Synthesis of star polymers by organized polymerization of macromonomers

Vinylbenzyl-terminated polyisoprene (PI) macromonomers were synthesized by the coupling reaction of the corresponding living anions with p-chloromethylstyrene. PI stars were prepared by the free-radical crosslinking of PI macromonomers with divinylbenzene (DVB) in n-heptane. No macrogelation was observed during polymerization. The radical copolymerization of PI macromonomer with DVB led to microgelation in micelles formed by PI macromonomers in the selective solvent (organized polymerization). The arm number of star polymers depended strongly on the concentration ratio of DVB to macromonomer ends.     

Kinetics of Free-Radical Polymerization of Vinylbenzyl-Terminated Macromonomers

Vinylbenzyl-terminated polystyrene (PS) macromonomers were prepared by the direct reaction of living PS anions with p-chloro-methylstyrene (CMS). The propagation rate constant (k _p) was obtained from free-radical polymerization of PS macromonomers in the presence of 1-buten-3-ol as a degradative chain transfer agent by using gel permeation chromatography (GPC) analysis. In this condition, the polymer radicals were terminated by a unimoiecular mechanism. Subsequently, we studied the radical propagation step of vinylbenzyl-terminated diblock poly[styrene (S)-b-isoprene (I)] and poly[S-b-2-vinylpyridine (2VP)] macromonomers in benzene. The vinylbenzyl groups at the terminal ends of diblock macromonomers apparently take the concentrated state in micelles. These results are discussed from the point of view of polymer-polymer reactions.     

New water soluble azobenzene-containing diblock copolymers: synthesis and aggregation behavior

Homopolymers of azobenzene (azo) methacrylates with different substituents and their diblock copolymers with poly(2-(dimethylamino)ethyl methacrylate p(DMAEMA) were synthesized via atom transfer radical polymerization (ATRP). Controlled/‘living’ ATRP of azo methacrylates were achieved up to ∼50% conversion, after which deviation occurred. It was found that the copolymerization rate of 6-[4-phenylazo]phenoxy]hexylmethacrylate (PPHM) from p(DMAEMA) macroinitiator was almost identical to that for the homopolymerization of PPHM monomer, with k_app∼0.0078 min⁻¹. For the copolymerizations, almost complete incorporation of the azo methacrylate monomers could be obtained with low molecular weight macroinitiator (PDMAEMA)-Cl, whereas macroinitiators of long chain length did not give full conversion, most likely due to chain floding and steric hindrance caused by the bulk azo monomers. Because azo monomers are highly hydrophobic, only the diblock copolymers with short azo segment were soluble in water which self-assembled into micellar particles. The effect of photo-induced trans-cis isomerization on lower critical solution temperature (LCST) and surface tension were studied. The LCST of the diblock copolymers increased upon irradiation by UV light due to the cis conformers being more hydrophilic. However, the trans-cis isomerization had only small effect on the critical micelle concentration (cmc) and γ_cmc of azo methacrylate block copolymers, due to the formation of compact core of the micelles. The formations of core-shell micelles were established from LLS and TEM studies. All the three azo methacrylate amphiphilic block copolymers formed hard core-shell micelles with relatively small R_h values of 31 nm for p(DMAEMA₁₇₂-b-BPHM₇), 26 nm for p(DMAEMA₁₇₂-b-CPHM₇) and 32 nm for p(DMAEMA₁₇₂-b-PPHM₉). Whereas for the azo acrylate copolymer, p(DMAEMA₁₇₂-b-BPHA₆), large micelles with R_h∼78 nm with loose core was formed.     

