Synthesis and characterization of dendritic-linear-dendritic triblock copolymers based on poly(amidoamine) and polystyrene

Dendritic-linear-dendritic triblock copolymers composed of linear polystyrene (PSt) and poly(amidoamine) dendrons have been successfully synthesized. Two bromines-terminated PSt with M_n = 13,000 was prepared by atom transfer radical polymerization (ATRP) using α,α′-dibromo-p-xylene as initiator. Then the terminal bromines at both ends of PSt chains were replaced by one imine group of piperazine (PZ), and further Michael addition reaction of terminal PZ with excess 1,3,5-triacryloylhexahydro-1,3,5-triazine (TT) produced the first generation (G₁) of the triblock copolymer. Continuous growth of dendrons from G1.5 to G4 at the both ends of PSt chains was carried out by the iterative Michael addition reactions with excess PZ and following TT. The ABA triblock copolymers composed of the G₁-G₄ dendrons and the linear PSt were obtained. Structures of the triblock polymers were characterized by GPC and ¹H NMR spectra. Thermal phase transitions of the polymers were studied by DSC measurements, and all of the copolymers displayed a glass transition temperature.     

Synthesis and properties of ABA-type triblock copolymers of poly(glycolide-co-caprolactone) (A) and poly(ethylene glycol) (B)

Poly(glycolide-co-caprolactone) (A)-poly(ethylene glycol) (B) ABA-type triblock copolymers (PGCE) were synthesized by bulk ring opening polymerization, using the hydroxyl endgroups of poly(ethylene glycol) (PEG) as initiator and stannous octoate as catalyst. The resulting copolymers were characterized by various analytical techniques. Gel permeation chromatographic analysis indicated that the polymerization product was free of residual monomers, PEG and oligomers. ¹H NMR and differential scanning calorimeter results demonstrated that the copolymers had a structure of poly(glycolide-co-caprolactone) (PGC) chains chemically attached to PEG segments. All the PGCE copolymers showed improved hydrophilicity in comparison with the corresponding PGC copolymers with the same molar ratio of glycolidyl and caproyl units. The microspheres of PGCE copolymer exhibited rough surfaces quite different from the smooth surface of PGC microspheres. This phenomenon was attentively ascribed to the highly swollen ability of PGCE copolymers and the freeze-drying process in the microspheres fabrication.     

Block and star block copolymers by mechanism transformation. VIII Synthesis and characterization of triblock poly(LLA-b-St-b-MMA) by combination of ATRP and ROP

Block copolymers, poly(LLA-b-St)s and triblock copolymes, poly(LLA-b-St-b-MMA)s have been synthesized by the combination of the atom transfer radical polymerization (ATRP) with ring-opening polymerization (ROP) using bifunctional initiator β-hydroxylethyl α-bromoisobutyrate (HEBIB) without intermediate function transformation. The molecular weight (MW) and the molecular weight distribution (MWD) can be controlled. The structures of the copolymers were confirmed by ¹H NMR spectroscopy and GPC.     

Crystallization behavior of oxyethylene/oxybutylene diblock and triblock copolymers

The crystallization behavior of poly(oxyethylene)-b-poly(oxybutylene) block copolymers with different compositions, morphologies and architectures (E_mB_n diblock copolymers and E_mB_nE_m, B_nE_mB_n triblock copolymers) were investigated and the effect of volume fraction and architecture on the crystallization temperature (T_c) in non-isothermal crystallization was determined. It is found that the E_mB_n diblock copolymers having long E blocks exhibit similar crystallization temperatures, irrespective of volume fraction and morphology, but for the block copolymers with shorter E blocks the crystallization temperature increases with both the volume fraction, φ_E, and the length, m, of the E block. Some block copolymers with extremely low T_c, which fall into the temperature range normally associated with homogenous nucleation, were chosen for time-resolved small-angle X-ray scattering (SAXS) and isothermal crystallization kinetics experiments. The results show that breakout crystallization occurs in all these block copolymers. Therefore, unlike E_mB_n/B_h blends, there is no obvious relationship between T_c and crystallization behavior in neat block copolymers and homogeneous nucleation does not definitely lead to confined crystallization. The values of χ_c/χ_ODT for all the block copolymers with hex and bcc morphology were also calculated. It is found that all the block copolymers have χ_c/χ_ODT<3, in agreement with the previously reported critical value and consistent with their breakout crystallization behavior.     

