Molecular Brushes Research Articles

Linear and Star Block Copolymers of Styrenic Macromonomers by Anionic Polymerization

We describe the synthesis of a number of linear and star block copolymers where at least one block consists of a polymacromonomer (molecular brush). The key to this general technique is the use of styrene-tipped macromonomers, which polymerize in a manner similar to styrene monomer. By this means well-defined styrenic macromonomers of polyisoprene, PI (sMMI), polybutadiene, PB (sMMB), and poly(styrene-b-isoprene), PSI (sMMSI), have been prepared by selective reaction of PILi, PBLi, and PSILi with the chlorine of 4-(chlorodimethylsilyl)styrene. These macromonomers and conventional monomers (styrene, isoprene, and butadiene) were homopolymerized or block copolymerized, by anionic polymerization high-vacuum techniques, to synthesize the following structures: PsMM-b-PM, PS-b-PsMM-b-PM, and PsMM1-b-PsMM2 where PsMM is PsMMI or PsMMB and PM is PS, PI, or PB. Furthermore, by utilizing the chlorosilane method, star polymers of the (PS)2(PsMMB) and (PsMMI-b-PI)3 type were synthesized. Attempts to prepare A3 and A4 star polymers with polymacromonomer arms were not entirely successful. All materials were characterized by size exclusion chromatography (SEC) with DRI and UV detectors, SEC with two-angle-laser-light-scattering detector (SEC/TALLS), and NMR spectroscopy. It was concluded that macromonomers can be used much like the conventional monomers under appropriate experimental conditions.

Effect of the carbohydrate side-chain on the conformation of a glycoconjugate polystyrene in aqueous solution

An oligomaltose-carrying polystyrene “glycoconjugate polystyrene” was synthesized by the homopolymerization of 4-vinylbenzylamine oligomaltonic amides, derived from maltose, maltotriose, maltopentaose, and maltoheptaose. The resultant amphiphilic glycoconjugate polystyrenes were dissolved in 0.1 M aqueous urea, and their structures characterized by small-angle X-ray scattering and molecular modeling. “Glycoconjugate polystyrene” was found to behave as a “molecular bottle brush”, composed of a large pseudo-helical polystyrene backbone and carbohydrate brushes. A large pseudo-helical polystyrene backbone is formed by a random sequence of TT, TG, and/or TTGG. The results indicate that the cross-section of a backbone chain with smaller oligosaccharide side-chains is obliged to expand more than that with longer side-chains. Even with rigid hydrophilic pendant oligosaccharide chains, the larger pseudo-helix of the main chain could orient the side-chains so as to envelop the hydrophobic backbone in aqueous solution. Thus the conformation of the main chain is determined not only by the chemical nature of an oligosaccharide chain but also by its length.

Initiation Efficiency in the Synthesis of Molecular Brushes by Grafting from via Atom Transfer Radical Polymerization

Grafting from via atom transfer radical polymerization (ATRP) was employed for the synthesis of densely grafted molecular brushes. A poly(2-(2-bromopropionyl)oxyethyl methacrylate) (PBPEM) macroinitiator was used to polymerize n-butyl acrylate (BA) with various degrees of polymerization. The brush syntheses proceeded with first-order kinetic behavior, and the molecular weights determined by gel permeation chromatography coupled with multiangle laser light scattering (GPC-MALLS) increased linearly with conversion. The PBA side chains (SCs) were cleaved by acid solvolysis, and the resulting products were characterized in order to study the initiation process in the synthesis of molecular brushes. By comparing the molecular weight of the cleaved SCs to their theoretical molecular weights (calculated assuming quantitative initiation), the grafting efficiency was determined as a function of monomer conversion. In the early stages of polymerization, the initiation efficiency is low but gradually increases to 87% at 12% monomer conversion. The results were compared to an analogous linear ATRP conducted under identical conditions, except that ethyl 2-bromopropionate was employed as the initiator. The initiation efficiency for the linear polymerization was consistently higher than that observed for the brushes. The difference was attributed to the congested environment (high local concentration of initiation sites) encountered in the case of grafting from a macroinitiator backbone, which led to slower deactivation of growing radicals at low conversion. The initiation efficiency was enhanced by increasing the rate of deactivation of the growing species or decreasing the rate of propagation.