CMC and dynamic properties of poly(VA-<Emphasis Type="Italic">b</Emphasis>-St) copolymer micelles for drug delivery

The polymeric micelles from amphiphilic block copolymer poly(vinyl alcohol-b-styrene) (poly(VA-b-St)) with different syndiotacticity of poly(vinyl alcohol) (PVA) block were prepared by dialysis against water. Critical micelle concentration (CMC) and dynamic properties of poly(VA-b-St) copolymeric micelles were investigated by fluorescence techniques. From the fluorescence emission spectrum measurements using pyrene as a fluorescence probe, the observed CMC value was in the range of 0.125–4.47 mg/L. The CMC value increased with decreasing the weight ratio of PS to PVA block and with increasing the syndiotacticity of PVA block. The rate of pyrene release was very slow for block copolymers containing PVA block with higher syndiotacticity, which indicates that their micelles have increased kinetic stability. This work was presented at 13 ^th YABEC symposium held at Seoul, Korea, October 20–22, 2007.     

Mechanism of particle formation in radical emulsion copolymerization of styrene with α-tert-butoxy-ω-vinylbenzyl-polyglycidol macromonomer

In the batch emulsion copolymerization of styrene and α-tert-butoxy-ω-vinylbenzyl-polyglycidol macromonomer, carried out at macromonomer concentrations exceeding the critical micelle concentration (c_mc), particles are formed by a two-step coagulative nucleation mechanism. This mechanism leaves its mark on morphology of particle interface, rate of polymerization and on molecular weight distribution of the obtained polymer. AFM studies revealed that the interface of particles is composed of objects with dimensions close to dimensions of the primary particles. Compartmentalization of styrene in the macromonomer micelles leads to the higher initial rate of styrene conversion than in the similar macromonomer free homopolymerization of styrene. The initial polymerization in the monomer-swollen macromonomer micelles, similar to the microemulsion polymerization, is responsible for the formation of the highest molecular weight component. In the mature particles there are two different polymerization loci: the interfacial layer and the core. This leads to bimodal molecular weight distribution of the formed polymer.     

Synthesis of poly(vinyl alcohol) core-polystyrene shell-type flower microgels

Four types of poly(vinyl alcohol-b-styrene-b-vinyl alcohol) (P(VA-b-S-b-VA)) triblock copolymers were synthesized by hydrolysis of poly(vinyl acetate-b-styrene-b-vinyl acetate) triblock copolymers prepared by radical living polymerization. The polyvinyl alcohol (PVA) core-polystyrene corona-type flower micelles were formed with ASA-4 in benzene at 25°C. The PVA core part of the micelle was crosslinked with hexamethylene diisocyanate in solution. The monodispersed spherical products (microspheres) were synthesized by crosslinking. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 63: 849–854, 1997     

Synthesis and self-assembly of a novel Y-shaped copolymer with a helical polypeptide arm

A novel biodegradable Y-shaped copolymer, poly(l-lactide)₂-b-poly(γ-benzyl-l-glutamic acid) (PLLA₂-b-PBLG), was synthesized by the ring-opening polymerization (ROP) of N-carboxyanhydride of γ-benzyl-l-glutamate (BLG-NCA) with centrally amino-functionalized poly(l-lactide), PLLA₂-NH₂, as a macroinitiator in a convenient way. The Y-shaped copolymer and its precursors were characterized by ¹H NMR, FT-IR, GPC, WAXD and DSC measurements. The self-assembly of the PLLA₂-b-PBLG copolymer in toluene and benzyl alcohol was examined. It was found that the self-assembly of the copolymer was dependent on solvent and on relative length of the PBLG block. For a copolymer with PLLA blocks of 26 in total degree of polymerization (DP), if the PBLG block was long enough (e.g., DP = 54 or more), the copolymer/toluene solution became a transparent gel at room temperature. In benzyl alcohol solution, only PLLA₂-b-PBLG containing ca. 190 BLG residues could form a gel; those with shorter PBLG blocks (e.g., DP = 54) became nano-scale fibrous aggregates and these aggregates were dispersed in benzyl alcohol homogeneously. Copolymers with short PBLG blocks behaved like a pure PLLA both in toluene and in benzyl alcohol. These experimental results were discussed and explained by virtue of the helical conformation of PBLG and the interactions between the solvents and the PLLA and/or PBLG segments.     

