Ring-opening polymerization of l-lactide in supercritical carbon dioxide using PDMS based stabilizers

Ring-opening suspension polymerization of l-lactide in supercritical CO₂ (scCO₂) was investigated in the presence of different stabilizer architectures based on poly(dimethyl siloxanes) (PDMS). Two amphiphilic AB type block copolymers, a graft copolymer, and an ester-capped PDMS were selected to find their efficacy as stabilizers for the synthesis of poly(l-lactide) (PLLA) in scCO₂. The stabilizer’s efficiency was analyzed in terms of the molecular weight, yield, and particle morphology of PLLA. The block copolymers, poly(dimethylsiloxane)-b-poly(acrylic acid) (PDMS-b-PAA) and poly(dimethylsiloxane)-b-poly(methacrylic acid) (PDMS-b-PMA) were found to be effective, leading to the formation of fine, discrete PLLA microparticles. On the other hand, the graft copolymer, poly(dimethylsiloxane-g-pyrrolidonecarboxylic acid) (PDMS-g-PCA) and acetylated PDMS (PDMS-OAc) failed to give an enough stabilization to the PLLA due to their short polymer-philic chains, resulting in hard agglomerates.     

Nitroxide‐mediated controlled statistical copolymerizations of N‐isopropylacrylamide with N‐tert‐butylacrylamide

The copolymerization of N‐isopropylacrylamide (NIPAM) and N‐tert‐butylacrylamide (TBAM) via conventional radical polymerization and nitroxide‐mediated polymerization (NMP) with N‐tert‐butyl‐N‐[1‐diethylphosphono‐(2,2‐dimethylpropyl)]nitroxide (SG1) was investigated. The monomer reactivity ratios were determined to be 0.58 and 1.00 for NIPAM and TBAM, respectively. The reactivities were approximately the same at 120 and 60 °C in N,N‐dimethylformamide (DMF) and toluene, respectively, for the conventional copolymerizations and in DMF at 120 °C for NMP. Controlled/living characteristics for NMP were achieved with a 2,2′‐azobisisobutyronitrile/SG1 bimolecular system and a unimolecular polystyrene [poly(STY)]–SG1 macroinitiator in the presence of excess free SG1. Block copolymers of poly(N‐isopropylacrylamide‐stat‐N‐tert‐butylacrylamide) [poly(NIPAM‐stat‐TBAM)] with styrene {poly(N‐isopropylacrylamide‐stat‐N‐tert‐butylacrylamide)‐block‐polystyrene [poly(NIPAM‐stat‐TBAM)‐block‐poly(STY)]} were obtained by chain extension of either poly(NIPAM‐stat‐TBAM)–SG1 with styrene or poly(STY)–SG1 with NIPAM/TBAM. A comparison of the number‐average molecular weight calculated from the end‐group content with the number‐average molecular weight measured by gel permeation chromatography for poly(NIPAM‐stat‐TBAM)‐block‐poly(STY)–SG1 indicated that nearly all poly(NIPAM‐stat‐TBAM) chains were capped by SG1 and were thus living. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 6410–6418, 2006     

Synthesis and aqueous solution properties of functionalized and thermoresponsive poly(D,L-lactide)/polyether block copolymers

Tri- and pentablock amphiphilic copolymers containing hydrophobic poly(D,L-lactide) block(s) and hydrophilic polyethers were synthesized in order to obtain new precursor architectures suitable for drug delivery systems. Polyglycidol-6-poly(ethylene oxide)-b-poly(D,L-lactide) possess high hydroxyl functionality provided by the linear polyglycidol block. Thus very stable hydroxyl functionalized micelles in aqueous media were obtained. On the other hand poly(D,L-lactide)-b-poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide)-b-poly(D,L-lactide) form temperature sensitive aggregates. The copolymers obtained were analyzed by SEC and NMR, and their aqueous solution properties were followed by cloud point measurements and determination of critical micellization temperature. TEM was used for particles visualization.     

