Synthesis of chiral amphiphilic diblock copolymers via consecutive RAFT polymerizations and their aggregation behavior in aqueous solution

Two chiral amphiphilic diblock copolymers with different relative lengths of the hydrophobic and hydrophilic blocks, poly(6‐O‐p‐vinylbenzyl‐1,2:3,4‐Di‐O‐isopropylidene‐D ‐galactopyranose)‐b‐poly(N‐isopropylacrylamide) or poly(VBCPG)‐b‐poly(NIPAAM) and poly(20‐(hydroxymethyl)‐pregna‐1,4‐dien‐3‐one methacrylate)‐b‐poly(N‐isopropylacrylamide) or poly(MAC‐HPD)‐b‐poly(NIPAAM) were synthesized via consecutive reversible addition‐fragmentation chain‐transfer polymerizations of VBCPG or MAC‐HPD and NIPAAM. The chemical structures of these diblock copolymers were characterized by ¹H nuclear magnetic resonance spectroscopy. These amphiphilic diblock copolymers could self‐assemble into micelles in aqueous solution, and the morphologies of micelles were investigated by transmission electron microscopy. By comparison with the lower critical solution temperatures (LCST) of poly(NIPAAM) homopolymer in deionized water (32 °C), a higher LCST of the chiral amphiphilic diblock copolymer (poly(VBCPG)‐b‐poly(NIPAAM)) was observed and the LCST increased with the relative length of the poly(VBCPG) block in the copolymer from 35 to 47 °C, respectively. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 7690–7701, 2008     

Amphiphilic diblock,triblock, and star block copolymers by living radical polymerization: Synthesis and aggregation behavior

Copper(I)‐mediated living radical polymerization was used to synthesize amphiphilic block copolymers of poly(n‐butyl methacrylate) [P(n‐BMA)] and poly[(2‐dimethylamino)ethyl methacrylate] (PDMAEMA). Functionalized bromo P(n‐BMA) macroinitiators were prepared from monofunctional, difunctional, and trifunctional initiators: 2‐bromo‐2‐methylpropionic acid 4‐methoxyphenyl ester, 1,4‐(2′‐bromo‐2′‐methyl‐propionate)benzene, and 1,3,5‐(2′‐bromo‐2′‐methylpropionato)benzene. The living nature of the polymerizations involved was investigated in each case, leading to narrow‐polydispersity polymers for which the number‐average molecular weight increased fairly linearly with time with good first‐order kinetics in the monomer. These macroinitiators were subsequently used for the polymerization of (2‐dimethylamino)ethyl methacrylate to obtain well‐defined [P(n‐BMA)_x‐b‐PDMAEMA_y]_z diblock (15,900; polydispersity index = 1.60), triblock (23,200; polydispersity index = 1.24), and star block copolymers (50,700; polydispersity index = 1.46). Amphiphilic block copolymers contained between 60 and 80 mol % hydrophilic PDMAEMA blocks to solubilize them in water. The polymers were quaternized with methyl iodide to render them even more hydrophilic. The aggregation behavior of these copolymers was investigated with fluorescence spectroscopy and dynamic light scattering. For blocks of similar comonomer compositions, the apparent critical aggregation concentration (cac = 3.22–7.13 × 10^?3 g L^?1) and the aggregate size (ca. 65 nm) were both dependent on the copolymer architecture. However, for the same copolymer structure, increasing the hydrophilic PDMAEMA block length had little effect on the cac but resulted in a change in the aggregate size. © 2002 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 40: 439–450, 2002; DOI 10.1002/pola.10122     

侧基带有L-苯丙氨酸的手性两亲嵌段共聚物的合成及其自组装行为

N3-苯丙氨酸与嵌段共聚物聚乙二醇-b-聚炔丙基缩水甘油(MPEO-b-PGPE)发生"click"反应,合成了具有光学活性的两亲嵌段共聚物聚乙二醇-B-聚L-苯丙氨酸三唑基缩水甘油(MPEO-b-PGTP),用1H-NMR和元素分析对其结构和组成进行表征.并对其自组装行为进行研究,滴体积法测定MPEO-b-PGTP溶...     

