Synthesis and Characterization of PNIPAm Core Cross‐Linked Star Polymers and Their Functionalization with Cyclodextrin

Poly(N‐isopropylacrylamide) (PNIPAm) core cross‐linked star (CCS) polymers (s‐(PNIPAm)_n) are synthesized by arm‐first ATRP method. The related synthesis conditions are investigated and optimized. By varying cross‐linker N,N‐methylenebisacrylamide (BIS) concentration, PNIPAm CCS polymers with about 47, 86, and 211 arms are synthesized. Then, under ATRP condition, the “living” sites at the core of s‐(PNIPAm)_n reacting with a monovinyl β‐cyclodextrin (β‐CD) monomer afford β‐CD functionalized s‐(PNIPAm)_n (CDF‐SPNIPAm). The structures of the star polymers are characterized. The results indicate that in CDF‐SPNIPAm, the ratio of β‐CD units to PNIPAm arm numbers could be up to 0.6:1. The fluorescence spectra of star polymer/ANS (8‐­anilino‐1‐naphthalenesulfonic acid ammonium salt hydrate) systems prove that the β‐CD moieties of CDF‐SPNIPAm are available for including guest molecules. By using pH‐sensitive adamantyl (Ada)‐terminated poly(4‐vinylpyridine) (Ada‐P4VP) (synthesized by ATRP strategy) as a model guest macromolecule, the host–guest complexation between β‐CD units of CDF‐SPNIPAm and adamantyl groups of Ada‐P4VP is confirmed via 2D NOESY ¹H NMR and DLS measurements. The results indicate that the presence of the Ada‐P4VP arms provides temperature‐responsive star polymers with pH sensitivity. Therefore, the β‐CD‐functionalized star PNIPAm could provide host macromolecular platform for constructing novel miktoarm star polymers.

     

Nanoprecipitation of PMMA Stabilized by Core Cross‐Linked Star Polymers

Nanoprecipitation of poly(methyl methacrylate) (PMMA) in the presence of water‐soluble core cross‐linked star (CCS) polymers is investigated. Slowly dropping water into DMF solutions containing PMMA and CCS polymers of varying compositions produces colloidally stable particles with CCS polymers being effectively incorporated. During the solvent‐shifting process, CCS polymers migrate to and concentrate at the particle surface to stabilize the formed particles, which is confirmed by X‐ray photoelectron spectroscopy, transmission electron microscopy, and contact angle measurements. This stabilizing effect is realized via the steric effect of the CCS polymers adsorbed at the particle surface.

     

Synthesis and Characterization of Polymer Brushes on Micromechanical Cantilevers

Summary: A technique to cover microelectromechanical systems (MEMS), such as micromechanical cantilever (MC) sensors, with a covalently bound brush layer has been developed. The polymer layer was grown using a “grafting‐from” synthesis of polymer brushes under mild conditions, by surface‐initiated atom transfer radical polymerization. Atomic force microscopy (AFM) and ellipsometry have revealed a uniform thickness of about 12 nm from which a grafting density of polymer brushes of 0.19 chains · nm^?2 was estimated. The coating with polymer brushes can be realized on a selected surface. It was shown that a single‐sided brush layer swells reversibly in toluene, resulting in a bending of the micromechanical cantilever.

Schematic representation of the PMMA brush synthesis on the MC surface, by surface‐initiated ATRP.     

Polystyrene‐block‐Polyisoprene Diblock‐Copolymer Micelles: Coupled Pressure and Temperature Effects

The structural changes of diblock‐copolymer micelles under pressures from 200 to 16 000 psi are investigated using small‐angle neutron scattering (SANS). Asymmetric polystyrene‐block‐polyisoprene (PS–PI) diblock copolymers are dissolved in decane, a selective solvent for PI, to form spherical micelles with a core of PS and a corona of PI. The micellar solutions are put under pressure at temperatures of 25 to 60 °C. At room temperature, elevating the pressure from 200 to 16 000 psi has no effect on the size of the micelles. While the micellar solutions remain stable, instantaneous association of micelles is detected. In contrast to micelles at atmospheric pressure, increasing the temperature at elevated pressures does not lead to dissociation of micelles; instead, the micelles aggregate and evolve into sheet‐like structures, reminiscent of a macroscopic phase separation. Furthermore, higher pressures lead to a smaller temperature range in which shape transitions take place.