Thermoresponsive core-shell-corona micelles of poly(ethyleneglycol)-b-poly(N-isopropylacrylamide)-b-polystyrene

Core-shell-corona micelles with a thermoresponsive shell self-assembled by triblock copolymer of poly(ethyleneglycol)-b-poly(N-isopropylacrylamide)-b-polystyrene (PEG₄₅-b-PNIPAM₁₆₈-b-PS₄₆) are studied by ¹H NMR, light scattering and atomic force microscopy. The thermoresponsive triblock copolymer, which has a relatively short hydrophobic PS block, can disperse in water at room temperature to form core-shell-corona micelles with the hydrophobic PS block as core, the thermoresponsive PNIPAM block as shell and the hydrophilic PEG block as corona. At temperature above lower critical solution temperature (LCST) of the PNIPAM block, the PNIPAM chains gradually collapse on the PS core to shrink the size and change the structure of the resultant core-shell-corona micelles with temperature increasing. It is found that there possibly exists an interface between the PNIPAM shell and PEG corona of the core-shell-corona micelles at temperature above LCST of the PNIPAM block.     

Preparation and characterization of graft copolymers of methyl methacrylate and poly(n-hexyl isocyanate) macromonomers

Living poly(n-hexyl isocyanate) (PHIC) was deactivated with methacryloyl chloride to produce methacryl-terminated poly(n-hexyl isocyanate) (PHIC-MA) rodlike macromonomers. Radical copolymerization of methyl methacrylate (MMA) with PHIC-MA was performed using 2,2′-azobis(isobutyronitrile) as an initiator in benzene at 60 °C to prepare poly(methyl methacrylate)-graft-poly(n-hexyl isocyanate) (PMMA-graft-PHIC) graft copolymers. The monomer reactivity ratios of MMA (M₁) and PHIC-MA (M₂) were evaluated as r₁=11.5 and r₂=∼0, exhibiting remarkably lower reactivity of PHIC-MA macromonomer than that of common macromonomers. The resultant graft copolymers were characterized using gel permeation chromatography equipped with low-angle laser light-scattering to determine the molecular weights, and equipped with a refractive index detector and an ultraviolet light detector to estimate a PHIC weight fraction of PMMA-graft-PHIC at the ith elution volume of the GPC chromatogram. There are 2-3 PHIC grafts per PMMA molecule, and the PHIC rodlike chains might be difficult to introduce into the PMMA main chains having higher molecular weights. A specific dimension of PMMA-graft-PHIC in solution was discussed in detail.     

Nanostructure and hydrogen bonding in interpolyelectrolyte complexes of poly(?-caprolactone)-block-poly(2-vinyl pyridine) and poly(acrylic acid)

Nanostructured poly(?-caprolactone)-block-poly(2-vinyl pyridine) (PCL-b-P2VP)/poly(acrylic acid) (PAA) interpolyelectrolyte complexes (IPECs) were prepared by casting from THF/ethanol solution. The morphological behaviour of this amphiphilic block copolymer/polyelectrolyte complexes with respect to the composition was investigated in a solvent mixture. The phase behaviour, specific interactions and morphology were investigated using differential scanning calorimetry (DSC), Fourier transform infrared (FTIR) spectroscopy, optical microscopy (OM), dynamic light scattering (DLS) and atomic force microscopy (AFM). Micelle formation occurred due to the aggregation of hydrogen bonded P2VP block and polyelectrolyte (PAA) from non-interacted PCL blocks. It was observed that the hydrodynamic diameter (D_h) of the micelles in solution decreased with increasing PAA content up to 40 wt%. After 50 wt% PAA content, D_h again increased. The micelle formation in PCL-b-P2VP/PAA IPECs was due to the strong intermolecular hydrogen bonding between PAA homopolymer units and P2VP blocks of the block copolymer. The penetration of PAA homopolymers into the shell of the PCL-b-P2VP block copolymer micelles resulted in the folding of the P2VP chains, which in turn reduced the hydrodynamic size of the micelles. After the saturation of the shell with PAA homopolymers, the size of the micelles increased due to the absorption of added PAA onto the surface of the micelles.     

