Synthesis and tunable thermoresponsive solution morphologies of 2,2-bis-methylolpropionic acid dendron-azobenzene-poly(N -isopropyl acrylamide) copolymers (original) (raw)
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Hydrophobic dendrons with different numbers and positions of azobenzenes as well as different groups-benzyl and dodecyl, on the periphery were synthesised and attached to poly(ethylene glycol) using copper-catalysed azide-alkyne cycloaddition to obtain linear-dendritic copolymers. Self-assembly of the polymers in aqueous solution was characterised using dynamic light scattering (DLS), transmission electron microscopy (TEM) and critical micelle concentration (cmc). Formation of H-aggregates during micellisation was shown for polymers with a higher number of azobenzene units. Photoisomerisation of azobenzene in the assemblies was studied and the rate constant of thermal photoisomerisation was calculated. Release of hydrophobic dye Nile red upon photoisomerisation of azobenzene occurred without disruption of micellar aggregates. Dye release varied with the pathway-thermal or visible light irradiation, followed for cis-trans isomerisation. The encapsulation capacity of the micelles and extent of dye release in either pathway were found to be influenced by the dendron structure. A polymer with a lower number of azobenzenes and aliphatic periphery on the dendron showed significantly different behaviour than polymers with a larger number of aromatic units.
Controlled micellar disassembly of photo-and pH-cleavable linear-dendritic block copolymers
A linear-dendritic block copolymer with both photo-and pH-cleavable linkages at the junction between the hydrophobic dendron and hydrophilic linear polymer was synthesised. The photocleavable o-nitro-benzyl group attached to alkyne was introduced at the focal point of polyester dendron and acid-degrad-able acetal linkage with terminal azide was introduced at one end of linear polyethylene glycol. The two segments were connected using click chemistry. The copolymer was found to self-assemble into micelle-like aggregates in dilute aqueous solution. Stimuli-responsive disassembly of the aggregates was studied by monitoring fluorescence emission of the encapsulated hydrophobic dye and dynamic light scattering (DLS). Products of acetal cleavage at acidic pH were analyzed by gel permeation chromatography and NMR spectroscopy. The photo and pH stimuli were applied separately as well as simultaneously to study the rate of dye release. The synergistic effect of the two stimuli on dye release was demonstrated and the non-cytotoxic nature of the polymer was shown using the MTT assay. Cell uptake of DOX-loaded micelles and photo-induced release of the drug resulting in significantly higher cyto-toxicity than by free DOX was demonstrated using MDA-MB-231 cells.
Macromolecules, 2003
Star-block copolymers (PEO3-b-PAA3) and dendrimer-like copolymers (PEO3-b-PAA6) consisting of three inner poly(ethylene oxide) (PEO) arms and either three or six peripheral poly(acrylic acid) (PAA) blocks were derived by a core-first approach. To this end, the OH end groups of three-arm PEO stars prepared anionically were derivatized into either three or six bromo-ester functions that served to grow the poly(tert-butyl acrylate) (PtBA) blocks by atom transfer radical polymerization (ATRP) in a controlled fashion. This could be achieved at 80°C in toluene in the presence of CuBr/pentamethyldiethylenetriamine (CuBr/PMDETA) as the catalyst system. Characterization by size exclusion chromatography and NMR of star-block copolymers (PEO 3-b-PtBA3) and dendrimer-like copolymers (PEO3-b-PtBA6) confirmed their well-defined character. Subsequent treatment with trifluoroacetic acid selectively hydrolyzed the PtBA blocks, leading to the targeted PEO3-b-PAA3 and PEO3-b-PAA6 compounds. Alternatively, an arm-first methodology utilizing a divinylic comonomer as the linking agent was applied to access star-block copolymers incorporating an inner PAA part and a peripheral PEO layer. To this end, preformed PEO-b-PtBA diblock copolymers were reacted with divinylbenzene in anisole in the presence of CuBr/PMDETA. Some of the factors controlling the formation of (PtBA-b-PEO) f stars were examined. These included the molar ratio of the linking agent to the diblock precursor, the molar mass of the latter species, and the reaction time. Finally, selective hydrolysis led to the expected double hydrophilic star-block copolymers noted (PAA-b-PEO) f.
