RAFT polymerization and self-assembly of thermoresponsive poly(N-decylacrylamide-b-N,N-diethylacrylamide) block copolymers bearing a phenanthrene fluorescent α-end group (original) (raw)
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Polymer, 2010
Phenanthrene a-end-labeled poly(N-decylacrylamide-b-N,N-diethylacrylamide) (PDcA n-b-PDEA m) block copolymers consisting in a highly hydrophobic block (n ¼ 11) and a thermoresponsive block with variable length (79 m 468) were synthesized by reversible addition-fragmentation chain transfer (RAFT) polymerization. A new phenanthrene-labeled chain transfer agent (CTA) was synthesized and used to control the RAFT polymerization of a hydrophobic acrylamide derivative, N-decylacrylamide (DcA). This first block was further used as macroCTA to polymerize N,N-diethylacrylamide (DEA) in order to prepare diblock copolymers with the same hydrophobic block of PDcA (number average molecular weight: M n ¼ 2720 g mol À1 , polydispersity index: M w /M n ¼ 1.13) and various PDEA blocks of several lengths (M n ¼ 10,000-60,000 g mol À1) with a very high blocking efficiency. The resulting copolymers self-assemble in water forming thermoresponsive micelles. The critical micelle concentration (CMC) was determined using Fö rster resonance energy transfer (FRET) between phenanthrene linked at the end of the PDcA block and anthracene added to the solution at a low concentration (10 À5 M), based on the fact that energy transfer only occurs when phenanthrene and anthracene are located in the core of the micelle. The CMC (w2 mM) was obtained at the polymer concentration where the anthracene fluorescence intensity starts to increase. The size of the polymer micelles decreases with temperature increase around the lower critical solution temperature of PDEA in water (LCST w 32 C) owing to the thermoresponsiveness of the PDEA shell.
Macromolecules, 2010
Block copolymers of poly(N-decylacrylamide-b-N,N-diethylacrylamide) (PDcA-b-PDEA), with different PDEA block lengths and a constant PDcA block labeled with a phenanthrene fluorescent dye at the PDcA R-chain-end were prepared by RAFT polymerization. These copolymers form star-like micelles in water, (critical micelle concentration below 0.1 g/L, determined using coumarine 153) with a PDcA insoluble core surrounded by a PDEA corona showing thermoresponsive properties. The kinetics of F€ orster resonance energy transfer (FRET) between the chain-end phenanthrene groups and anthracene loaded into the hydrophobic core of the micelles in water, was analyzed using a new model for energy transfer in spherical nanodomains. This model takes into account the Poisson distribution of the acceptors in the micelle population and the existence of two phenanthrene states with different fluorescence lifetimes. The analysis yields the radius of the micelle core, R c =2.7 (0.1 nm, with no need for deuteration of the core block. The result is compared with the value obtained by extrapolation of the light scattering data using the star micelle model, R c (DLS)=3.0 nm. The model for star-like micelles also yields a solvent-corona interaction parameter that changes with temperature due to the thermoresponsive nature of PDEA.
Polymer, 2009
Well-defined pH-and thermo-multi-responsive fluorescent micelles based on the self-assembly of diblock copolymers poly[(N-isopropyl-acrylamide-co-N-vinylcarbazole)-b-2-(dimethylamino)ethyl acrylate], (PNIPAAM-co-PNVC)-b-PDMAEA, are described. The diblock copolymers are prepared via the reversible addition fragmentation chain transfer (RAFT) copolymerization of N-isopropyl-acrylamide (NIPAAM) and N-vinylcarbazole (NVC) followed by chain extension in presence of 2-(dimethylamino)ethyl acrylate) (DMAEA). The micelles are formed in aqueous solutions in a wide range of temperature (25-60 C), and their sizes increase from 40 to 65 nm when varying pH from basic to acidic. The cross-linking of the PDMAEA-containing shell with 1,2-bis(2-iodoethoxy)ethane (BIEE) results in spherical soft nanoparticles which size is increased by 20-25% when compared to the micelles. The presence of NVC in concentrations as low as 4% in the core of the micelles allow the nanoparticles to be tagged by fluorescence, making them well suited for therapeutic applications.