     

A Cleavable Network Based on Crosslinked Star Polymers Containing Acid‐Labile Diacetal Crosslinks: Synthesis,Characterization and Hydrolysis

A hydrolyzable model network comprising interconnected star polymers was prepared by the sequential group transfer polymerization of methyl methacrylate and the acid‐labile diacetal‐based dimethacrylate crosslinker bis[(2‐methacryloyloxy)ethoxymethyl] ether. In contrast to other polymer networks previously synthesized by our group, all the branching points of this polymer network were found to hydrolyze under mildly acidic conditions, giving a linear copolymer with the theoretically expected molecular weight and composition. The ease of hydrolysis of this polymer network renders it a good candidate for use in the biomedical field. The characterization of the synthesized network, its linear and star polymer precursors and the hydrolysis products of the network and its precursors, by a variety of techniques, established the successful synthesis and hydrolysis of this well‐defined polymer nanostructure.

     

SAXS from Four‐Arm Polyelectrolyte Stars in Semi‐Dilute Solutions

We have performed small‐angle X‐ray scattering experiments on semi‐dilute solutions of highly charged star polyelectrolytes. Poly(sodium acrylate) (PANa) and poly(cesium acrylate) (PACs) stars with four arms were successfully synthesized by a combination of atom transfer radical polymerization and chemical modifications. Over a wide range of polyelectrolyte concentration C_p, these two systems as well as their equivalent linear polyelectrolytes were investigated at different ionic strengths. Scattering experiments show the existence of a scattering peak denoted as q_max, which disappears with the addition of a simple electrolyte, evidencing the electrostatic character of the interactions. We have also studied the effect of the charge parameter and the nature of the counterion (Na and Cs) on the scattering properties of these star polyelectrolytes. In the case of PACs, q_max scales with the polyelectrolyte concentration as C_p^1/2 over the whole range of studied concentrations, whereas it scales as C_p^1/2 (for C_p < 45 mg · ml^?1) and C_p^1/4 (for C_p > 45 mg · ml^?1) in the case of PANa.

Variation of q_max as a function of polyelectrolyte concentration C_p for PA1Cs (○) and PA2Cs (?) stars (error bars are indicated).     

Nanoscopic Polymer Particles with a Well‐Defined Surface: Synthesis,Characterization, and Properties

We review our recent work on polymeric latex particles having a well‐defined surface of grafted polymers. Two types of particles are discussed: i) particles with a thermosensitive shell, and ii) particles bearing a dense layer of linear polyelectrolyte chains on their surface (“spherical polyelectrolyte brush”). Both types of particles consist of a polystyrene core onto which either linear or crosslinked chains are grafted. The synthesis of these core–shell particles proceeds in a two‐step process which is reviewed in detail. In particular, we discuss the photo‐emulsion polymerization used for the synthesis of the spherical polyelectrolyte brushes. Most of the studies of these particles have been done in aqueous suspension. Here the equilibrium structure of the surface layer of the particles has been investigated by dynamic light scattering and by small‐angle X‐ray scattering. Results deduced from the flow behavior of the particles in dilute solution can directly be derived from static data. Very recent data refer to the interaction of the spherical polyelectrolyte brushes with solid interfaces. It is demonstrated that particles bearing positively charged polyelectrolyte chains interact strongly with negatively charged surfaces. The strong interaction that is directly evident from AFM studies can be used to generate 2D networks of the particles on the surface.

Schematic representation of the synthesis of defined spherical polyelectrolyte brushes.     

Synthesis and Characterization of Styrene/Butyl Acrylate Linear and Star Block Copolymers via Atom Transfer Radical Polymerization