A covalent-chemistry approach to giant macromolecules and their wetting behavior on solid substrates.

Synthetic polymers commonly lack any sizable extension into the second dimension (thickness) and are best described as more or less flexible threads. Recently there have been two synthetic approaches to address the problem of how to make macromolecules “thick”: one is referred to as molecular brushes, the other as dendromized polymers. Dendromized polymers (denpols) differ from molecular brushes in that their side chains are not linear but highly and regularly branched (dendrons), which results in a considerable density of molecular segments in the direct neighborhood of the central backbone. Denpols, especially those of higher generation, have the built-in character of a cylindrical object, a feature which found application in basic experiments in nanoscience in which the bottom-up approach was investigated. These objects can reach lengths of several hundred nanometers and widths of 5–7 nm depending on their chemical structure and generation number and, in this regard, are amongst the largest molecules ever prepared. It seems difficult, for both synthetic and space reasons, to extend the denpol concept to a degree such that the dimensions of biologically relevant cylindrical objects such as the tobacco mosaic virus (TMV envelope: diameter 18 nm, length ca. 300 nm) can be mimicked. Therefore, we set out to merge the brush and denpol concepts. Herein, we report a joint synthetic and scanning force microscopy (SFM) inves-

Conformational Switching of Molecular Brushes in Response to the Energy of Interaction with the Substrate

Cylindrical brush molecules adsorbed on a surface change their contour length and then switch conformation from rodlike to globular upon decrease of the surface energy of the substrate. The conformational changes result from partial desorption of poly(n-butyl acrylate) side chains as the surface pressure drops from 23.7 to 3.1 mN/m and the energy of interaction between the side chains and the substrate decreases from 89.7 mJ/m2 to 69.1 mJ/m2. At the lowest value of the interaction energy, one observes a coexistence of rodlike and globular molecules. This result is in agreement with the theoretical prediction of the rod-globule transition of surface confined brush molecules as a conformational phase transition of the first order.

Multiarm Molecular Brushes: Effect of the Number of Arms on the Molecular Weight Polydispersity and Surface Ordering

Individual molecules of multiarm starlike molecular brushes were visualized by atomic force microscopy. In the studied series of brushes, the number of arms varied from one for a linear chain to four, while the length of the side chains was kept approximately constant. Molecular visualization provided a unique opportunity for independent size characterization of the brush arms separately from that of the entire molecule. In agreement with the Schulz-Flory theory for chain coupling, the polydispersity of the total length was significantly lower than that of the arm length. The variation in polydispersity had an effect on molecular ordering. Lateral compression of the starlike brushes caused a transition from an extended dendritic-like conformation to a compact disklike conformation. In contrast to one-, two-, and three-arm brushes, the four-arm molecules with a lower polydispersity index of 1.04 demonstrated local hexagonal order.

Tadpole Conformation of Gradient Polymer Brushes

Cylindrical molecular brushes, each with a gradient of grafting density along the backbone, demonstrated a transition from rodlike to tadpole conformations. Using atomic force microscopy (AFM), a coexistence of two conformational phases was observed within individual molecules adsorbed on a mica substrate. These observations were made by compressing monolayers on the surface of water and then transferring a sample of this monolayer to a mica substrate for AFM studies. Upon compression, the rod−globule transition occurs at the end where the brush is densely grafted, leaving a molecule with a globular “head” and an extended “tail”a so-called tadpole conformation. This research demonstrates the asymmetric changes in the molecular conformation, which is one of the prerequisites for directed motion.

Spontaneous Curvature of Comblike Polymers at a Flat Interface

Spontaneous curvature of molecular brushes adsorbed on a flat substrate was observed by scanning force microscopy for comblike polymers of poly(butyl acrylate). This phenomenon was analyzed theoretically for a 2D model of a brush molecule in the regime of strong adsorption. Although all side chains were confined to the substrate plane, they were allowed to flip over and change their position with respect to the backbone. Instability of the straight backbone was caused solely by the entropic elasticity of the side chains: smaller extension of the side chains was attained upon their uneven distribution relative to the backbone. An equilibrium, i.e., spontaneous, curvature resulted from competition between elasticity of the side chains and stabilizing factors such as inherent stiffness of the backbone, mixing entropy of the side chains and excluded volume interactions of distant parts of the brush. The curvature radius is found to be proportional to the degree of polymerization of the side chains. The effect of polydispersity of the side chains is discussed.