Synthesis and self-assembly behaviors of well-defined poly(lauryl methacrylate)-block-poly[N-(2-methacryloylxyethyl)pyrrolidone] copolymers

A series of structurally controllable poly(lauryl methacrylate)-b-poly[N-(2-methacryloylxyethyl)pyrrolidone], PLMA-b-PNMP, diblock copolymers were synthesized by reversible addition–fragmentation chain transfer polymerization. The self-assembly behaviors of PLMA-b-PNMP in a selective solvent, tetrahydrofuran (THF), were studied by employing static light scattering, dynamic light scattering, and transmission electron microscopy in detail. The relationships between the aggregation parameters, such as critical micelle concentration and the aggregation number (N _agg), and the molecular structure were established. It was found that spherical micelles can be formed once the solvophobic block length of poly[N-(2-methacryloylxyethyl)pyrrolidone] is larger than 215. Moreover, extremely small and monodisperse gold nanoparticles (<2 nm) were synthesized by employing PLMA-b-PNMP diblock copolymers in THF.     

Synthesis of carbosilane block copolymers by means of a living anionic polymerization of 1,1-diethylsilacyclobutane

Block polymerization of 1,1-diethylsilacyclobutane with styrene derivatives and methacrylate derivatives was investigated. Sequential addition of styrene to a living poly(1,1-diethylsilabutane), which was prepared from phenyllithium and 1,1-diethylsilacyclobutane in THF–hexane at −48°C, gave poly(1,1-diethylsilabutane)-b-polystyrene. Similarly, addition of 4-(tert-butyldimethylsiloxy)styrene to the living poly(1,1-diethylsilabutane) provided poly(1,1-diethylsilabutane)-b-poly(4-(tert-butyldimethylsiloxy)styrene). Poly(1,1-diethylsilabutane)-b-poly(methyl methacrylate) was obtained by treatment of living poly(1,1-diethylsilabutane) with 1,1-diphenylethylene followed by an addition of methyl methacrylate. Poly(1,1-diethylsilabutane)-b-poly(2-(tert-butyldimethylsiloxy)ethyl methacrylate) was also synthesized by adding 2-(tert-butyldimethylsiloxy)ethyl methacrylate to the living poly(1,1-diethylsilabutane) which was end-capped with 1,1-diphenylethylene in the presence of lithium chloride. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 2699–2706, 1998     

Silica nanoparticle covered with mixed polymer brushes as Janus particles at water/oil interface

Silica nanoparticles (NSiO₂) are modified with mixed polymer brushes derived from a block copolymer precursor, poly(methyl methacrylate)-b-poly(glycidyl methacrylate)-b-poly(tert-butyl methacrylate) with short middle segment of PGMA, through one step ??grafting-onto?? approach. The block polymer precursors are prepared via reversible addition?Cfragmentation chain transfer-based polymerization of methyl methacrylate, glycidyl methacrylate, and tert-butyl methacrylate. The grafting is achieved by the reaction of epoxy group in short PGMA segment with silanol functionality of silica. After hydrolysis of poly(tert-butyl methacrylate) segment, amphiphilic NSiO₂ with ??V-shaped?? polymer brushes possessing exact 1:1 molar ratio of different arms were prepared. The functionalized particles self-assemble at oil/water interfaces to form stable large droplets with average diameter ranging from 0.15?±?0.06 to 2.6?±?0.75?mm. The amphiphilicity of the particles can be finely tuned by changing the relative lengths of poly(methyl methacrylate) and poly(methacrylic acid) segments, resulting in different assembly behavior. The method may serve as a general way to control the surface property of the particles.     