Macromolecules, 1992
A-B block copolymers of polystyrene-block-poly(methacry1ic acid) and polystyrene-blockpoly(tert-butyl methacrylate), both with short-chain oligovinyl-2-naphthalene moieties attached to the end of the polystyrene block, were prepared by anionic polymerization. After hydrolysis of the poly(tert-butyl methacrylate) blocks, micelles were prepared from the pol~t~~e-block-poly(methac~lic acid) copolymers and the photophysical properties were studied ae a function of different ratios of the solvent system 1,4dioxane/water. The fluorescence data were compared to that of micelles formed from the polystyreneblock-poly(tert-butyl methacrylate) copolymers in organic solvent mixtures of 1,Cdioxanelmethanol. It was found that intramolecular excimer formation (which is controlled by the mobility of the pendant fluorescent groups and the polymer chain dynamics) is sterically hindered in the micellar corea as they b m e more compact. Both steady-state and time-resolved excimer fluorescence are eignificantly ianuenced by the gradual collapse of the micellar cores and the increase in segment density within the cores with an increasing content of water. Changes in lifetimes and preexponentid factors for naphthalene fluorescence (monomer as well as excimer) were found to be sensitive indicators of micelle formation.
The Journal of Physical Chemistry B, 2010
Fluorescent probes, coumarin 153 (C153) and octadecylrhodamine B (ORB), were used to study the selfassembly in water of poly(N-decylacrylamide)-block-poly(N,N-diethylacrylamide), (PDcA 11-block-PDEA 295 ; M n) 40 300 g mol-1 ; M w /M n) 1.01). From the variation of both the fluorescence intensity and the solvatochromic shifts of C153 with polymer concentration, the critical micelle concentration (CMC) was determined as 1.8 (0.1 µM. On the other hand, steady-state anisotropy measurements showed the presence of premicellar aggregates below the CMC. Time-resolved fluorescence anisotropy evidenced that ORB is located in the premicellar aggregates and the micelle core, while C153 is partitioned between the aggregates and the water phase. The micelle core contains both semicrystalline and amorphous regions. In the semicrystalline regions the probes cannot rotate, while in the amorphous regions the rotational correlation times correlate well with the hydrodynamic volume of the probes. The amorphous region of the micelle core is relatively fluid, reflecting the large free-volume accessible to the probes.
Macromolecules, 2002
The micellization behavior of a hydrophobically modified double tagged polystyrene-blockpoly(methacrylic acid) diblock copolymer, PS-N-PMA-A was studied in 1,4-dioxane-H2O mixtures by light-scattering and fluorescence techniques. This polymer was fluorescently tagged by a naphthalene moiety at the junction of the blocks and by anthracene at the end of the PMA block. The behavior of a single-tagged sample, PS-N-PMA, and low-molar-mass analogues were studied for comparison. Multimolecular polymeric micelles with compact PS cores and PMA shells may be prepared indirectly by dialysis from 1,4-dioxane-rich mixtures as water is a strong selective precipitant for the PS block. In both types of micelles, the naphthalene tags are trapped in a nonpolar and fairly viscous core/shell interfacial region. The hydrophobic anthracene tags in PS-N-PMA-A are at the ends of the water-soluble PMA blocks and tend to avoid the bulk polar solvent, burying themselves into the shell. The collapse of a fraction of the PMA chains is an enthalpy-driven process, but it is entropically unfavorable, and the distribution of the anthracene tags in the shell is a result of the enthalpy-to-entropy interplay. Measurements of direct nonradiative excitation energy transfer (NRET) were performed on PS-N-PMA-A to estimate the distribution of the anthracene-tagged PMA ends in the shell. The experimental fluorometric data show that the anthracene tags penetrate deeply into the shell in water-rich solvents, although there is considerable fluctuation in the distance of closest approach to the excited naphthalene. We find that the collapsed PMA chains and loops in the shell results in the counterintuitive effect that the hydrodynamic radius is significantly increased compared to the corresponding unmodified PS-N-PMA.
Macromolecules, 1991
A-B-A block copolymers (A = poly(methacry1ic acid), B = polystyrene) have been prepared by anionic polymerization. These amphiphilic copolymers can form stable micelles in water/l,l-dioxane mixtures as well as in water or an aqueous buffer. These micelles are presumed to have a polystyrene core and poly(methacry1ic acid) shell. The ability of the micelles to solubilize and release hydrophobic species was studied by fluorescence methods, primarily using pyrene as a fluorescence probe. The following processes were studied: (1) the effect of pyrene loading on monomer/excimer fluorescence ratio and quenching by Cu2+; (2) the rate of exchange between micelles containing pyrene and other aromatic species by the time dependence of either their monomer/excimer ratio or sensitized fluorescence after mixing micelles; (3) the time dependence of the fluorescence quenching of pyrene following the addition of small molecules (Nfl-dimethylaniline, CC4). The following conclusions were obtained (1) a significant fraction (ca. 20-30%) of the pyrene molecules were on or near the polystyrene-water interface (this depends on loading); (2) diffusion of the probe out of the micelle is the rate-determining step in the release and exchange of large hydrophobes. This process is very slow in ita later phases and probably represents slow diffusion from the core of the polystyrene region of the micelle.