Synthesis and characterization of dual stimuli-responsive block copolymers based on poly(N-isopropylacrylamide)-b-poly(pseudoamino acid)

The current study synthesized amphiphilic thermal/pH-sensitive block copolymers PNiPAAm-b-PHpr by condensation polymerization of trans-4-hydroxy-l-proline (Hpr) initiated from hydroxy-terminated poly(N-isopropylacrylamide) (PNiPAAm) as the macroinitiator in the presence of the catalyst, SnOct₂. ¹H NMR, FTIR, and gel permeation chromatography (GPC) characterized these copolymers. Their solutions showed reversible changes in optical properties: transparent below a lower critical solution temperature (LCST) and opaque above the LCST. The LCST values depended on the polymer composition and the media. With critical micelle concentrations (CMCs) in the range of 1.23-3.73 mg L⁻¹, the block copolymers formed micelles in the aqueous phase owing to their amphiphilic characteristics. Increased hydrophobic segment length or decreased hydrophilic segment length in an amphiphilic diblock copolymer produced lower CMC values. The current work proved the core-shell structure of micelles by ¹H NMR analyses of the micelles in D₂O. Transmission electron microscopy analyzed micelle morphology, showing a spherical core-shell structure. The micelles had an average size in the range of 170˜210 nm (blank), and 195˜280 nm (with drug). Observations showed high drug entrapment efficiency and drug-loading content for the drug micelles.     

Synthesis of poly(ethylene oxide)-block-poly(methyl methacrylate)-block-polystyrene triblock copolymers by two-step atom transfer radical polymerization

The synthesis of ABC triblock copolymer poly(ethylene oxide)-block-poly(methyl methacrylate)-block-polystyrene (PEO-b-PMMA-b-PS) via atom transfer radical polymerization (ATRP) is reported. First, a PEO-Br macroinitiator was synthesized by esterification of PEO with 2-bromoisobutyryl bromide, which was subsequently used in the preparation of halo-terminated poly(ethylene oxide)-block-poly(methyl methacrylate) (PEO-b-PMMA) diblock copolymers under ATRP conditions. Then PEO-b-PMMA-b-PS triblock copolymer was synthesized by ATRP of styrene using PEO-b-PMMA as a macroinitiator. The structures and molecular characteristics of the PEO-b-PMMA-b-PS triblock copolymers were studied by FT-IR, GPC and ¹H NMR.     

Synthesis of poly(trioctylammonium p-styrenesulfonate) homopolymers and block copolymers by RAFT polymerization

A method to prepare sulfonated polystyrene-containing block copolymers has been investigated by neutralizing styrene sulfonic acid with trioctylamine to produce the hydrophobic monomer trioctylammonium p-styrenesulfonate (SS-TOA). This monomer was polymerized by reversible addition fragmentation chain transfer (RAFT) polymerization to produce PSS-TOA homopolymers. A PSS-TOA homopolymer was then used as a macro-RAFT agent for the polymerization of styrene to prepare poly(trioctylammonium p-styrenesulfonate)-block-poly(styrene) (PSS-TOA-b-PS). These block copolymers could be ion-exchanged to produce either the hydrophilic sodium salt form of PSS or a hydrophobic quaternary ammonium salt. This approach will be useful for preparing PSS-containing block copolymers with a range of hydrophobic blocks for applications such as ion-exchange membranes.     