Macromolecular engineering by controlled/living ionic and radical polymerizations

Various methods for the synthesis of well‐defined (co)polymers with controlled dimension, polydispersity, topology, composition and functionality are discussed. They include controlled/living vinyl polymerization using anionic, cationic and radical intermediates being in equilibria with dormant species. Special emphasis is placed on the radical polymerization and on the needs for the comprehensive structure property correlation.     

Novel segmented copolymers by combination of controlled ionic and radical polymerizations

Transformation of different living and non‐living polymerization mechanisms to controlled/“living” atom transfer radical polymerization (ATRP) in order to prepare block and graft copolymers is described. The synthesis and characterization of macroinitiators and the resulting segmented copolymers is discussed.     

Peptide-polymer bioconjugates: hybrid block copolymers generated via living radical polymerizations from resin-supported peptides

A novel strategy for the preparation of peptidic-synthetic bioconjugate block copolymers is based upon sequential condensation and living radical addition polymerizations, each performed upon a solid support.     

Solvent effect on the aggregation behavior of rod-coil diblock copolymers

The water-induced aggregation behavior of rod-coil diblock copolymers based on poly(ethylene oxide) (PEO) and poly{(+)-2,5-bis[4'-((S)-2-methylbutoxy)phenyl]styrene} (PMBPS), PEO104-b-PMBPS53, was investigated in the common solvent THF and in the selective solvent dioxane. Before adding water, PEO104-b-PMBPS53 stayed as single polymer chains no matter what conformation the PEO block took (i.e., either the random coil conformation in THF or the compact globule conformation in dioxane). The critical water content ( approximately 6 wt %) at which PEO104-b-PMBPS53 began to aggregate was also similar in both solvents, indicating that PMBPS dominated the aggregation process. However, the size, the size distribution, and the morphology of aggregates in THF/water were quite different from those in dioxane/water. Narrowly distributed spheres with Rh approximately 20 nm were observed in dioxane, whereas in THF, a bimodal distribution peaked at 3 and approximately 300 nm, was observed. The results from 2D wide-angle X-ray diffraction and polarized optical microscopy demonstrated that the PMBPS blocks packed in a parallel pattern upon aggregation in dioxane/water. The anisotropic disclike structures observed in THF/water also indicated the orientation of the PMBPS blocks upon forming aggregates in dilute solution.     

New amphiphilic diblock copolymers: surfactant properties and solubilization in their micelles

Several series of amphiphilic diblock copolymers are investigated as macrosurfactants in comparison to reference low-molar-mass and polymeric surfactants. The various copolymers share poly(butyl acrylate) as a common hydrophobic block but are distinguished by six different hydrophilic blocks (one anionic, one cationic, and four nonionic hydrophilic blocks) with various compositions. Dynamic light scattering experiments indicate the presence of micelles over the whole concentration range from 10(-4) to 10 g x L(-1). Accordingly, the critical micellization concentrations are very low. Still, the surface tension of aqueous solutions of block copolymers decreases slowly but continuously with increasing concentration, without exhibiting a plateau. The longer the hydrophobic block, the shorter the hydrophilic block, and the less hydrophilic the monomer of the hydrophilic block is, the lower the surface tension is. However, the effects are small, and the copolymers reduce the surface tension much less than standard low-molar-mass surfactants. Also, the copolymers foam much less and even act as anti-foaming agents in classical foaming systems composed of standard surfactants. The copolymers stabilize O/W emulsions made of methyl palmitate as equally well as standard surfactants but are less efficient for O/W emulsions made of tributyrine. However, the copolymer micelles exhibit a high solubilization power for hydrophobic dyes, probably at their core-corona interface, in dependence on the initial geometry of the micelles and the composition of the block copolymers. Whereas micelles of copolymers with strongly hydrophilic blocks are stable upon solubilization, solubilization-induced micellar growth is observed for copolymers with moderately hydrophilic blocks.     

Liquid crystalline side chain-coil diblock copolymers by living free radical polymerization

A series of LC-coil diblock copolymers was prepared using living radical polymerization. The resulting polymers were shown to possess a smectic mesophase and a lamellar microstructure by X-ray diffraction. The microstructure slowly develops on annealing as suggested by DSC studies.     