Summary: Well‐defined styrene (S) and butyl acrylate (BA) linear and star‐like block copolymers are synthesized via atom transfer radical polymerization (ATRP) using di‐ and trifunctional alkyl halide initiators employing the Cu/PMDETA (N,N,N′,N″,N″‐pentamethyldiethylenetriamine) catalyst system. Initial addition of Cu^II deactivator and utilization of halogen exchange techniques suppresses the coupling of radicals and improves cross‐propagation to a large extent, which results in better control over the polymerization. Two types of star‐like PBA/PS block copolymers are prepared by using core‐first techniques: a trifunctional PBA or PS macroinitiator extended with the other monomer. Block copolymers with a well‐defined structure and low polydispersity (PDI = ) are obtained in both cases. A trifunctional PBA₃ macroinitiator with = 136 000 g · mol^?1 and PDI = 1.15 is extended to (PBA‐PS)₃ star‐like block copolymer with = 171 100 g · mol^?1 and PDI = 1.15. A trifunctional PS₃ macroinitiator with = 27 000 g · mol^?1 and PDI = 1.16 g · mol^?1 is extended to (PS‐PBA)₃ with = 91 500 g · mol^?1 and PDI = 1.40. The individual star‐like macromolecules as well as their aggregates are visualized by atomic force microscopy (AFM) where the PS and PBA adopt the globular and extended conformation, respectively. For the PBA core star block copolymers, PS segments tend to aggregate either intramolecularly or intermolecularly. PS core star block copolymers form aggregates with a PS core and emanating PBA chains. Most aggregates have ‘n × 3’ arms but minor amounts of ‘defective’ stars with 4, 5, 8, or 11 arms are also observed. The AFM analysis shows that PS core star block copolymers contain about 92% three‐arm block copolymers, and the efficiency of cross‐propagation is 97.3%.

Schematic representation of the synthesis of BA/S star‐like block copolymers by ATRP, and their resultant AFM images.     

Polymerization of Styrene Sulfonate Ethyl Ester and Styrene Sulfonate Dodecyl Ester by ATRP: Synthesis and Characterization of Polymer Brushes

Summary: Cylindrical polymer brushes from PSSE and PSSD have been synthesized by a ‘grafting from’ approach by ATRP in solution. Starting from macroinitiators with 570 and 1 600 repeat units, and 10–100% reactive site density, PSSD brushes with 10–40% grafting density and PSSE brushes with 10–100% grafting density are obtained. The brushes have been characterized by ¹H NMR spectroscopy, GPC‐MALLS, analytical ultracentrifugation, AFM, and TEM.

Phase AFM image of a PSSD brush. The image was obtained using a ‘supercantilever’ (tapping mode, mica).     

Ultra‐Small‐Angle X‐Ray Scattering Study of PET/PC Nanolayers and Comparison to AFM Results

The forced assembly of two immiscible polymers, produced by layer‐multiplying co‐extrusion, is analyzed by means of USAXS. Comparison of scattering and AFM results sheds light on many details of the nanolayered structure in PET/PC films. The role played by the volume concentration and cold crystallization of PET on the experimental scattering is discussed. The appearance of at least two scattering maxima in all cases, corresponding to higher orders of the same repeating distance, accounts for the high regularity of the developed nanostructure. It is finally shown that long spacing values, derived from a localized area in AFM, are in a good agreement with the USAXS values averaged over much larger areas.

     

Reaction Kinetics and Thermodynamic Aspects of Thermoreversibly Cross‐Linked Polymer Networks

The Diels–Alder reaction of furan and maleimide is used to prepare reversibly cross‐linking polymer gels consisting of furan‐bearing polymer chains and bismaleimides. The roles of chain length, stoichiometry, and concentration on gelation kinetics and other properties are evaluated. Additionally, kinetics of the Diels–Alder reactions in the gelling system are compared with the kinetics of small molecule systems. By combining spectroscopic and rheological techniques, a mechanochemical analysis is performed.

     

Investigation of Thermoresponsive Nano‐Confined Polymer Brushes by AFM‐Based Force Spectroscopy

PNIPAM polymer brushes are synthesized in nanopores of track‐etched membranes. The internal structure of these nano‐confined brushes is studied by AFM force spectroscopy experiments as a function of the synthesis conditions. The approach force vs. distance profiles are fitted using the Alexander‐de Gennes model to determine the grafting density, and the worm‐like chain model is used to estimate the chain length from the retract profiles. The grafting density for brushes synthesized within 80 nm pores is ten times lower than that obtained in 330 nm pores. For the same polymerization time, the chain lengths are much lower if the synthesis is carried out in small pores. The elastic properties of the brushes are also directly related to the confinement conditions.     