ABA triblock copolymers from poly(p‐dioxanone) and poly(ethylene glycol)

Poly(p‐dioxanone)–poly(ethylene glycol)–poly(p‐dioxanone) ABA triblock copolymers (PEDO) were synthesized by ring‐opening polymerization from p‐dioxanone using poly(ethylene glycol) (PEG) with different molecular weights as macroinitiators in N₂ atmosphere. The copolymer was characterized by ¹H NMR spectroscope. The thermal behavior, crystallization, and thermal stability of these copolymers were investigated by differential scanning calorimetry and thermogravimetric measurements. The water absorption of these copolymers was also measured. The results indicated that the content and length of PEG chain have a greater effect on the properties of copolymers. This kind of biodegradable copolymer will find a potential application in biomedical materials. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102:1092–1097, 2006     

Morphological and mechanical study of nanostructured epoxy systems modified with amphiphilic poly(ethylene oxide-b-propylene oxide-b-ethylene oxide)triblock copolymer

Thermosetting systems based on DGEBA epoxy resin and poly(ethylene oxide-b-propylene oxide-b-ethylene oxide) (EPE) triblock copolymer were prepared and investigated. Different mixtures were obtained by using different contents of EPE block copolymer in order to study the influence of the modifier on the properties of the final materials. All thermosetting systems were prepared without using any solvent and were cured at ambient temperature, taking into account the lower critical solution temperature (LCST) behavior of the block copolymer. DSC results indicated that the addition of block copolymer affected to the curing reaction time and to the glass transition temperature of the mixtures and also the miscibility of EPE triblock copolymer in the epoxy resin was proved. The morphologies studied by AFM and TEM showed clear nanostructuration up to 25 wt % EPE content. The addition of 5 and 15 wt % of EPE block copolymer led to a considerable improvement in the toughness of the materials. When EPE block copolymer was added to the epoxy resin, the surface became more hydrophilic and the UV–vis transmittance decreased slightly maintaining a high level of transparency.     

Paclitaxel-incorporated nanoparticles using block copolymers composed of poly(ethylene glycol)/poly(3-hydroxyoctanoate)

Block copolymers composed of poly(3-hydroxyoctanoate) (PHO) and methoxy poly(ethylene glycol) (PEG) were synthesized to prepare paclitaxel-incorporated nanoparticle for antitumor drug delivery. In a ¹H-NMR study, chemical structures of PHO/PEG block copolymers were confirmed and their molecular weight (M.W.) was analyzed with gel permeation chromatography (GPC). Paclitaxel as a model anticancer drug was incorporated into the nanoparticles of PHO/PEG block copolymer. They have spherical shapes and their particle sizes were less than 100 nm. In a ¹H-NMR study in D₂O, specific peaks of PEG solely appeared while peaks of PHO disappeared, indicating that nanoparticles have core-shell structures. The higher M.W. of PEG decreased loading efficiency and particle size. The higher drug feeding increased drug contents and average size of nanoparticles. In the drug release study, the higher M.W. of PEG block induced the acceleration of drug release rate. The increase in drug contents induced the slow release rate of drug. In an antitumor activity study in vitro, paclitaxel nanoparticles have practically similar anti-proliferation activity against HCT116 human colon carcinoma cells. In an in vivo animal study using HCT116 colon carcinoma cell-bearing mice, paclitaxel nanoparticles have enhanced antitumor activity compared to paclitaxel itself. Therefore, paclitaxel-incorporated nanoparticles of PHO/PEG block copolymer are a promising vehicle for antitumor drug delivery.     