Photo-switchable azobenzene-functionalized block copolymers from atom transfer radical polymerizations

Diblock copolymers composed of monomers of tert-butyl acrylate and a side-chain azobenzenecontaining monomer, 4-[(E)-(4-nitrophenyl)diazenyl]phenyl prop-2-enoate were synthesized using atom transfer radical polymerization technique. Experimental strategy involved synthesis of block of tert-butyl acrylate macroinitiator followed by addition of second block of azobenzene-containing monomer to prepare desired block-copolymer. GPC analysis indicated narrow molecular weight distributions with degree of polymerization found in good agreement with targeted value. Prepared block copolymers of varying chain lengths can potentially be used to obtain morphologies that can find useful applications for biomedical applications including intriguing photo-switchable drug delivery systems.     

Novel fluoroalkyl end-capped amphiphilic diblock copolymers with pH/temperature response and self-assembly behavior

A series of fluoroalkyl end-capped diblock copolymers of poly[2-(N,N-dimethylamino)ethyl methacrylate] (PDMAEMA or PDMA) and poly[2-(N,N-diethylamino)ethyl methacrylate] (PDEAEMA or PDEA) have been synthesized via oxyanion-initiated polymerization, in which a potassium alcoholate of 4,4,5,5,6,6,7,7,7-nonafluoro-1-heptanol (NFHOK) was used as an initiator. The chemical structures of the NFHO-PDMA-b-PDEA and NFHO-PDEA-b-PDMA depended on the addition sequence of the two monomers and the feeding molar ratios of [DMA] to [DEA] during the polymerization process. These copolymers have been characterized by (1)H NMR and (19)F NMR spectroscopy and gel permeation chromatography (GPC). The aggregation behavior of these copolymers in aqueous solutions at different pH media was studied using a combination of surface tension, fluorescence probe, and transmission electron microscopy (TEM). Both diblock copolymers exhibited distinct pH/temperature-responsive properties. The critical aggregation concentrations (cacs) of these copolymers have been investigated, and the results showed that these copolymers possess excellent surface activity. Besides, these fluoroalkyl end-capped diblock copolymers showed pH-induced lower critical solution temperatures (LCSTs) in water. TEM analysis indicated that the NFHO-PDMA(30)-b-PDEA(10) diblock copolymers can self-assemble into the multicompartment micelles in aqueous solutions under basic conditions, in which the pH value is higher than the pKa values of both PDMA and PDEA homopolymers, while the NFHO-PDEA(10)-b-PDMA(30) diblock copolymers can form flowerlike micelles in basic aqueous solution.     

Controlling polymer structures by atom transfer radical polymerization and other controlled/living radical polymerizations

Fundamentals of controlled/living radical polymerization (CRP) and Atom Transfer Radical Polymerization (ATRP), relevant to the synthesis of controlled polymer structures are described. Macromolecular brushes with star like structure are used as an example to illustrate synthetic power of ATRP.     

Synthesis and aggregation behavior of four-arm star amphiphilic block copolymers in water

We report the first synthesis of amphiphilic four-arm star diblock copolymers consisting of styrene (STY) and acrylic acid (AA) made using reversible addition-fragmentation chain transfer (RAFT; Z group approach with no star-star coupling). The polymerization proceeded in an ideal "living" manner. The size of the poly(AA(132)-STY(m)4 stars in DMF were small and close to 7 nm, suggesting no star aggregation. Slow addition of water (pH = 6.8) to this mixture resulted in aggregates of 15 stars per micelle with core-shell morphology. Calculations showed that the polyAA blocks were slightly extended with a shell thickness of 15 nm. Treatment of these micelles with piperidine to cleave the block arms from the core resulted in little or no change on micelle size or morphology, but the polyAA shell thickness was close to 29 nm (33 nm is the maximum at full extension) suggesting a release of entropy when the arms are detached from the core molecule. In this work we showed through the use of star amphiphilic polymers that the micelle size, aggregation number, and morphology could be controlled.     