Tailor‐Made Antimicrobial/Antiviral Star Polymer via ATRP of Cyclodextrin and Guanidine‐Based Macromonomer

Novel star‐like polymers are prepared via atom transfer radical polymerization (ATRP) of polyhexamethylene guanidine hydrochloride (PHMG) macromonomer and acrylamide (AM) using β‐cyclodextrin (CD) with 8‐active and 5‐active sites as a macroinitiator. The resulting star‐like polymers are characterized by gel permeation chromatography (GPC) and ¹H NMR and are used for deactivating bacteria and viruses. It is found that star polymers with comparable amounts of PHMG possess excellent antimicrobial activity, which, however, strongly depends on the topological structure (i.e., the arm number and the monomer ratio) of the composing copolymers. The in vitro antibacterial activities of the synthesized polymers are investigated against Escherichia coli in terms of the minimum inhibitory concentration (MIC), whereas the antiviral activity of star copolymers is assessed via a plaque assay against non‐enveloped adenovirus (ADV). The results show that the highest antimicrobial activity is achieved by the star‐like copolymer with the monomer ratio of 20:3 (AM:PHGM, mol/mol), while the number of functional arms is fixed at 8. The incorporation of PHMG also renders the star copolymer highly antiviral, thus permitting it to be used as an effective antibacterial/antiviral agent for various applications.

     

Iridium(III) Complexes with PEO and PS Polymer Macroligands and Light‐Emitting Properties: Synthesis and Characterization

Summary: On the basis of terpyridine functionalized poly(ethylene oxide) (PEO) and poly(styrene) (PS), a series of light‐emitting iridium(III) compounds was effectively synthesized. The respective iridium(III) target compounds were prepared by grafting chloro‐bridged precursor complexes [Ir(ppy)₂‐μ‐Cl]₂ (ppy = phenylpyridine) and [Ir(ppy‐CHO)₂‐μ‐Cl]₂ (ppy‐CHO = 4‐(2‐pyridyl)benzaldehyde) onto terpyridine functionalized PEO and PS tails. 1D and 2D NMR characterization was performed revealing the expected resonances. Gel permeation chromatography (GPC) proved the stability and purity of the targeted materials. Preliminary investigations of the light‐emitting properties were carried out by standard methods such as UV‐vis and steady‐state luminescence spectroscopy. The morphology and the quality of films of these iridium(III) compounds were furthermore investigated using AFM. Improved stability on the electrode surface was illustrated using cyclic voltammetry. One of the polymer materials was compared to the neat complex, which showed quick degradation.

Green and orange light‐emitting iridium(III) complexes with two cyclometalating ligands and one terpyridine ligand with PS tails.     

Methacryloyl and/or Hydroxyl End‐Functional Star Polymers Synthesized by ATRP Using the Arm‐First Method

Star polymers with methacryloyl groups at the chain ends were synthesized by esterification of hydroxyl end‐functional star polymers. First, the linear hetero‐telechelic macroinitiator (MI) was prepared by ATRP using a functional initiator. The obtained MI was chain‐extended and cross‐linked with DVB and then esterified into methacryloyl chain‐end functionality. The degree of esterification was controlled by changing the initial molar ratios of methacryloyl chloride to hydroxyl groups. The methacryloyl‐containing star polymers were cross‐linked either intermolecularly or intramolecularly under heating or UV irradiations, depending on the concentrations of star polymers during the cross‐linking reactions.

     

Effects of Intermicellar Interactions on the Dissociation of Block Copolymer Micelles: SANS and NMR Studies

The effects of intermicellar interactions on the dissociation of block copolymer micelles of poly­styrene‐block‐polyisoprene in a selective solvent, decane, are investigated using small‐angle neutron scattering (SANS) and ¹H NMR spectroscopy. This well‐studied polymer is used as a model system to correlate intermicellar interactions with overall micellar stability. Decane is a preferential solvent for polyisoprene (PI) and drives the association of the polystyrene (PS) blocks, resulting in spherical micelles with a PS core and a Gaussian PI corona. The dissociation of the PS–PI micelles is triggered by increasing temperature, while the intermicellar interactions are controlled by varying the polymer concentration and modulating temperature. With increasing temperature, the cores of the micelles first swell, followed by a breakdown to smaller micelles, with similar shapes, that eventually dissociate into single molecules. Herein, it is shown for the first time that enhancing the intermicellar interaction delays the dissociation process of the micelles.