Thermosensitive polyion complex micelles prepared by self-assembly of two oppositely charged diblock copolymers

Polyion complex (PIC) micelles were spontaneously formed in aqueous solutions through electrostatic interaction between two oppositely charged block copolymers, poly(N-isopropylacrylamide)-b-poly(L-glutamic acid) and poly(N-isopropylacrylamide)-b-poly(L-lysine). Their controlled synthesis was achieved via the ring opening polymerization of N-carboxyanhydrides (NCA), ε-benzyloxycarbonyl-L-lysine (Lys(Z)-NCA) or γ-benzyl-L-glutamate (BLG-NCA) with amino-terminated poly(N-isopropylacrylamide) macroinitiator and the subsequent deprotection reaction. The formation of PIC micelles was confirmed by dynamic light scattering and transmission electron microscopy. Turbidimetric characterization suggested that the formed PIC micelles had a concentration-dependent thermosensitivity and their phase transition behaviors could be easily adjusted either by the block length of coplymers or the concentration of micelles.     

Model block–graft copolymer via anionic living polymerization: Preparation and characterization of polystyrene-block-[poly(p-hydroxystyrene)-graft-poly(ethylene oxide)]-block-polystyrene

A model graft copolymer in which position of graft points was set to the center of a backbone molecule was prepared via anionic living polymerization. Polystyrene-block-poly(p-tert-butoxystyrene)-block-polystyrene (PSt-b-PBSt-b-PSt) was prepared by three-stage sequential addition. The tert-butyl group was removed from PBSt by hydrogen bromide to yield PSt-b-PHSt-b-PSt, having a poly(p-hydroxystyrene) (PHSt) block. The hydroxyl group of PHSt was reacted with dimeric potassium dianions of 1, 1-diphenylethylene (DPE-K) or cumyl potassium (cumyl K) to yield the corresponding macromolecular initiators of PSt-b-PHSt⁻K⁺-b-PSt containing the potassium alkoxide ion of PHSt. The newly formed alkoxide groups and remaining initiators of DPE-K or cumyl K are capable of initiating the additionally introduced ethylene oxide (EO). Thus, two block–graft copolymers of polystyrene-block-[poly(p-hydroxystyrene)-graft-poly(ethylene oxide)]-block-polystyrene (PSt-b-(PHSt-g-PEO)-b-PSt) were prepared by a “grafting from” process (backbone initiation). A PSt-b-PHSt-b-PSt backbone (M_n = 1.7₅ × 10⁵ by osmometry and M_w/M_n = 1.0₈ by GPC), and two PSt-b-(PHSt-g-PEO)-b-PSt block–graft copolymers (M_n = 2.4₅ × 10⁵ by osmometry and M_w/M_n < 1.1₀ by GPC) had narrow molecular weight distributions. A relationship between nonquantitative metallation and spacing of the graft points on a backbone molecule was discussed in detail. Two benzene-cast films formed clear microphase-separated structures of lamellar structure. The dependence of composition on the morphology of the block–graft copolymers was found to differ from that of common block copolymers. A degree of crystallinity of PEO segment and lamellar thickness of PEO phase serving as graft molecule were also found to differ from those of homo PEO and/or PEO segment in common block copolymer. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 3021–3034, 1998     

Effect of stereocomplex formation between enantiomeric poly(l,l-lactide) and poly(d,d-lactide) blocks on self-organization of amphiphilic poly(lactide)-block-poly(ethylene oxide) copolymers in dilute aqueous solution

The effect of crystallization of a hydrophobic poly(lactide) block on the self-organization of biocompatible and biodegradable amphiphilic poly(lactide)-block-poly(ethylene oxide) (PLA-b-PEO) copolymers in a dilute aqueous solution has been investigated. It was demonstrated that the co-crystallization of poly(L,L-lactide) [P(L,L)LA] and poly(d,d-lactide) [P(d,d)LA] chains under equimolar mixing of P(L,L)LA₄₆-b-PEO₁₁₃ and P(d,d)LA₅₆-b-PEO₁₁₃ copolymers resulted in the formation of stable and spontaneously water-redispersible stereocomplex micelles with semicrystalline P(L,L)LA/P(d,d)LA cores. It was shown that the P(L,L)LA₄₆ / P(d,d)LA₅₆-b-PEO₁₁₃ stereo-complex micelles produced by dialysis can be potential vehicles for the anticancer agent oxaliplatin     