Collection of Czechoslovak Chemical Communications, 2002
The micellization behavior of a hydrophobically modified polystyrene-block-poly(methacrylic acid) diblock copolymer, PS-N-PMA-A, tagged with naphthalene between blocks and with anthracene at the end of the PMA block, was studied in 1,4-dioxane-methanol mixtures by light scattering and fluorescence techniques. The behavior of a single-tagged sample, PS-N-PMA, and low-molar-mass analogues was studied for comparison. Methanol-rich mixtures with 1,4-dioxane are strong selective precipitants for PS. Multimolecular micelles with compact PS cores and PMA shells may be prepared indirectly by dialysis from 1,4-dioxane-rich mixtures, or by a slow titration of copolymer solutions in 1,4-dioxane-rich solvents with methanol under vigorous stirring. In tagged micelles, the naphthalene tag is trapped in nonpolar and fairly viscous core/shell interfacial region. In hydrophobically modified PS-N-PMA-A micelles, the hydrophobic anthracene at the ends of PMA blocks tends to avoid the bulk polar solvent and buries in the shell. The distribution of anthracene tags in the shell is a result of the enthalpy-to-entropy interplay. The measurements of direct nonradiative excitation energy transfer were performed to estimate the distribution of anthracene-tagged PMA ends in the shell. The experimental fluorometric data show that anthracene tags penetrate into the inner shell in methanol-rich solvents. Monte Carlo simulations were performed on model systems to get reference data for analysis of time-resolved fluorescence decay curves. A comparison of experimental and simulated decays indicates that hydrophobic traps return significantly deep into the shell (although not as deep as in + The study is a part of the long-term Research Plan of the School of Science No. MSM 113100001. aqueous media). The combined light scattering, fluorometric and computer simulation study shows that the conformational behavior of shell-forming PMA blocks in non-aqueous media is less affected by the presence of nonpolar traps than that in aqueous media.
The Journal of Physical Chemistry B, 2003
The structure and behavior of amphiphilic block copolymer micelles with partly hydrophobically modified polyelectrolyte shells were studied in 1,4-dioxane-water mixtures and in purely aqueous media by a combination of several experimental techniques. The studied hybrid micelles are formed by 20 wt % of a modified polystyrene-block-poly(methacrylic acid), PS-N-PMA-A, double-tagged by one pendant naphthalene between blocks and one anthracene at the end of the PMA block and by 80% of either nontagged PS-PMA or polystyrene-block-poly(ethylene oxide), PS-PEO. The cores of micelles contain pure PS, while the shells contain either PMA-A/PMA or PMA-A/PEO mixed chains. The double tagging by naphthalene and anthracene allows for a nonradiative energy transfer (NRET) study aimed at the estimate of donor-trap distances within one micelle. The fluorometric study suggests that the hydrophobic anthracene tag at the end of shell-forming PMA block tries to avoid the aqueous medium and is buried in the shell, forcing the PMA chain to loop back toward the core. Since the stability of hybrid micellar solutions is guaranteed by favorable interactions of stretched unmodified shell-forming chains (which are in excess in the system) with the aqueous solvent, the reduced entropy of the loop-forming chains does not play such an important role as in micelles with 100% tagging. Hence, we conclude that a higher fraction of the anthracene-tagged chains may return closer to the core-corona interface than in the case of 100% tagged micelles. † Part of the special issue "International Symposium on Polyelectrolytes".
Time-resolved fluorescence study of micellizing block copolymers
Journal of Molecular Structure, 1990
ABSTRACT We have studied the dynamics of polystyrene-block-hydrogenated polyisoprene samples, fluorescently labelled on the polystyrene block, by steady-state and time-resolved fluorometry. In selective precipitants for the labelled block, fluorescent probes are trapped and immobilized in compact micellar cores. The rotation of pendant fluorophors is frozen. However, the fast torsional vibrations depolarize partially the fluorescence. As the rotation of micelles is slow as compared with the fluorescence life-time, a significant residual anisotropy is observed. In good solvents for both blocks, fluorescent probes in expanded copolymer coils are exposed to solvent molecules and free to rotate.