Core Cross-linked Star Polymers Research Articles

Synthesis and Characterization of Star Polymer with Comb-like Poly(lauryl methacrylate) Arms by Atom Transfer Radical Polymerization via One-pot ‘Arm-First’ Method

Atom transfer radical polymerization (ATRP) was used to prepare core crosslinked star polymers with comb-like poly (lauryl methacrylate) (LMA) arms by one-pot “arm-first” method, involving the synthesis of comb-like PLMA arms, followed by their crosslinking, using a mixture of LMA monomer and ethylene glycol dimethacrylate (EGDMA) crosslinker. By adjusting the feeding time and level of EGDMA, a series of star-like polymers with various comb-like arms length and number can be obtained. The molecular architecture including radius of gyration (Rg), hydrodynamic radius (Rh) and intrinsic viscosity (ηi) etc. were characterized by a triple-detector gel permeation chromatography (GPC) equipped with a refractive index detector, viscometer detector and a multi-angle static laser light scattering (MALLS) detector. The thermal property and shearing stability of these star-like polymers were also investigated.

Organic Catalyst-Mediated Ring-Opening Polymerization for the Highly Efficient Synthesis of Polyester-Based Star Polymers

A facile, highly efficient, and metal-free synthesis of well-defined polyester-based core cross-linked star (CCS) polymers with yields of up to 96 % was achieved via an organic catalyst (i.e., methanesulfonic acid) mediated ring-opening polymerization (ROP) at room temperature, through either a two-pot or a one-pot, two-step strategy. CCS polymers with narrow molecular weight distributions (PDI ≤ 1.3) and macroinitiator (MI) conversions of 90-96% were prepared using poly(ε-caprolactone) (PCL) MIs with molecular weights ranging from 9.9 to 36.2 kDa and [4,4'-bioxepane]-7,7'-dione (BOD) as the cross-linker. Furthermore, transesterification was identified as being responsible for the small percentage of unincorporated low molecular weight polymer remaining and star-star couplings in the star formation. Compared to CCS polymers synthesized via the methanesulfonic acid-mediated ROP, CCS polymers prepared via ROP mediated by high transesterification rate catalysts (i.e., stannous octoate (Sn(Oct)2)) suffer from much lower star purity (ca. 70%) and star-star coupled products due to more prominent transesterification side-reactions.

One-pot RAFT synthesis of core cross-linked star polymers of polyPEGMA in water by sequential homogeneous and heterogeneous polymerizations

The arm-first approach is well accepted for the synthesis of core cross-linked star (CCS) polymers, which typically involves two separate steps, i.e., the initial synthesis and purification of the arm polymers followed by a second step synthesis of CCS. We report here a one-pot approach for the RAFT synthesis of a well-defined CCS in water without the separation and purification of the arm polymers. This novel approach consists of a homogeneous polymerization step to prepare the arm polymer, polyPEGMA, and a heterogeneous polymerization step to assemble the arm polymers into CCS polymers. Various parameters affecting the formation of the CCS were studied, including the type of cross-linker, spacing monomer, arm polymer concentration and arm polymer molecular weight. Cross-linkers of longer length provided the flexibility required to form high-quality CCS polymers. The use of spacing monomers in conjugation with cross-linkers reduced the stress during cross-linking and also provided enhanced driving force to produce well-defined CCS polymers. High-quality CCS polymers were prepared at relatively high polymer concentration (≥10%). The higher molecular weight arm polymer exhibited increased steric hindrance for CCS formation. Overall, this one-pot approach is highly efficient, environmentally benign and amenable to a variety of reaction conditions.

Honeycomb Films from Perfluoropolyether-Based Star and Micelle Architectures

A perfluoropolyether-b-poly(t-butyl acrylate) (PFPE-b-PtBA) block copolymer macroinitiator was used to prepare both core cross-linked star (CCS) polymers and micelles, whereby the outer shell and core, respectively, are comprised of fluorinated segments. The star polymer complete with PFPE outer shell was synthesised via atom transfer radical polymerisation (ATRP) and the arm-first approach, through cross-linking of the PFPE-b-PtBA macroinitiator with ethylene glycol diacrylate (EGDA). Alternatively, the PFPE-b-PtBA block copolymer could be self-assembled in benzene to form micelles with a PtBA shell and PFPE core. Both the micelle and CCS polymer were subsequently fabricated into non-cracking honeycomb (HC) patterned films on both planar and non-planar surfaces via the ‘Breath Figure’ (BF) technique using a static casting system.