     

Rapid Synthesis and MALDI‐ToF Characterization of Poly(ethylene oxide) Multiarm Star Polymers

Multiarm PEO star polymers with a purely aliphatic polyether structure have been synthesized using hyperbranched polyglycerol (PG) with different molecular weights as a multifunctional initiator. Different degrees of deprotonation of the initiator were studied with respect to molecular weight control. The results show that the degree of deprotonation is a crucial parameter for the synthesis of well‐defined polymers with controlled molecular weights. Partial deprotonation of the PG hydroxyl groups (5–8%) was proven to represent an optimum for the synthesis of star polymers with molecular masses close to the theoretical values. Molecular weights of the stars ranged between 9 000 and 30 000 g · mol^?1. MALDI‐ToF spectra confirmed that the PEO arms in the star polymers possess homogeneous lengths.

     

Characterization of Linear and 3‐Arm Star Block Copolymers by Liquid Chromatography at Critical Conditions

Summary: LCCC for polyMA homopolymers was established in order to analyze the polyMA‐polySt linear and star block copolymers. The validity of the assumption that under the LCCC for polyMA, the polyMA segment in the polyMA‐containing block copolymer is chromatographically “invisible” was verified. It was found that within the scale of investigation ( ), the molecular weight and architecture of the polyMA segments had no evident influence on the retention behavior of the polySt‐polyMA block copolymers and the polyMA block in the copolymer was “invisible”. The critical conditions of polyMA were used for quantitative analysis of the polySt block in the linear and 3‐arm star polyMA‐polySt block copolymers, which were synthesized by AGET ATRP in miniemulsion. It was shown that the copolymer had completely different elution peak from its MI. The calculated molecular weights of polySt blocks in the block copolymers were similar to those obtained from normal SEC analysis. Transferring the eluates from the LCCC (the first dimension) column to a SEC column (the second dimension) produced LCCC × SEC two‐dimensional chromatogram, which contained information on both chemical composition and molecular weight of the synthesized copolymers. The combination of these liquid chromatography methods clearly confirmed the high initiation efficiency of the polyMA MIs during the synthesis of block copolymers and the presence of a byproduct formed by radical‐radical coupling.

     

Multi‐Arm Star Polyglycerol‐block‐poly(tert‐butyl acrylate) and the Respective Multi‐Arm Poly(acrylic acid) Stars

Summary: Well‐defined multi‐arm star block copolymers, polyglycerol‐block‐poly(tert‐butyl acrylate) (PG‐b‐PtBA), with average arm‐numbers of 17, 27, 36, 66 and 90 arms, respectively, have been prepared by atom transfer radical polymerization (ATRP) of tBA in acetone, using a core‐first strategy. After hydrolysis with excess concentrated HCl in refluxing dioxane, full hydrolysis of the tert‐butyl ester groups was achieved, resulting in multi‐arm star polyelectrolytes, polyglycerol‐block‐poly(acrylic acid) (PG‐b‐PAA). The hyperbranched macroinitiators employed were prepared on the basis of hyperbranched polyglycerols via esterification with 2‐bromoisobutyryl bromide. Both CuBr/PMDETA and CuBr/Me₆TREN catalyst systems have been employed for ATRP of tBA. CuBr/PMDETA was found to permit good control. Polydispersity indices for the new multi‐arm stars were mainly in the range of 1.22 to 1.4, and the absolute data were in agreement with the calculated values. Moreover, kinetic curves show a linear dependence of ln([M]₀/[M]_t) on time, confirming that the polymerizations are controlled.

Relationship between arm numbers of the multi‐arm stars and maximum conversion achieved.     

Morphology Control of Highly Sulfonated Block Copolymers by a Simple Thermal Process

Well‐defined sulfonated block copolymers were prepared by direct thermolysis with block copolymers of n‐butyl acrylate (nBA) and neopentyl styrene sulfonated (NSS), which were synthesized by Cu‐based living radical polymerization ( <1.20). A simple thermal process for 10 min at 150 °C completely deprotected the neopentyl groups in the poly(NSS) block segment to give fully sulfonated polystyrene backbone. SAXS profile of the block copolymer with 47 wt.‐% of poly(NSS) showed lamella structure, which appeared more clearly with long ranged order after sulfonation of the block copolymer.