Journal of Polymer Science Part A-polymer Chemistry, 2008
Two series of amphiphiles composed of hydrophilic poly(amidoamine) dendrons (from the first to the third generation) as the shell and hydrophobic aromatic chromophores (3,6-di(maleimidyl)-9-phenyl carbazole and 9-(4′-maleimidyl phenyl)-3-maleimidyl carbazole) as the central scaffold were synthesized. The effect of dendritic architecture on the photophysical properties and the self-assembly behavior of these amphiphiles were studied by UV–vis absorption spectroscopy, fluorescence spectroscopy, and transmission electron microscopy (TEM) measurements. Both the generation of dendritic shell and the location of dendrons at the chromophoric scaffold had great effect on the photophysical properties of these amphiphiles. In addition, different spherical aggregates were formed from these amphiphiles in the aqueous solution at different concentrations. Because of the combined effects of steric hindrance and architecture of dendritic shells, the amphiphiles from G2 dendron with central chromophore self-organized into ordered aggregates more readily than that from G1 and G3. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 4584–4593, 2008
Journal of Polymer Science Part A: Polymer Chemistry, 2006
This article describes the syntheses and solution behavior of model amphiphilic dendritic-linear diblock copolymers that self-assemble in aqueous solutions into micelles with thermoresponsive shells. The investigated materials are constructed of poly(benzyl ether) monodendrons of the second generation ([G-2]) or third generation ([G-3]) and linear poly(N-isopropylacrylamide) (PNIPAM). [G-2]-PNIPAM and [G-3]-PNIPAM dendritic-linear diblock copolymers have been prepared by reversible addition-fragmentation transfer (RAFT) polymerizations of N-isopropylacrylamide with a [G-2]-or [G-3]-based RAFT agent, respectively. The critical micelle concentration (cmc) of [G-3]-PNIPAM 220 , determined by surface tensiometry, is 6.3 Â 10 À6 g/mL, whereas [G-2]-PNIPAM 235 has a cmc of 1.0 Â 10 À5 g/mL. Transmission electron microscopy results indicate the presence of spherical micelles in aqueous solutions. The thermoresponsive conformational changes of PNIPAM chains located at the shell of the dendritic-linear diblock copolymer micelles have been thoroughly investigated with a combination of dynamic and static laser light scattering and excimer fluorescence. The thermoresponsive collapse of the PNIPAM shell is a two-stage process; the first one occurs gradually in the temperature range of 20-29 8C, which is much lower than the lower critical solution temperature of linear PNIPAM homopolymer, followed by the second process, in which the main collapse of PNIPAM chains takes place in the narrow temperature range of 29-31 8C. V
Hydrophobic dendrons based on different branching patterns, viz. 3,5-di-and 3,4,5-trisub-stituted phenyl rings, consist of the same backbone but exhibit different sizes, shapes, and hydrophobic densities. These dendrons are attached to poly(ethylene glycol) and the core properties of the copolymer micelles are investigated in tetrahydrofuran (THF)/water mixtures by neutron scattering. Two polymers with intermediate hydrophobicity are studied further with variations in the solvent composition and the temperature. The aggregation numbers for 3,4,5-based dendron copolymers are lower, with more THF molecules of solvation compared with the 3,5-based dendron copolymer, the difference being greater at higher generations due to different molecular shapes. The micellar core size increases in small steps with dendron size so that dye encapsulation is tuned.
European Polymer Journal, 2014
We report on the synthesis of poly(N-isopropyl acrylamide)-block-poly(n-butyl acrylate) (PNIPAm-b-PnBA) amphiphilic block copolymers and their temperature-responsive selfassembly behavior in aqueous solution. Well-defined PNIPAm-b-PnBA copolymers have been synthesized by a two-step RAFT polymerization scheme. The self-assembly behavior was studied by means of static and dynamic light scattering, 1 H NMR and fluorescence spectroscopy and transmission electron microscopy. The results show that already below the lower critical solution temperature (LCST) of PNIPAm, association of the PNIPAm blocks with hydrophobic dodecyl end groups of the charge transfer agent leads to the formation of loose aggregates of PNIPAm-b-PnBA micelles, the size and density of which increase with the increasing length of the PNIPAm block. The collapse of the PNIPAm blocks above the LCST leads to the decrease of the aggregates' size and the increase of their density, but the collapsed PNIPAm chains do not allow for interpenetration of the micellar shells and no further aggregation occurs.