Novel supramolecular block copolymer containing organic–inorganic pentablock copolymer by ATRP of styrene and vinyl acetate using polydimethylsiloxane/cyclodextrin inclusion complexes as macroinitiator

Organic–inorganic pentablock copolymers have been synthesized via atom transfer radical polymerization (ATRP) of styrene (St) and vinyl acetate (VAc) monomers at 60 °C using CuCl/N,N,N′,N″,N″-pentamethyldiethylenetriamine as a catalyst system initiated from boromoalkyl-terminated poly(dimethylsiloxane) (PDMS)/cyclodextrins macroinitiator (Br-PDMS/γ-CD). Br-PDMS-Br was reacted with γ-CD in different conditions with inclusion complexes being characterized through hydrogen nuclear magnetic resonance (¹H NMR) and differential scanning calorimetry (DSC). Resulting Br-PDMS-Br/γ-CD inclusion complexes were taken as macroinitiators for ATRP of St and VAc. Well-defined poly(styrene)-b-poly(vinyl acetate)-b-poly(dimethylsiloxane/γ-cyclodextrin)-b-poly(vinyl acetate)-b-poly(styrene) (PSt-b-PVAc-b-PDMS/γ-CD-b-PVAc-b-PSt) pentablock copolymer was characterized by ¹H NMR, gel permeation chromatograph (GPC) and DSC. There was a good agreement between the number-average molecular weight calculated from ¹H NMR spectra and that of theoretically calculated. Pentablock copolymers consisting of Br-PDMS-Br/γ-CD inclusion complex as central blocks (inorganic block) and PVAc and PSt as terminal blocks were synthesized by this technique. PSt-b-PVAc-b-PDMS/γ-CD-b-PVAc-b-PSt pentablock copolymer can undergo a temperature-induced reversible transition upon heating of the copolymer complex from white complex at 22 °C to green complex in 55 °C which characterized with XRD and ¹H NMR. XRD showed a change in crystallinity percent of St peak with changing the temperature which calculated by Origin75 software.     

Synthesis of urethane—acrylic multi-block copolymers via electrochemically mediated ATRP

The electrochemically mediated atom transfer radical polymerisation (eATRP) of n-butyl acrylate was investigated under a variety of catalyst concentrations. Poly(n-butyl acrylate)-block-polyurethane-block-poly(n-butyl acrylate) copolymers were prepared via electrochemically mediated atom transfer radical polymerisation (eATRP) using only 7 × 10^?6 mole % of Cu^II complex. The successful chain extension and formation of penta-block copolymers confirmed the living nature of the poly(alkyl acrylates) prepared by eATRP. In this work, the tri-block and penta-block urethane-acrylate copolymers were synthesised for the first time by using tertiary bromine-terminated polyurethane macro-initiators as transitional products reacting with n-butyl acrylate, and subsequently with tert-butyl acrylate in the presence of the Cu^IIBr₂/TPMA catalyst complex. The results of ¹H NMR spectral studies support the formation of tri-block poly(n-butyl acrylate)-block-polyurethane-block-poly(n-butyl acrylate) copolymers, and penta-block poly(tert-butyl acrylate)-block-poly(n-butyl acrylate)-block-polyurethane-block-poly(n-butyl acrylate)-block-poly(tert-butyl acrylate) copolymers.     

