and high-generation dendrimers, with their intrinsic viscosity depending primarily on arm length while not on arm number or polymer molecular weight.

Multiarm star poly(glycidol)‐block‐poly(ε‐caprolactone) of different arm lengths and their use as modifiers of diglycidylether of bisphenol a thermosets

AbstractWell‐defined multiarm star copolymers, hyperbranched poly(glycidol)‐b‐poly(ε‐caprolactone), with an average of 100–110 arms per molecule and a molecular weight of arms of 3000 g/mol (PGOH‐b‐PCL30) and 1000 g/mol (PGOH‐b‐PCL10) were synthesized by cationic ring‐opening polymerization of ε‐caprolactone from a poly(glycidol) core and used to modify diglycidylether of bisphenol A formulations. The curing process, studied by dynamic scanning calorimetry, was only slightly retarded when PGOH‐b‐PCLx were added to the formulation. By rheometry, the effect of this new topology and the arm length on the complex viscosity (η*) and gelation of the reactive mixture was analyzed in detail. The addition of these new reactive modifiers decreases the global shrinkage and increases the conversion at gelation. In addition, the modified thermosets have an improved reworkability. The homogeneity of pure DGEBA and modified thermosets was proved by dynamic thermomechanical analysis and electronic microscopy (FESEM). Addition of star‐like structures led to a more toughened fracture of the thermoset in comparison to pure DGEBA. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011

Phase behavior and self-assembly of PSn(P2VP-b-PAA)n multiarmed multisegmented star terpolymers with ampholytic arms

We report on the phase behavior and self-assembly phenomena of (polystyrene)n(poly(2-vinylpyridine)-b-poly(acrylic acid))n, PSn(P2VP-b-PAA)n, heteroarm star block terpolymers, in DMF/water solvent mixture. The stars were prepared using a one-pot, four-step anionic polymerization method following the “in–out” approach. These multiarmed, multisegmented star terpolymers self-assemble in selective media exhibiting pH responsiveness owing to the ampholytic nature of their diblock copolymer arms. Two soluble and two insoluble states were observed depending upon the pH of the medium. At low pH (pH 1.0), core–shell unimolecular micelles, with a hydrophobic PS core and P2VP, PAA segments in the shell, were formed. At pH 3.0, H-bonding and electrostatic interactions between the P2VP and PAA segments led to compact spheres. Finally, at pH 8.0 the stars were transformed from a bis-hydrophilic state to bis-hydrophobic state, leading to a network-like intermicellar assembly.

Synthesis of thermoresponsive unimolecular polymeric micelles with a hydrophilic hyperbranched poly(glycidol) core

This paper describes the reversible phase transition behavior of a thermoresponsive poly(N-isopropylacrylamide) (PNIPAM) shell at the surface of a hydrophilic core. Reversible addition-fragmentation transfer (RAFT) polymerization of N-isopropylacrylamide was conducted using a hydrophilic hyperbranched poly(glycidol) (HPG)-based macroRAFT agent. At lower temperatures (<30 °C), the resultant multiarm star block copolymer (HPG–PNIPAM) exists as unimolecular micelles, with hydrophilic HPG as the core and a densely grafted PNIPAM brush as the shell. In laser light scattering (LLS) studies, the concentration used for HPG–PNIPAM is 5 × 10−6 g ml−1, to avoid any possible aggregation between dendritic unimolecular micelles above the lower critical solution temperature (∼32 °C) of PNIPAM. What we observe for the phase transition of HPG–PNIPAM involves only unimolecular process. A combination of dynamic and static LLS studies of HPG–PNIPAM in aqueous solution reveals a reversible phase transition on heating and cooling. Dendritic multiarm star block copolymer hyperbranched poly(glycidol)–poly(N-isopropylacrylamide) (HPG–PNIPAM) was prepared by the reversible addition-fragmentation transfer polymerization technique. In aqueous solution, the unimolecular polymeric micelles HPG–PNIPAM have a core-shell nanostructure, with hydrophilic HPG as the core and thermoresponsive PNIPAM as the shell. Dynamic and static laser light scattering measurements show a unimolecular process of reversible phase transition behavior for the unimolecular polymeric micelles in dilute solution.

Synthesis and drug release properties of novel pH- and temperature-sensitive copolymers based on a hyperbranched polyether core

Multi-arm star amphiphilic hyperbranched copolymers with poly(2-(dimethylamino) ethyl methacrylate) (PDMAEMA) shell and hyperbranched poly(3-ethyl-3-(hydroxymethyl)oxetane) (HBPO) core were synthesized by reversible addition–fragmentation chain transfer method. The hyperbranched copolymers were further modified by succinic anhydride (SUC) to obtain the novel pH- and thermosensitive hyperbranched copolymer HBPO-star-PDMAEMAs-SUC. The composition and morphology of synthesized copolymers were investigated by 1H NMR, dynamic light scattering, and transmission electron microscopy. These copolymers exhibited phase transitions in response to pH and temperature. The pH-dependent release properties of the drug-loaded micelles were also investigated using indomethacin (IND) as a model drug. The IND-loaded micelles displayed a rapid drug release at an alkaline pH.

Multiarm star-like polydimethylsiloxanes based on dendrimers of the sixth generation

A series of star-like 128-arm polydimethylsiloxanes was synthesized by the anionic polymerization of hexamethylcyclotrisiloxane using the polylithium initiator, which was obtained from the carbosilane dendrimer of the sixth generation. The measured values of molecular weights and intrinsic viscosity made it possible to characterize the samples prepared as dense globular macromolecules. The behavior of the synthesized multiarm stars at the water-air interface was studied, and the cross-section surface areas of the macromolecules were calculated. A comparison of the obtained sizes with the calculated values suggests some deformation of the dendrite core due to the interaction of the arms with the water surface.

Multiarm star polymers with POSS at the periphery

AbstractAzide end‐functionalized polyhedral oligomeric silsesquioxane (POSS‐N3) was incorporated into the periphery of well‐defined alkyne‐polystyrene50‐poly(divinyl benzene) (alkyne‐PS50‐polyDVB) and alkyne‐poly(tert‐butyl acrylate)43‐poly(divinyl benzene) (alkyne‐PtBA43‐polyDVB) multiarm star polymers via highly efficient azide‐alkyne click reaction, resulting in POSS‐PS50‐polyDVB and POSS‐PtBA43‐polyDVB multiarm star block copolymers respectively, in the solution of tetrahydrofuran/N,N‐dimethyl formamide, CuBr/N,N,N′,N″,N″‐pentamethyldiethylenetriamine (PMDETA) at room temperature for 24 h. Linear precursors and star polymers obtained in this study were characterized 1H NMR, gel permeation chromatography (GPC), and triple detection GPC (TD‐GPC). Absolute molecular weight, hydrodynamic radius, and intrinsic viscosity ([η]) values for all star polymers were determined by TD‐GPC. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2010