Interaction of Bovine Serum Albumin and Human Blood Plasma with PEO-Tethered Surfaces: Influence of PEO Chain Length, Grafting Density, and Temperature

Solid surfaces are modified by grafting poly(ethylene oxide), PEO, to influence their interaction with indwelling particles, in particular molecules of bovine serum albumin and human plasma proteins. As a rule, the grafted PEO layers suppress protein adsorption. The suppression is most effective when the PEO layer is in a molecular brush conformation having a reciprocal grafting density (area per grafted PEO chain) less than the dimensions of the protein molecules. Nevertheless, the protein molecules may penetrate the PEO brush to some extent. For a given grafting density, the penetration is facilitated by increasing thickness of the brush. Tenuous brushes of reciprocal grafting densities exceeding the protein molecular dimensions enhance protein adsorption. The results point to a weak attractive interaction between PEO and protein. The protein repellency of a densely PEO-brushed surface is ascribed to a high activation energy for the protein molecules to enter the brush. Varying the temperature between 22 and 38 degrees C does not significantly affect the range of grafting density over which the brush changes from protein-attractive to protein-repellent.

Temperature-Dependent Conformational Change of PNIPAM Grafted Chains at High Surface Density in Water

Poly(N-isopropylacrylamide) (PNIPAM) exhibits a lower critical solution temperature (LCST) of {approx}30 C in water that is attributed to alterations in the hydrogen-bonding interactions of the amide group. PNIPAM in various forms has been explored for a variety of applications including controlled drug delivery, solute separation, tissue culture substrates, and controlling the adsorption of proteins, blood cells, and bacteria. Grafting PNIPAM onto surfaces is a promising strategy for creating responsive surfaces, since the physical properties of PNIPAM are readily controlled by changing the temperature. Considerable effort has been devoted to studying variations in chain conformations with temperature (T) in PNIPAM-based materials. Kubota et al. studied conformational changes of PNIPAM free chains with temperature for molecular weights ranging from 1.63 x 10{sup 6} to 2.52 x 10{sup 7} g/mol (M{sub w}/M{sub n} > 1.3) in water using laser light scattering. They reported a decrease in the radius of gyration (R{sub g}) as the solution temperature increased above the LCST. The magnitude of the effect was more pronounced with increasing molecular weight, ranging up to a factor of two for the highest molecular weight sample. In a similar study, Wu et al. observed a decrease in R{sub g} of a factor of sevenmore » for a high molecular weight PNIPAM sample with very low polydispersity (M{sub w} = 1.3 x 10{sup 7} g/mol, M{sub w}/M{sub n} < 1.05). Regarding grafted PNIPAM chains, Kidoaki et al. recently employed an iniferter-based graft polymerization method to generate a dense, high molecular weight brush and reported changes in the thickness measured by AFM. The thickness of the grafted layer was obtained from AFM images of the boundary between grafted and nongrafted (ablated by laser light) regions. They found that the swollen film thickness decreased by a factor of {approx}2 with increasing temperature from 25 to 40 C for samples with a range of dry film thickness from 250 to 1500 {angstrom}. More recently, Balamurugan et al. used surface plasmon resonance (SPR) to probe conformational changes in a PNIPAM brush grafted onto a gold layer by atom transfer radical polymerization (ATRP). For a sample with a dry film thickness of 517 {angstrom}, the SPR measurements indicated a significant contraction (extension of the layer with increasing/decreasing) temperature through the transition. Quantification of the change in profile characteristics was not reported, but it was noted that the change in the SPR signal occurred over a much broader range of temperature (15-35 C) than is typical of the transition for free chains in bulk solution. No systematic study of detailed PNIPAM chain conformations has yet been reported as a function of the two critical brush parameters, the surface density and molecular weight. A recent theoretical analysis by Baulin and Halperin has identified the surface density as a critical parameter demarcating different regimes of behavior. This arises from the concentration dependence of the Flory {chi} parameter as obtained from a recent phase behavior study of free chains in solution. Little attention has been paid to the surface density in previous experimental studies of grafted PNIPAM chains. We have begun a systematic study of the temperature-dependent conformational changes of PNIPAM grafted chains in water as a function of surface density and molecular weight using neutron reflection (NR). In previous work, we investigated the conformational changes of PNIPAM chains tethered to silicon oxide using two methods. The first was the 'grafting from' method in which N-isopropylacrylamide monomers were polymerized from the silicon surface with a chain transfer, free-radical technique. In the second method, preformed PNIPAM chains with carboxylic acid end groups associated with terminal hydroxyl groups of a mixed self-assembling monolayer. Detailed concentration profiles of the PNIPAM brushes were determined in D{sub 2}O as a function of temperature and also in d-acetone at room temperature. Profiles were obtained in the two solvents in order to investigate the role of the solvent in mediating interactions. The profiles in D{sub 2}O were bilayers, composed of a very thin layer with higher concentration at the surface and a low concentration layer extending well into the subphase. The very thin, higher concentration surface layer was attributed to attractive segment-surface interactions. The profiles in acetone were smoothly decaying single-layer profiles. The low segment concentration at the surface in acetone indicated that the surface density of these brushes was rather low. The dry film thicknesses were less than 40 {angstrom}, much lower than in the study of Kidoaki et al. On the basis of the molecular weights and dry film thicknesses, the surface density ({sigma}, chains/{angstrom}{sup 2}) ranged from 1 x 10{sup -4} to 2 x 10{sup -4} for those samples.« less