Synthesis and self-assembly of poly(ethylene glycol)-block-poly(N-3-(methylthio)propyl glycine) and their oxidation-sensitive polymersomes

Amphiphilic block copolymers poly(ethylene glycol)-block-poly(N-3-(methylthio)propyl glycine) (PEG-b-PMeSPG) were synthesized via ring-opening polymerization of N-3-(methylthio)propyl glycine N-thiocarboxyanhydride (MeSPG-NTA) initiated by amino-terminated PEG. The self-assemblies of three PEG-b-PMeSPG copolymers with different PMeSPG block lengths were first prepared by nanoprecipitation method using THF and DMF, respectively, as the organic solvent, and their morphologies were studied by Cryo-EM and DLS. To prepare polymersomes loaded with glucose oxidase (GOx), double emulsion method followed by extrusion treatment was employed. The oxidation-responsive disruption of polymersomes was achieved upon the introduction of glucose because of the oxidants generated in-situ by GOx/glucose.     

Organic/inorganic hybrid star-shaped block copolymers of poly(l-lactide) and poly(N-isopropylacrylamide) with a polyhedral oligomeric silsesquioxane core: Synthesis and self-assembly

The star-shaped organic/inorganic hybrid poly(l-lactide) (PLLA) based on polyhedral oligomeric silsesquioxane (POSS) was prepared using octa(3-hydroxypropyl) polyhedral oligomeric silsesquioxane as initiator via ring-opening polymerization (ROP) of l-lactide (LLA). The molecular weight of POSS-containing star-shaped hybrid PLLA (POSSPLLA) can be well controlled by the feed ratio of LLA to initiator. The POSSPLLA was further functionalized into the macromolecular reversible addition-fragmentation transfer (RAFT) agent for the polymerization of N-isopropylacrylamide (NIPAM), leading to the POSS-containing star-shaped organic/inorganic hybrid amphiphilic block copolymers, poly(l-lactide)–block–poly(N-isopropylacrylamide) (POSS(PLLA–b–PNIPAM)). The self-assembly behavior of POSS(PLLA–b–PNIPAM) block copolymers in aqueous solution was investigated by dynamic light scattering (DLS), transmission electron microscopy (TEM) and atomic force microscopy (AFM). DLS showed the PNIPAM block in the aggregates is temperature-responsive and its phase-transition is reversible. TEM proved that the star-shaped POSS(PLLA–b–PNIPAM) amphiphilic block copolymers can self-assemble into the vesicles in aqueous solution. The vesicular wall and coronas are composed of the hydrophobic POSS core and PLLA, and hydrophilic PNIPAM blocks, respectively. Therefore, POSSPLLA and POSS(PLLA–b–PNIPAM) block copolymers, as a class of novel organic–inorganic hybrid materials with the advantageous properties, can be potentially used in biological and medical fields.     

Thermal degradation kinetics of thermoresponsive poly(N-isopropylacrylamide-co-N,N-dimethylacrylamide) copolymers prepared via RAFT polymerization

Reversible addition-fragmentation chain transfer polymerization at 70 °C in N,N-dimethylformamide was used to prepare poly(N-isopropylacrylamide-co-N,N-dimethylacrylamide) copolymers in various compositions to afford well-defined polymers with pre-determined molecular weight, narrow molecular weight distribution, and precise chain end structure. The copolymer compositions were determined by ¹H NMR spectroscopy. The reactivity ratios of N-isopropylacrylamide (NIPAM) and N,N-dimethylacrylamide (DMA) were calculated as r _NIPAM = 0.838 and r _DMA = 1.105, respectively, by the extended Kelen–Tüdös method at high conversions. The lower critical solution temperature of PNIPAM can be altered by changing the DMA content in the copolymer chain. Differential scanning calorimetry and thermogravimetric analysis at different heating rates were carried out on these copolymers to understand the nature of thermal degradation and to determine its kinetics. Different kinetic models were applied to estimate various parameters like the activation energy, the order, and the frequency factor. These studies are important to understand the solid state polymer degradation of N-alkyl substituted polymers, which show great potential in the preparation of miscible polymer blends due to their ability to interact through hydrogen bonding.     