Amphiphilic heteroarm star polymer synthesized by RAFT dispersion polymerization in water/ethanol solution

Well-defined amphiphilic heteroarm core cross-linked star (CCS) polymer was efficiently synthesized by RAFT-mediated arm-first strategy in dispersion polymerization, and its direct self-assembly in water was demonstrated.

Synthesis and Visualization of a Core Cross-Linked Star Polymer Carrying Optically Active Rigid-Rod Helical Polyisocyanide Arms and Its Chiral Recognition Ability

Synthesis and Visualization of a Core Cross-Linked Star Polymer Carrying Optically Active Rigid-Rod Helical Polyisocyanide Arms and Its Chiral Recognition Ability

Folic Acid Conjugated Amino Acid-Based Star Polymers for Active Targeting of Cancer Cells

Amino acid-based core cross-linked star (CCS) polymers (poly(L-lysine)(arm)poly(L-cystine)(core)) with peripheral allyl functionalities were synthesized by sequential ring-opening polymerization (ROP) of amino acid N-carboxyanhydrides (NCAs) via the arm-first approach, using N-(trimethylsilyl)allylamine as the initiator. Subsequent functionalization with a poly(ethylene glycol) (PEG)-folic acid conjugate via thiol-ene click chemistry afforded poly(PEG-b-L-lysine)(arm)poly(L-cystine)(core) stars with outer PEG coronas decorated with folic acid targeting moieties. Similarly, a control was prepared without folic acid, using just PEG. A fluorophore was used to track both star polymers incubated with breast cancer cells (MDA-MB-231) in vitro. Confocal microscopy and flow cytometry revealed that the stars could be internalized into the cells, and higher cell internalization was observed when folic acid moieties were present. Cytotoxicity studies indicate that both stars are nontoxic to MDA-MB-231 cells at concentrations of up to 50 μg/mL. These results make this amino acid-based star polymer an attractive candidate in targeted drug delivery applications including chemotherapy.

Enthalpy and Volume Relaxation of Core-Crosslinked Star Polystyrene/Poly(methyl methacrylate) Blends

Blends of linear and core-crosslinked star (CCS) polymers are prepared. The relaxation and thermal properties of the blends are determined using conventional and modulated-temperature DSC and modulated-temperature thermomechanometry. Addition of CCS polymers increase the glass transition temperature while decreasing the enthalpy and the linear coefficient of thermal expansion, suggesting that the hyperbranched polymers restrict matrix chain motions. Isothermal annealing increased the Tg and ΔH and decreased sub-Tg α due to the reduced free volume and mobility within the polymer films. Volume relaxation during annealing is observed using mT-TM, while a stretched-exponential function is utilised to interpret the data. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Synthesis of a Star Polymer Library with a Diverse Range of Highly Functionalized Macromolecular Architectures

An efficient and versatile synthetic route toward highly functionalized core cross-linked star (CCS) polymers with interesting structures and properties is presented using an alkyne CCS polymer as scaffold. The alkyne CCS polymer scaffold was initially prepared via an improved arm-first approach, through ring-opening polymerization (ROP) of 4,4′-bioxepanyl-7,7′-dione (BOD) with a poly(caprolactone-b-propargyl methacrylate) macroinitiator and stannous triflate (Sn(OTf)2) catalyst. Highly functionalized fluorescent, saccharide and amphiphilic CCS polymers were synthesized by grafting the alkyne CCS polymer with the corresponding azido substituted compounds via copper catalyzed 1,3-dipolar azide−alkyne cycloaddition (CuAAC), ‘click’ chemistry. The resulting corona-functionalized CCS polymers were characterized via GPC, 1H NMR spectroscopic analysis and DLS. 1H NMR spectroscopic analysis revealed that the grafting efficiency (i.e., click efficiency) ranged from 10 to >99% and was highly dependent on the structure and functionality of the azido compounds. This equates to the grafting of 45 to 450 functional compounds onto the CCS polymer scaffolds corona. The results indicate that the click functionalization efficiency is closely related to the molecular size of the azido compounds. Other than size, factors including molecular structure, compatibility and synergistic driving forces, such as the formation of potential inclusion complexes, are also found to affect the functionalization efficiencies.

Highly Efficient Synthesis of Low Polydispersity Core Cross‐Linked Star Polymers by Ru‐Catalyzed Living Radical Polymerization

The efficient formation of low polydispersity core cross-linked star (CCS) polymers via controlled/living radical polymerization (LRP) and the arm-first approach was found to be dependent on the mediating catalyst system. The Ru catalyst, Ru(Ind)Cl(PPh₃)₂ Cat. 1, and tertiary amine co-catalyst were used to synthesize highly living poly(methyl methacrylate) (PMMA) macroinitiators, which were then linked together with ethylene glycol dimethacrylate (EGDMA) to form PMMA(arm)PEGDMA(core) CCS polymers. The quantitative and near-quantitative synthesis of CCS polymers were observed for low to moderate molecular weight macroinitiators (M(n) = 8 and 20 kDa), respectively. Lower conversions were observed for high-molecular weight macroinitiators (M(n) ≥ 60 kDa). Overall, an improvement of between 10 and 20% was observed when comparing the Cat. 1 system to a conventional Cu-catalyzed system. This significant improvement in macroinitiator-to-star conversion is explained in the context of catalyst system selection and CCS polymer formation.