Cylindrical molecular brushes under poor solvent conditions: microscopic observation and scaling analysis.

Axial contraction of cylindrical molecular brushes of polymethylmethacrylate was observed by static light scattering and scanning force microscopy. Single brush molecules were visualized as worm-like particles whose length was almost three times shorter than the contour length of the backbone. A somewhat larger length was measured by light scattering in a good solvent. A scaling approach has been used to analyze the driving forces for the axial contraction and the conformation of the molecular brushes.

The Strength of the Macromonomer Strategy for Complex Macromolecular Architecture: Molecular Characterization, Properties and Applications of Polymacromonomers

AbstractThe efficiency of the macromonomer (MM) synthetic strategy for the creation of nonlinear complex macromolecular architectures (miktoarm stars, α,ω‐branched polymers, random/exact comb and graft copolymers, dendritic polymers, molecular brushes) is reviewed. In addition, the solution/bulk properties and the potential applications of polymacromonomers (PMM), as well as new synthetic ideas are presented. The synthesis of macromonomers and living polymacromonomers in situ leads to many novel linear and nonlinear structures, as for example PMM‐b‐PS‐b‐PMM, (PS)2PMM. The use of multichlorosilylstyrenes as monomer/linking agents opens new ways for structures with multichain macromonomeric building blocks.The use of multichlorosilylstyrenes as monomer/linking agents opens new ways for structures having multichain macromonomeric building blocks.magnified imageThe use of multichlorosilylstyrenes as monomer/linking agents opens new ways for structures having multichain macromonomeric building blocks.

Effect of Initiation Conditions on the Uniformity of Three-Arm Star Molecular Brushes