Double-polyelectrolyte, like-charged amphiphilic diblock copolymers: swollen structures and pH- and salt-dependent lyotropic behavior

We consider a symmetrical poly(styrene- stat-(acrylic acid))- block-poly(acrylic acid), i.e., PSAA- b-PAA, diblock copolymer, with a molar fraction phi AA = 0.42 of acrylic acid, in the more hydrophobic PSAA statistical first block. We investigate its structural behavior at constant concentration in water using small-angle neutron scattering (SANS) by varying (i) the ionization of its acrylic acid motives via the pH by adding NaOH and (ii) the ionic strength of the solution by increasing the NaCl salt concentration c S. We present the resulting morphological phase diagram {pH, c S}, in which we identified two different lamellar phases presenting a smectic long-range order at small-to-intermediate ionizations and a spherical phase with a liquid-like short-range order at larger ionization. In the low-ionization regime, the first lamellar phase comprises a water-free PSAA lamellar core surrounded by a dense poly(acrylic acid) brush swollen with water. Its mostly hydrophobic core still being glassy, this phase is unable to reorganize and is frozen in. A detailed analysis of the SANS data shows the osmotic nature of the polyelectrolyte brush, in which the Na+ counterions are confined so that local electroneutrality is satisfied. Above the pH at which the PSAA statistical block starts ionizing, the PSAA lamellar core melts. The second lamellar phase identified then comprises a PSAA core thinner than that of the frozen-in previous phase, implying a significant increase of the core/water interface and a decrease of the brush surface density. The transition from the first lamellar phase to the second one can be quantitatively shown to result from the balance between the two contributions: (i) the extra interfacial cost between the thinner core and water and (ii) the associated gain in entropy of mixing for the counterions confined inside the brush. At even higher ionization, the diblocks finally form spherical objects with a very small, pH-dependent aggregation number and reach an apparent onset of self-association. When the highest ionization investigated is reached, the cores of these final spherical core-shell objects are found to contain a significant amount of water. We thereby demonstrate that at constant concentration, pH, and ionic strength both trigger a transition from frozen to molten hydrophobic phases as well as unexpected morphological transitions.     

Mechanisms and kinetics of living radical polymerizations

The polymerization rates and activation processes of several variants of living radical polymerization (LRP) are discussed on the basis of recent experimental and theoretical results. Because of bimolecular termination, which is inevitable in LRP as well as in conventional radical polymerization, the time‐conversion curves of LRP have several characteristic features depending on the experimental conditions, such as the presence or absence of conventional initiation. Despite the presence of termination (and initiation, in some cases), polymers obtained by LRP can have a low polydispersity, provided that the number of terminated chains is small compared to the number of potentially active chains. A large rate constant of activation, k_act, is another fundamental requisite for low polydispersities. Systematic experimental investigation into k_act has clarified the exact mechanisms of activation in several LRP systems. The magnitudes of k_act was found to largely differ from system to system.     

Novel diblock copolymer‐grafted multiwalled carbon nanotubes via a combination of living and controlled/living surface polymerizations

Diels–Alder cycloaddition reactions were used to functionalize multiwalled carbon nanotubes (MWNTs) with 1‐benzocylcobutene‐1′‐phenylethylene (BCB‐PE) or 4‐hydroxyethylbenzocyclobutene (BCB‐EO). The covalent functionalization of the nanotubes with these initiator precursors was verified by FTIR and thermogravimetric analysis (TGA). After appropriate transformations/additions, the functionalized MWNTs were used for surface initiated anionic and ring opening polymerizations of ethylene oxide and ε‐caprolactone (ε‐CL), respectively. The OH‐end groups were transformed to isopropylbromide groups by reaction with 2‐bromoisobutyryl bromide, for subsequent atom transfer radical polymerization of styrene or 2‐dimethylaminoethyl methacrylate to afford the final diblock copolymers. ¹H NMR, differential scanning calorimetry (DSC), TGA, and transmission electron microscopy (TEM) were used for the characterization of the nanocomposite materials. TEM images showed the presence of a polymer layer around the MWNTs as well as the dissociation of MWNT bundles. Consequently, this general methodology, employing combinations of different polymerization techniques, increases the diversity of diblocks that can be grafted from MWNTs. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 1104–1112, 2010     

pH/enzyme/light triple-responsive vesicles from lysine-based amphiphilic diblock copolymers