Nitroxide-mediated precipitation polymerization of styrene in supercritical carbon dioxide: Effects of monomer loading and nitroxide partitioning on control

Nitroxide-mediated solution and precipitation polymerizations of styrene in toluene and supercritical carbon dioxide (scCO₂), respectively, using the nitroxides N-tert-butyl-N-[1-diethylphosphono-(2,2-dimethylpropyl)]nitroxide (SG1) and 2,2,5-trimethyl-4-phenyl-3-azahexane-3-nitroxide (TIPNO) are presented. Solution polymerizations are compared with simulations using PREDICI software, revealing that differences in the polymerization behaviours between the SG1- and TIPNO-mediated systems cannot be rationalized based on literature rate coefficients and the ideal mechanism for nitroxide-mediated polymerization. Nitroxide and monomer partitioning between the polymer particles and the continuous phase play important roles in the precipitation polymerizations in scCO₂. Loss of control (broader molecular weight distributions) as a result of nitroxide partitioning is accentuated at low monomer loading, and is significantly more pronounced for TIPNO than SG1. However, at higher monomer loading the level of control was superior in scCO₂ compared to in the corresponding solution polymerizations for both nitroxides, most likely caused by an increase in the number of activation-deactivation cycles experienced by any given chain during its growth.     

Surface patterns of block copolymers in thin layers after vapor treatment

A systematic study of formation of surface patterns in block copolymer thin layers after their exposure to solvent vapors was performed. The studied effect involves layers of thickness approximately equal to the ordering size of polymers - about 45 nm. Experiments were performed on three styrene - methacrylate derivative block copolymers, synthesized by living anionic polymerization: poly(4-octylstyrene)-block-poly(butyl methacrylate), poly(4-fluorostyrene)-block-poly(butyl methacrylate) and poly(p-octylstyrene)-block-poly(methyl methacrylate). The polymers were exposed to vapors of chloroform, 1,4-dioxane, hexane, acetone and tetrahydrofuran.     

Macromolecular surfactants for supercritical carbon dioxide applications: Synthesis and characterization of fluorinated block copolymers prepared by nitroxide‐mediated radical polymerization

Poly(perfluorooctyl‐ethylenoxymethylstyrene) (PFDS) and poly(1,1,2,2‐tetrahydroperfluorodecyl acrylate) (PFDA) homopolymers as well as poly(styrene)‐b‐poly(perfluorooctyl‐ethylenoxymethylstyrene) (PS‐b‐PFDS) and poly(styrene)‐b‐poly(1,1,2,2‐tetrahydroperfluorodecyl acrylate) acrylate) (PS‐b‐PFDA) block copolymers of various chain lengths were synthesized by nitroxide‐mediated radical polymerization in the presence of either 2,2,6,6‐tetramethyl‐1‐piperidinyloxy free radical (TEMPO) in the case of FDS monomer or N‐tert‐butyl‐N‐(1‐diethylphosphono‐2,2‐dimethylpropyl)‐N‐oxyl (DEPN) in the case of the FDA monomer. The molar composition of the block copolymers was determined by elemental analysis and proton NMR while the blocky structure was checked by SEC analysis in trifluorotoluene. Block copolymers PS‐b‐PFDS (3.6K/60K) and PS‐b‐PFDA (3.7K/43K) were soluble in neat CO₂ at moderate pressure and temperature, indicating the formation of micelles. Similar block copolymers with a longer PS block such as PS‐b‐PFDA (9.5K/49K), corresponding to a lower CO₂‐philic/CO₂‐phobic balance, were insoluble in neat CO₂ but could be solubilized in the presence of styrene as a cosolvent. Additionally, surface and bulk properties of PS‐b‐PFDA were investigated, indicating the same surface tension as for the PFDA homopolymer (γ_LV = 10.3 mN/m) and a bulk nanostructured morphology. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 3537–3552, 2004     

Synthesis of amphiphilic multiblock and triblock copolymers of polydimethylsiloxane and poly(N,N‐dimethylacrylamide)