A series of three-arm star densely grafted molecular brushes with poly(n-butyl acrylate) side chains were prepared by atom transfer radical polymerization (ATRP). The three-arm macroinitiators of poly(2-bromopropionyloxyethyl methacrylate) were prepared by ATRP of 2-trimethylsilyloxyethyl methacrylate using trifunctional initiators 1,1,1-tris(4-(2-bromoisobutyryloxy)phenyl)ethane and different catalytic systems, i.e., a CuIX/dNbpy complex (X = Br, Cl) with and without CuIIX2 (X = Br, Cl) as deactivators, followed by the subsequent esterification with 2-bromopropionyl bromide. The kinetics exhibited first-order reaction with respect to the monomer concentration in all systems. Polymerization rates decreased with the catalyst in the order of CuBr > CuBr/CuBr2 > CuCl > CuCl/CuCl2. The different initiation conditions also affect the polydispersities of the final brushes, although in all cases the number-average molecular weight (Mn) increases with the increasing monomer conversion. AFM measurements of multiarm brushes enabled quantitative analysis of the length distribution. Because of visualization of individual starlike molecules, one could separately analyze the brush arms and the whole molecule. The polydispersities of the arm length decreased in the order of CuBr > CuBr/CuBr2 > CuCl > CuCl/CuCl2. The CuCl/CuCl2 system seems to be the best catalyst for the star synthesis yielding brushes with narrow length distribution. CuBr is a relative “worst” catalyst system in which several coupled molecular brushes were observed, while some arms were missing. These led to the broadest distribution of arm length. Similar analysis for four-arm molecular brushes was performed. The polydispersity of the total length was significantly lower than those of the arm length. Here, the total length polydispersity displayed amazing agreement with Schulz−Flory theory for chain coupling.

Synthesis and Visualization of Densely Grafted Molecular Brushes with Crystallizable Poly(octadecyl methacrylate) Block Segments

Densely grafted molecular brushes with AB and ABA “block−graft” architectures, i.e., pODMA-b-p(BPEM-g-nBuA) and pODMA-b-p(BPEM-g-nBuA)-b-pODMA, were synthesized by the “grafting from” approach using atom transfer radical polymerization (ATRP). The backbone macroinitiators were prepared in three steps. First, ATRP of ODMA or HEMA-TMS was carried out from mono- or difunctional initiators to form macroinitiators, followed by ATRP of HEMA-TMS or ODMA initiated from these macroinitiators to obtain pODMA-b-pHEMA-TMS or pODMA-b-pHEMA-TMS-b-pODMA macromolecular backbone precursors. Finally, these precursors were esterified to pODMA-b-pBPEM and pODMA-b-pBPEM-b-pODMA backbone macroinitiators. The pnBuA side chains were grafted from the pBPEM segments of these backbone macroinitiators by ATRP. AFM measurement of the final molecular brushes demonstrated that the backbone of the brushes, the pBPEM segment, was almost completely stretched on the mica surface. pODMA segments tended to associate, due to crystallization o...

Synthesis of Molecular Brushes with Gradient in Grafting Density by Atom Transfer Polymerization

Macromolecular brushes with a gradient of side-chain spacing along the backbone have been synthesized by the “grafting from” approach using atom transfer radical polymerization. A macroinitiator was prepared in two steps, first by conducting a gradient copolymerization of MMA and HEMA-TMS followed by transformation of the resulting poly(MMA-grad-HEMA-TMS) to poly(MMA-grad-BPEM). The gradient composition of the macroinitiator was a forced gradient formed by continuous feeding of HEMA-TMS during MMA polymerization. The gradient structure was characterized by monitoring monomer conversion (GC) and molecular weight evolution (GPC) during copolymerization. AFM measurements demonstrated the characteristic anisotropy of the molecular structure by resolving individual brush molecules with a bulky head and a thin tail.

Effect of surface-grafted molecular brushes on the adhesion performance of bonded polymers and composite interfaces

This paper reviews the theoretical principles of the macromolecular design of polymer interface/interphase systems for obtaining maximum adhesion and fracture performance of composite materials and adhesively bonded assemblies. Subsequently, a relatively simple and industry-feasible technology for surface grafting molecular brushes is discussed in detail and supported by a range of experimental examples. It is shown, in agreement with contemporary theory, that the use of chemically attached graft chemicals of controlled spatial geometry and chemical functionality enables a significant increase in the strength and fracture energy of the interphase, to the point of cohesive fracture of the substrate, or that of an adjacent medium such as adhesive, elastomer or matrix material. This occurs even after prolonged exposure of investigated systems to adverse environments such as hot water.