Synthesis of poly(tetrahydrofuran-b-ε-caprolactone) macromonomer via the Sml2-induced transformation

Summary A novel well-defined macromonomer consisting of different types of monomers in polymerization mechanisms was synthesized for the first time through the SmI₂-induced transformation. The macromonomer, -methacryloylpoly-(tetrahydrofuran-b--caprolactone), was prepared by the reaction of methacryloyl chloride with living poly(tetrahydrofuran-b--caprolactone) [poly(THF-b-CL)] which was obtained by the two-electron reduction of the cationic growing center of poly(THF) by samarium iodide (SmI₂) followed by the polymerization of CL. ¹H NMR analysis indicated the quantitative introduction of the methacryloyl group onto the polymer end. The molecular weight distribution of the macromonomer was relatively narrow, and the unit ratio of THF to CL could be controlled by both polymerization time of THF and the amount of CL, resulting from the living nature of both CL- and THF-polymerizations. Radical copolymerization of the produced macromonomers with methyl methacrylate in the presence of AIBN resulted in a polymethacrylate backbone grafted with poly(THF-b-CL) block copolymers.     

Microsphere synthesis by emulsion copolymerizations of styrene with poly(methacrylic acid) macromonomers

The polymer microspheres were synthesized by emulsion copolymerizations of styrene with vinyl benzyl terminated poly(methacrylic acid) (PMA) macromonomers in an aqueous ethanol solution. In these copolymerization systems, the macromonomer acted not only as a comonomer but also as a stabilizer. The particle diameters obtained from the water‐soluble initiator system showed smaller values than those obtained from the water‐insoluble initiator system. These polymer microspheres had a very narrow particle‐size distribution. The particle diameters depended strongly on the feed amount and molecular weight of PMA macromonomers, water fraction in the mixed solvent, and pH in the medium. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 85: 900–905, 2002     

Functionalization of amphiphilic coil-rod-coil triblock copolymer poly(2-vinylpyridine)-b-poly(n-hexyl isocyanate)-b-poly(2-vinylpyridine) with florescence moiety and C60

We recently achieved quantitative synthesis of an amphiphilic coil-rod-coil triblock copolymer, poly(2-vinylpyridine)-b-poly(n-hexyl isocyanate)-b-poly(2-vinylpyridine), by coupling in situ living diblock copolymer poly(2-vinylpyridine)-b-poly(n-hexyl isocyanate) (P2VP-b-PHIC) using malonyl chloride in the presence of pyridine. This led to the introduction of an active methylene group that is a site for further functionalization in the rod block. The Michael addition reaction of the triblock copolymer with 7-(4-trifluoromethyl) coumarin acrylamide led to copolymer bearing a fluorescent pendent in the rod block. The fluorescent labeled copolymers were isolated in ∼94% yields. Similarly C₆₀ pendent was introduced to the rod block by the Bingel reaction. The yields of C₆₀ functionalized copolymers were ∼54%. The precursor and functionalized amphiphilic coil-rod-coil copolymer show diverse morphologies, such as micelles and vesicles by simply changing the solvent. For the C₆₀ functionalized block copolymer, structural constraints in micelles and vesicles prevented C₆₀ pendents to aggregate.     