The formation of core cross-linked star polymer and nanogel assemblies facilitated by the formation of dynamic covalent imine bonds

Reversible addition-fragmentation chain transfer (RAFT) polymerization has been utilised to prepare diblock copolymer chains possessing ‘inert’ blocks of polystyrene and ‘reactive’ blocks displaying aldehyde or amino functional groups. These polymer chains were shown to cross-link through the formation of imine bonds in organic solvents to form kinetically stable polymer assemblies possessing core-cross-linked star (CCS) polymers architectures which display a size-dependency upon the concentration at which the cross-linking reactions are performed. Evidence was found for the formation of a 2-armed species as a side-product in the formation of CCS polymers. RAFT polymerization was utilised to prepare a series of methacrylate copolymers possessing only ‘reactive’ blocks displaying a significantly lower density of aldehyde and amino functional groups than the diblock copolymers utilized in the formation of CCS polymers. These copolymers were shown to cross-link through the formation of imine bonds in organic solvents to form polymer assemblies possessing nanogel architectures. The lower density of functional groups was required to promote the formation of discrete nanogel assemblies. This cross-linking also displayed a size-dependency upon the concentration at which the cross-linking reactions are performed. Macroscopic gelation of these polymer chains was observed at higher concentrations of the copolymer chains. Both the CCS polymers and nanogels underwent structural reconfiguration to linear polymer chains through component exchange facilitated by trans-imination with a small molecule amine. We speculate that the formation of both the CCS polymers and nanogels is under kinetic control on account of the lack of reversibility of imine bonds in organic solvents.

Star polymers composed entirely of amino acid building blocks: a route towards stereospecific, biodegradable and hierarchically functionalized stars

Highly functionalized water soluble core cross-linked star (CCS) polymers having degradable cores and hierarchical functionalities spanning from the peripheral groups along the arms to the core have been synthesized entirely from amino acid building blocks. The core-isolated moieties were shown to undergo further reactions, such as click chemistry, as well as being capable of encapsulating water-insoluble drugs.

Efficient and versatile synthesis of star polymers in water and their use as emulsifiers

Core cross-linked star polymers of low polydispersity were efficiently prepared in high yield by RAFT-mediated emulsion and dispersion polymerizations in water at high solid content. These star polymers were demonstrated to be effective emulsifiers, and the emulsion was successfully used as template to fabricate polymer particles.

PH triggered self-assembly of core cross-linked star polymers possessing thermoresponsive cores

Core cross-linked star polymers possessing responsiveness to pH and temperature stimuli have been prepared, and we demonstrate how changes to pH and temperature can be used to trigger the release and uptake of a hydrophobic dye.

Thermosets with core–shell nanodomain by incorporation of core crosslinked star polymer into epoxy resin

Core crosslinked star (CCS) polymers, which have crosslinked poly (divinyl benzene-co-styrene) [P(DVB-St)] core and multiple arms of polystyrene-b-poly(ethylene oxide) diblock copolymer (PEO-b-PS) [denoted as PEO-b-PS/P(DVB-St) CCS], were synthesized via atom transfer radical polymerization(ATRP). PEO-b-PS/P(DVB-St) CCS polymer was spherical with average diameters of scores of nanometers from transmission electron microscopy (TEM) and dynamic light scattering (DLS), and blended with diglycidyl ether of bisphenol (DGEBA) and 4,4′-diamino diphenyl methane (DDM) in tetrahydrofuran (THF). With 5 or 10 wt% PEO-b-PS/P(DVB-St) CCS polymer, spherical core–shell nanodomains with average diameters of 29 or 32 nm were observed from atomic force microscopy (AFM), which were randomly distributed in the resultant thermosets. Considering the difference in miscibility of the epoxy with P(DVB-St) and PEO-b-PS after and before curing reaction, a reaction-induced microphase separation (RIMPS) mechanism was proposed to account for the formation of the core–shell nanodomains in the thermosets. During curing, the RIMPS of PS subchain occurred but was confined by P(DVB-St) core, resulting in formation of thermoplastic PS shell around the crosslinked core. Such core–shell nanodomain could be easily etched away by THF, whereas the control thermosets containing PEO/P(DVB-St) CCS polymer could not be etched by THF. The glass transition temperatures (Tgs) of the epoxy thermosets containing PEO-b-PS/P(DVB-St) CCS polymer were significantly improved compared with pure epoxy thermosets.