Painting with a molecular brush: Genomic/proteomic interfacing to define the drug action profile of novel solid-tumor selective anticancer agents

XK469 is an investigational anticancer agent that exhibits antiproliferative activity in tumor-bearing animal models. We examined the drug-action profile of this agent at the molecular level regarding alterations induced in gene expression and proteins in HCT-116 human colon adenocarcinoma cells. We used a unique cDNA microarray (GeneMapTM Cancerarray) comprising 1152 human tumor-related genes and 2-D gel electrophoresis, respectively, following a 24-hour exposure to a drug concentration that killed a two-log fraction of HCT-116 clonogenic cells. Functional gene cluster profile (FGCP) analysis of the 71 out of 1152 genes that displayed a >2-fold increase or decrease in expression (over untreated control) identified a drug-specific involvement of the MAPK signal transduction pathway. MAPK signaling together with the involvement of ubiquitin proteins from 2-D gel electrophoresis suggest a novel drug-action profile at the molecular level for the in vitro antiproliferative activity of XK469. Cytometry 47:72–79, 2002. © 2001 Wiley-Liss, Inc.

Painting with a molecular brush: Genomic/proteomic interfacing to define the drug action profile of novel solid‐tumor selective anticancer agents

XK469 is an investigational anticancer agent that exhibits antiproliferative activity in tumor-bearing animal models. We examined the drug-action profile of this agent at the molecular level regarding alterations induced in gene expression and proteins in HCT-116 human colon adenocarcinoma cells. We used a unique cDNA microarray (GeneMap(TM) Cancerarray) comprising 1152 human tumor-related genes and 2-D gel electrophoresis, respectively, following a 24-hour exposure to a drug concentration that killed a two-log fraction of HCT-116 clonogenic cells. Functional gene cluster profile (FGCP) analysis of the 71 out of 1152 genes that displayed a >2-fold increase or decrease in expression (over untreated control) identified a drug-specific involvement of the MAPK signal transduction pathway. MAPK signaling together with the involvement of ubiquitin proteins from 2-D gel electrophoresis suggest a novel drug-action profile at the molecular level for the in vitro antiproliferative activity of XK469.

Formation of pore‐filled microfiltration membranes using a combination of modified interfacial polymerization and grafting

AbstractA novel method of fabricating pore‐filled membranes has been developed by coating a hydrophobic polyolefinic microfiltration membrane with polyethylenimine (PEI) to produce a chemically active surface. An evaporative coating technique was used to coat the internal surface of a polyolefinic membrane with PEI in chloroform (CHCl3). The coated PEI was then crosslinked by naphthalene‐1,4‐disulfonyl chloride in carbon tetrachloride (CCl4). The incorporation of the PEI coating changed the properties of the base membrane from hydrophobic to relatively hydrophilic; the nitrogens could be titrated, and the coating could not be washed out. 4,4′‐Azo‐bis(4‐cyanovaleryl chloride), a heat‐sensitive radical source, was incorporated into the coated membrane. The coated membrane, with incorporated azo compound, was heated to 75 °C in the presence of acrylic acid, 4‐(vinylpyridine), or styrene to give the corresponding grafted, pore‐filled membranes. The membranes exhibited hydraulic fluxes and pH valve effects consistent with molecular brushes grafted within the pores of the membranes. © 2001 John Wiley &amp; Sons, Inc. J Polym Sci Part A: Polym Chem 40: 242–250, 2002

Synthesis of Molecular Brushes with Block Copolymer Side Chains Using Atom Transfer Radical Polymerization

Brush macromolecules having poly(n-butyl acrylate-block-styrene) and poly(styrene-block-n-butyl acrylate) side chains have been synthesized by the “grafting from” approach using atom transfer radical polymerization (ATRP). The molecular weights of the resulting polymers were characterized by gel permeation chromatography (GPC) using refractive index and multiangle light scattering detection. The block copolymer side chains were cleaved from the backbone and analyzed by GPC, confirming the synthesis of well-defined copolymer brushes. Visualization of individual molecules by atomic force microscopy (AFM) enabled analysis of the conformation and microstructure of the brush macromolecules on mica surface. The brushes with the pnBuA core were almost fully stretched, while the inverted structure with the pS core exhibited longitudinal contraction compared to the contour length of the main chain. In addition, the poly(n-butyl acrylate-block-styrene) brushes demonstrated a characteristic necklace morphology which...