Macromolecules, 1999
Amphiphilic diblock copolymers were synthesized based on poly(2-ethyl-2-oxazoline) (PEtOz) as a hydrophilic block and aliphatic polyesters such as poly(L-lactide) (PLA) or poly(-caprolactone) (PCL) as a hydrophobic block. Their micellar characteristics in an aqueous phase were investigated by using dynamic light scattering and fluorescence techniques. The block copolymers formed micelles in the aqueous phase with critical micelle concentrations (cmcs) in the range of 1.0-8.1 mg/L. The cmc values become lower upon increasing the length of the hydrophobic block. The mean diameters of the micelles were in the range of 108-192 nm, with a narrow distribution. In general, the micelle size increased as the hydrophobic PLA or PCL block became larger. The partition equilibrium constants, K v, of pyrene in the micellar solutions of the block copolymers were from 1.79 × 10 5 to 5.88 × 10 5. For each block copolymer system of PEtOz-PLA or PEtOz-PCL, the Kv value increased as the length of the hydrophobic block increased. The steady-state fluorescence anisotropy values (r) of 1,6-diphenyl-1,3,5-hexatriene (DPH) were 0.265-0.284 in PEtOz-PLA solution and 0.189-0.196 in PEtOz-PCL solution. The anisotropy values of PEtOz-PLAs were higher than those of PEtOz-PCLs. The anisotropy values were independent of the length of the hydrophobic block when the chemical structures of the hydrophobic blocks were identical. The micelles underwent hydrogen bonding at pH <3.5 with poly(acrylic acid), which produced polymer complex precipitates that could be reversibly dispersed as micelles at pH >3.8.
Journal of Polymer Science Part A: Polymer Chemistry, 2008
A series of stable and translucent colored nanolatex, that is, colloidal aqueous suspensions of dye-tagged polymer nanoparticles (NPs) in the 15-to 20-nm diameter range, have been prepared by covalent attachment of azobenzene chromophores to the surface of reactive NPs. Primary crosslinked NPs bearing chlorobenzyl groups were produced by microemulsion copolymerization of styrene and vinylbenzylchloride. Amine-functionalized NPs were obtained after a second functionalization step with polyamines (cyclam and polypropyleneimine dendrimers of first and third generations). Dye-doped particles were obtained by reacting pyridylazo-dimethylaminobenzene (PADA) with chlorobenzyl-NPs and by reacting amine-reactive dimethylaminoazobenzene dyes (DABsyl, DAB-ITC) as well as Disperse Red 1 acrylate with polyamine-coated NPs. Regardless the dye solubility, the grafting readily proceeded in aqueous suspensions at room temperature in the presence of a cationic surfactant without added solvent. Purple, red, and orange suspensions (maximum absorption around 550, 500, 430 nm), with dye loads ranging from 0.3 to 1.2 mmol/g, corresponding to 400-1800 azobenzene residues per NP, are obtained. The reported results indicate that functional polymer NPs, with remarkably accessible multiple anchoring sites, are useful building blocks. V V C 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 3375-3386, 2008 Synthesis of the Model N-Benzylpyridinium Derivative 1
Macromolecules, 1993
The reactivity of benzylic dendritic polyethers toward linear polymers was investigated using coupling reactions of preformed dendritic and linear blocks in solution and in the melt. It was found that the rate constants for the Williamson reaction of poly(ethy1ene oxide)s (PEO) or poly(ethy1ene glyco1)s (PEG) with dendritic bromides of various sizes increased with the length of the linear block and the generation of the dendrimer. This anomalous behavior is attributed to the increased reactivity of the PEO and PEG alcoholate anions due to the solvation of the counterion by the linear block and to the conformation changes occurring after attachment of the first dendritic block to PEG. It was shown that the functional group at the 'focal point" of the dendrimer preserves ita accessibility and reactivity even in highly restrictive medium and is able to participate in transesterification reactions with PEO and PEG in the melt. Thus, block copolymers that differ by a single linking bond between the linear and dendritic blocks were formed.