Grafted polytriphenylamines synthesized by atom transfer radical polymerization in tandem with oxidative polycondensation

Arylamine polymers are among the most studied conducting polymers due to a whole range of interesting properties and applications. In this paper, we describe the synthesis of soluble and processable electroactive poly(triphenylamine-g-oligostyrene) by two polymerization methods in tandem. In the first step, oligostyrene macromonomers with well-defined molecular weight, polydispersity and chain-end functionality were obtained by radical-controlled polymerization (i.e., atom transfer radical polymerization, ATRP) of styrene. The styrene oligomerization was carried out using a new triphenylamine-based initiator, [4-(diphenylamino)benzyl 2-bromo-2-methylpropanoate], cuprous bromide as co-initiator and bipyridine as ligand, at 100 °C, in bulk. Using two feed molar ratios of components: [styrene]₀:[initiator]₀:[CuBr]₀:[bipyridine]₀, two macromonomers with M _n = 9,900; M _w/M _n = 1.25 and M _n = 3,460; M _w/M _n = 1.22, respectively, were synthesized. The presence of triphenylamine moiety at one end of the macromonomer allowed the chemical and electrochemical polymerization of macromonomers to polytriphenylamine brushes having short oligostyrene grafts in every structural unit. The oligostyrene substituents confer processability and good solubility in common organic solvent characteristics for polystyrene while preserving the optoelectronic properties of arylamine-conjugated main chain. The chemical oxidative polymerization of macromonomer was carried out using iron (III) chloride dissolved in nitrobenzene. The redox behavior of the initiator, macromonomer and graft polymer was investigated by cyclic voltammetry. In all cases, it was observed that the reversible oxido-reduction of arylamine sites was accompanied by irreversible oxidative electropolymerization by free-para positions of triphenylamine substituents and deposition of thin films on working electrode. The structures of the initiator, macromonomers and graft copolymers were determined by Fourier transform infrared, ¹H and ¹³C NMR spectroscopy. The absorption and fluorescence spectroscopy have revealed absorption and emission maxima characteristics for triphenylamine group. DSC studies have confirmed glass transitions characteristics for styrene oligomers.     

The influence of hydrophobic substitution on self-association of poly(ethylene oxide)-b-poly(n-alkyl glycidyl carbamate)s-b-poly(ethylene oxide) triblock copolymers in aqueous media

A series of amphiphilic poly(ethylene oxide)-b-poly(n-alkyl glycidyl carbamate)s-b-poly(ethylene oxide) triblock copolymers were synthesized by reaction between poly(ethylene oxide)-b-polyglycidol-b-poly(ethylene oxide) precursor copolymer and four n-alkyl isocyanates: ethyl, propyl, butyl and pentyl. After dissolution in water at room temperature the copolymers spontaneously form micelles. The critical micellization concentrations were determined by UV-VIS spectroscopy. The dimensions of the micelles, the aggregation numbers, and in some cases the micellar shape were determined by dynamic and static light scattering in a relatively broad temperature range. Special attention has been paid to the influence of the number of the carbon atoms in the alkyl chains, and respectively, the relative hydrophobicity of the middle block upon the self-association process. Clouding transition was observed for all of the copolymers, the clouding point being dependent upon the length of the alkyl chain.     

Removal of perylene from water using block copolymer nanospheres or micelles

Water-soluble nanospheres with poly(acrylic acid) (PAA) coronas were prepared from poly(2-cinnamoylethyl methacrylate)-block-PAA (PCEMA-b-PAA) and P(CEMA-ran-OEMA)-b-PAA, where “ran” denotes the random incorporation of 2-octanoylethyl methacrylate (OEMA) into the PCEMA block. These nanospheres and polystyrene-block-PAA micelles uptake perylene, a polycyclic aromatic hydrocarbon (PAH), from water. The nanospheres or micelles, with the sorbed perylene, are precipitated by CaCl₂. These nanospheres may be useful in concentrating PAHs present in trace amounts in water for chemical analysis or in the reclamation of water contaminated by PAHs. Investigated in this article are factors that govern the capacities of the nanospheres and micelles, and the critical calcium concentration for inducing nanosphere or micelle precipitation. © 1998 John Wiley & Sons, Inc. J. Appl. Polym. Sci. 70: 397–408, 1998     