Synthesis of Core Cross-Linked Star Polymers Consisting of Well-Defined Aromatic Polyamide Arms

Homopolymer-arm, block-arm, and miktoarm star polymers consisting of poly(N-octyl-m-benzamide) and poly(N-H-m-benzamide) were synthesized by means of a core cross-linking method. Macromonomers (MM) with the styryl terminal moiety were synthesized by chain-growth condensation polymerization of 3-(alkylamino)benzoic acid esters 1 in the presence of phenyl 4-vinylbenzoate as an initiator, and copolymerization with N,N′-methylenebis(acrylamide) as a divinyl monomer in the presence of 2,2′-azobis(isobutyronitrile) at 60 °C yielded the corresponding star polymers. In the synthesis of star polymers containing poly(N-H-m-benzamide) arms, the 4-(octyloxy)benzyl group on the amide nitrogen of the obtained star polymers was removed with trifluoroacetic acid. The number of arms per molecule, determined by multiangle laser light scattering, varied in the range of 36−100 depending on the N-alkyl group of 1 and the molecular weight of MM. The 1H NMR spectra of the star polymers in dimethyl sulfoxide revealed that the po...

1,1‐Diphenyl Ethylene‐Mediated Radical Polymerisation: A General Non‐Metal‐Based Technique For The Synthesis Of Precise Core Cross‐Linked Star Polymers

This communication details the successful synthesis of low polydispersity core cross-linked star (CCS) polymers via DPE-mediated polymerisation. We demonstrate the ability to produce poly(methyl methacrylate) and poly(acrylonitrile) CCS polymers that are currently inaccessible via the two most common non-metal-based controlled radical polymerisation techniques (NMP and RAFT polymerisations).

The formation of core cross-linked star polymers containing cores cross-linked by dynamic covalent imine bonds

Diblock copolymers possessing amino or aldehyde functions within one of their blocks were prepared using RAFT polymerization techniques. These polymers were shown to cross-link through dynamic imine bonds to form core cross-linked star polymers which display a size-dependency upon the concentration at which the cross-linking reactions are performed.

Degradable star polymers with high “click” functionality

AbstractDegradable polyester‐based star polymers with a high level of functionality in the arms were synthesized via the “arms first” approach using an acetylene‐functional block copolymer macroinitiator. This was achieved by using 2‐hydroxyethyl 2′‐methyl‐2′‐bromopropionate to initiate the ring‐opening polymerization (ROP) of caprolactone monomer followed by an atom transfer radical polymerization (ATRP) of a protected acetylene monomer, (trimethylsilyl)propargyl methacrylate. The hydroxyl end‐group of the resulting block copolymer macroinitiator was subsequently crosslinked under ROP conditions using a bislactone monomer, 4,4′‐bioxepanyl‐7,7′‐dione, to generate a degradable core crosslinked star (CCS) polymer with protected acetylene groups in the corona. The trimethylsilyl‐protecting groups were removed to generate a CCS polymer with an average of 1850 pendent acetylene groups located in the outer block segment of the arms. The increased functionality of this CCS polymer was demonstrated by attaching azide‐functionalized linear polystyrene via a copper (I)‐catalyzed cycloaddition reaction between the azide and acetylene groups. This resulted in a CCS polymer with “brush‐like” arm structures, the grafted segment of which could be liberated via hydrolysis of the polyester star structure to generate molecular brushes. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 1485–1498, 2009

Fabrication of Reversibly Crosslinkable, 3‐Dimensionally Conformal Polymeric Microstructures

AbstractMultifaceted porous materials were prepared through careful design of star polymer functionality and properties. Functionalized core crosslinked star (CCS) polymers with a low glass transition temperature (Tg) based on poly(methyl acrylate) were prepared having a multitude of hydroxyl groups at the chain ends. Modification of these chain ends with 9‐anthracene carbonyl chloride introduces the ability to reversibly photocrosslink these systems after the star polymers were self‐assembled by the breath figure technique to create porous, micro‐structured films. The properties of the low Tg CCS polymer allow for the formation of porous films on non‐planar substrates without cracking and photo‐crosslinking allows the creation of stabilized honeycomb films while also permitting a secondary level of patterning on the film, using photo‐lithographic techniques. These multifaceted porous polymer films represent a new generation of well‐defined, 3D microstructures.