The synthesis and spectroscopic characterization of a new family of amphiphilic multiblock and triblock copolymers is described. The synthetic methodology rests on the preparation of telechelic multifunctional and difunctional chain transfer agents easily available in two synthetic steps from commercially available polydimethylsiloxane‐containing starting materials. Telechelic polymers thus synthesized are used as macromolecular chain transfer agents in the reversible addition fragmentation chain transfer (RAFT) polymerization of N,N‐dimethylacrylamide (DMA) enabling the synthesis of (AB)_n‐type multiblock and ABA‐type triblock copolymers of varying compositions possessing monomodal molecular weight distribution. (AB)_n multiblock copolymers [(PDMA‐b‐PDMS)_n] were prepared with between 52 and 95 wt % poly(dimethylacrylamide) with number average molecular weights (M_n) between 14,000 and 86,000 (polydispersities of 1.20–2.30). On the other hand, ABA block copolymers with DMA led to amphiphilic block copolymers (PDMA‐b‐PDMS‐b‐PDMA) with M_n values between 9000 and 44,000 (polydispersities of 1.24–1.62). © Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 7033–7048, 2008     

Synthesis and micelle formation of triblock copolymers of poly(methyl methacrylate)-b-poly(ethylene oxide)-b-poly(methyl methacrylate) in aqueous solution

Amphiphilic triblock copolymers of poly(methyl methacrylate)-b-poly(ethylene oxide)-b-poly(methyl methacrylate) (PMMA-b-PEO-b-PMMA) with well-defined structure were synthesized via atom transfer radical polymerization (ATRP) of methyl methacrylate (MMA) initiated by the PEO macroinitiator. The macroinitiator and triblock copolymer with different PMMA and/or PEO block lengths were characterized with ¹H and ¹³C NMR and gel permeation chromatography (GPC). The micelle formed by these triblock copolymers in aqueous solutions was detected by fluorescence excitation and emission spectra of pyrene probe. The critical micelle concentration (CMC) ranged from 0.0019 to 0.016 mg/mL and increased with increasing PMMA block length, while the PEO block length had less effect on the CMC. The partition constant K_v for pyrene in the micelle and in aqueous solution was about 10⁵. The triblock copolymer appeared to form the micelles with hydrophobic PMMA core and hydrophilic PEO loop chain corona. The hydrodynamic radius R_h,app of the micelle measured with dynamic light scattering (DLS) ranged from 17.3 to 24.0 nm and increased with increasing PEO block length to form thicker corona. The spherical shape of the micelle of the triblock copolymers was observed with an atomic force microscope (AFM). Increasing hydrophobic PMMA block length effectively promoted the micelle formation in aqueous solutions, but the micelles were stable even only with short PMMA blocks.     

Shell-crosslinked nanoparticles through self-assembly of thermoresponsive block copolymers by RAFT polymerization

A reversible addition-fragmentation chain transfer (RAFT) agent, the methyl-2-(n-butyltrithiocarbonyl)propanoate (MBTTCP) has shown to be efficient in controlling the polymerization of N,N-dimethylacrylamide (DMA), N-isopropylacrylamide (NIPAM) and N-acryloyloxysuccinimide (NAS). Two different strategies have been studied to synthesize block copolymers based on one PNIPAN block and the other a random copolymer of DMA and NAS. When a PNIPAM trithiocarbonate-terminated is used as macromolecular chain transfer agent for the polymerization of a mixture of NAS and DMA, well-defined P(NIPAM-b-(NAS-co-DMA)) block copolymers were obtained with a low polydispersity index. These thermoresponsive block copolymers dissolved in aqueous solution at 25 °C and self-assembled into micelles when the temperature was raised above the LCST of the PNIPAM block. The micelle shell containing NAS units was further crosslinked using a primary diamine in order to get shell-crosslinked nanoparticles. Upon cooling below the LCST of PNIPAM this structure may easily reorganize to form nanoparticles with a water filled hydrophilic core.