Evaluation of the Physicochemical Properties,Pharmacokinetics, and In Vitro Anticancer Effects of Docetaxel and Osthol Encapsulated in Methoxy Poly(ethylene glycol)-b-Poly(caprolactone) Polymeric Micelles

Docetaxel (DTX), a taxane-based anticancer drug, and osthol (OTH), a coumarin-derivative compound, have shown anticancer effects against different types of cancers through various mechanisms. However, these drugs have low solubility in water and low oral bioavailability, and thus their clinical application is difficult. To overcome these problems, we encapsulated DTX and OTH in methoxy poly(ethylene glycol)-b-poly(caprolactone) (mPEG-b-PCL) and conducted studies in vitro and in vivo. We selected a 1:4 ratio as the optimal ratio of DTX and OTH, through combination index analysis in A549 cancer cells, and prepared micelles to evaluate the encapsulation efficiency, drug loading, particle size, and zeta potential. The in vitro drug-release profile showed that DTX/OTH-loaded mPEG-b-PCL micelles could slowly release DTX and OTH. In the clonogenic assay, DTX/OTH-loaded mPEG-b-PCL micelles showed 3.7 times higher inhibitory effect than the DTX/OTH solution. Pharmacokinetic studies demonstrated that micelles in combination with DTX and OTH exhibited increased area under curve and decreased clearance values, as compared with single micelles.     

Microstructure and performance of block copolymer modified epoxy coatings

The effects of two diblock copolymers, poly(ethylene-alt-propylene)-b-poly(ethylene oxide) (PEP–PEO) and poly(1,2-butadiene)-b-poly(2-vinyl pyridine) (PB–P2VP) on the mechanical properties of epoxy coatings were studied. Both modifiers self-assembled into spherical micelles of 10–20 nm diameter in cured bulk epoxy. This morphology was preserved in 15 μm thick coatings; however, micelle segregation to the coating/substrate interface was also observed. The critical strain energy release rate, G_1c, of bulk thermosets was enhanced by up to fivefold with the addition of block copolymers. Likewise, the abrasive wear resistance of thin coatings increased with modifier inclusion. The results showed that at 5 wt.% of loading, block copolymers were able to impart a 40% increase in abrasive wear resistance to modified coatings over neat ones. Block copolymer modifiers did not sacrifice the modulus and glass transition temperature of bulk thermosets and coatings, or the hardness and transparency of coatings.     

Poly(ε-caprolactone)-block-poly(N-vinyl pyrrolidone) diblock copolymers grafted from macrocyclic oligomeric silsesquioxane

Poly(ε-caprolactone)-block-poly(N-vinyl pyrrolidone) diblock copolymers grafted from macrocyclic oligomeric silsesquioxane (MOSS) (denoted MOSS[PCL-b-PVPy]₁₂) were synthesized via the sequential polymerizations involving ring-opening polymerization (ROP) of ε-caprolactone (CL) and RAFT/MADIX polymerization of N-vinyl pyrrolidone (NVP). The organic-inorganic brush-like diblock copolymers were characterized by means of nuclear magnetic resonance spectroscopy (NMR) and gel permeation chromatography (GPC). Small angle X-ray scattering (SAXS) showed that all the MOSS[PCL-b-PVPy]₁₂ was microphase-separated in the amorphous state. The microphase-separated morphologies were quite dependent on the length of PVPy blocks and the crystallization behavior of PCL subchains was significantly affected by the lengths of PVPy subchains. In aqueous solutions, the MOSS[PCL-b-PVPy]₁₂ can be self-assembled into the polymeric micelles as evidenced by dynamic light scattering (DLS) and transmission election microscopy (TEM). The critical micelle concentrations of the brush-like diblock copolymers increased with increasing the lengths of PVPy blocks. It is proposed that the stability of the micellar cores was increased with the macrocyclic molecular brush structure of the diblock copolymers and the formation of the MOSS aggregates via MOSS–MOSS interactions.