We report the synthesis of pH- and enzyme-responsive amphiphilic diblock copolymers through reversible addition-fragmentation chain transfer polymerization of a lysine-derived methacrylate monomer comprising p-nitrobenzyl carbamate (pNBC) functionality using a poly(ethylene glycol)-modified macro-chain transfer agent. Depending on the hydrophobic block length, the diblock copolymers self-assemble to form spherical micelles, wormlike micelles, and bilayered vesicles in the aqueous solution. The responsive behaviors of the polymeric vesicles to pH, enzyme, and light are investigated in detail. As the pH lowers to pH 5.0, the polymeric vesicles undergo a morphological transition from vesicles to spherical micelles. In the presence of nitroreductase and a cofactor NADH, the decomposition of pNBC releases the ε-NH₂ of the lysine moiety and hence induces the generation of the vesicles with crosslinked membranes at pH 7.4. Moreover, owing to the degradation of pNBC moiety under UV irradiation, the polymeric vesicles also demonstrate a photo-responsive feature. As the irradiation time prolongs, it is observed a light-triggered morphological transition from vesicles to wormlike micelles with network-like structures.     

Stereoblock copolymers and tacticity control in controlled/living radical polymerization

Three controlled/living radical polymerization processes, atom transfer radical polymerization (ATRP), reversible addition-fragmentation transfer (RAFT) polymerization, and nitroxide-mediated polymerization (NMP), were investigated for the polymerization of N,N-dimethylacrylamide in the presence of Lewis acids known to enhance isotacticity, such as yttrium trifluoromethanesulfonate (Y(OTf)(3)) and ytterbium trifluoromethanesulfonate (Yb(OTf)(3)). Poly(N,N-dimethylacrylamide) with controlled molecular weight, low polydispersity (M(w)/M(n) < 1.2), and a high proportion of meso dyads ( approximately 85%) was prepared by ATRP (with initiating system methyl 2-chloropropionate/CuCl/Me(6)TREN) and RAFT (with cumyl dithiobenzoate transfer agent) in the presence of Y(OTf)(3). The combination of NMP (using N-tert-butyl-1-diethylphosphono-2,2-dimethylpropyl nitroxide, SG1) and a Lewis acid complexation technique led to less precise control over chain architecture and microstructure ( approximately 65% meso dyads), as compared to RAFT/Y(OTf)(3) or ATRP/Y(OTf)(3). The latter two systems were used for the first one-pot synthesis of stereoblock copolymers by radical polymerization. Well-defined stereoblock copolymers, atactic-b-isotactic poly(N,N-dimethylacrylamides), were obtained by adding Y(OTf)(3) at a given time to either RAFT or ATRP polymerizations, initially started without the presence of the Lewis acid.     

Physical deposition behavior of charged amphiphilic diblock copolymers: Effect of charge distribution and electric field

Coarse-grained molecular dynamics simulations are used to investigate physical deposition behavior of charged amphiphlic diblock copolymers. The effects of solvent selectivity, charge distribution in amphiphlic diblock copolymers, and electric field strength on deposition conformations are studied qualitatively. Flat amphiphilic bilayers, which consist of hydrophilic monolayer and hydrophobic brush, are formed by physical deposition of charged amphiphlic diblock copolymers in nonselective solvents. For physically deposited amphiphlic diblock copolymers in selective solvents, amphiphilic bilayers consist of disc-shaped hydrophilic monolayers and hydrophobic nanospheres are found. This study sheds light on the formation of various amphiphlic diblock copolymer deposition conformations in different solvents and interaction mechanism of different components. Furthermore, the evolution of physical deposition process of charged amphiphlic diblock copolymers layer offers new insight to the controlling of amphiphilic bilayer thickness, hydrophobic nanosphere size, and interface property of depositional amphiphlic diblock copolymers.