Synthesis and characterization of poly(ether sulfone) copolymers

Poly(ether sulfone) copolymers I–V were synthesized by the nucleophilic substitution reaction of 4,4-dichlorodiphenyl sulfone with varying mole proportions of 4,4-isopropylidene diphenol (bisphenol A) and 4,4-dihydroxydiphenyl sulfone (bisphenol S) using sulfolane as the solvent in the presence of anhydrous K₂CO₃. The polymers were characterized by different physicochemical techniques. The glass transition temperature was found to decrease with increase in the concentration of bisphenol A units in the polymers. All polymers were found to be amorphous. Thermogravimetric studies showed that all the polymers were stable up to 400°C with a char yield of about 36% at 900°C in a nitrogen atmosphere. ¹³C-NMR spectral analysis reveals that bisphenol S-based triads are preferentially formed compared to bisphenol-A triads, indicating greater reactivity of bisphenol S toward dichlorodiphenyl sulfone. The overall activation energy for the thermal decomposition of bisphenol A-based polymer (1) is much higher than that of bisphenol S-based polymer ( II ). This was attributed to the modification of the backbone of polymer I during the initial cleavage of the C—CH3 bond of the isopropyledene group. Polymer II decomposes by cleavage of the C—SO2 bond. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 69: 743–750, 1998     

Synthesis and characterization of poly(ethylene glycol)-b-poly (l-lactide)-b-poly(l-glutamic acid) triblock copolymer

A biodegradable amphiphilic triblock copolymer of poly(ethylene glycol)-b-poly(l-lactide)-b-poly(l-glutamic acid) (PEG-b-PLLA-b-PLGA) was obtained by catalytic hydrogenation of poly(ethylene glycol)-b-poly(l-lactide)-b-poly(γ-benzyl-l-glutamic acid) (PEG-b-PLLA-b-PBLGA) synthesized by the ring-opening polymerization (ROP) of N-carboxyanhydride of γ-benzyl-l-glutamate (BLG-NCA) with amino-terminated MPEG-b-PLLA-NH₂ as a macroinitiator. MPEG-b-PLLA-NH₂ converted from MPEG-b-PLLA-OH first reacted with tert-Butoxycarbonyl-l-phenylalanine (Phe-^NBOC) and dicyclohexylcarbodiimide (DCC) and then deprotected the tert-butoxycarbonyl group. MPEG-b-PLLA-OH was prepared by ROP of l-lactide with monomethoxy poly(ethylene glycol) in the presence of stannous octoate. The triblock copolymer and its diblock precursors were characterized by ¹H NMR, FTIR, GPC and DSA (drop shape analysis) measurements. The lengths of each block polymers could be tailored by molecular design and the ratios of feeding monomers. The triblock polymer PEG-b-PLLA-b-PLGA containing carboxyl groups showed obviously improved hydrophilic properties and could be a good potential candidate as a drug delivery carrier.     

Frustrated structures of polycaprolactam and poly(p-benzamide) in their rod-coil-rod triblock copolymers

A series of rod-coil-rod triblock copolymers were synthesized by two-step polycondensation with polycaprolactam (PA6) as the flexible block and poly(p-benzamide) (PBA) as the rod. Proton nuclear magnetic resonance (¹H NMR), UV-vis spectrophotometry and differential scanning calorimetric (DSC) were first performed to determine the fundamental molecular structure and thermal property of each polymer. Through wide angle X-ray scattering (WAXS), and Fourier transformation infrared spectroscopy (FT-IR) measurements, frustrated structures of both components were studied. For PA6, a quasi-γ mesomorphic order was found in the transition-region nearby the PBA domain which is more favored with the increase of the PBA content owing to stretching from the PBA rod block and different cross-section areas of the rod and coil chains. On the other hand, the mesomorphic order of PA6 segments imposes stretching and constraint on the PBA blocks, which leads to a reduction of order of PBA block when the volume fraction of rod reaches approximately 45%. α crystals of PA6 can form only in the triblock copolymer with low volume fraction of PBA, which exhibit the Brill transition feature during the heating process though this transition ends prematurely as the melting of crystals.     

Near-surface morphology effect on tack behavior of poly(styrene-b-butadiene-b-styrene) triblock copolymer/rosin films

The correlation between near-surface morphology and tack behavior of poly(styrene-b-butadiene-b-styrene) triblock copolymer (SBS)/rosin ester films was investigated using probe tack tests, transmission electron microscopy and small-angle X-ray scattering. The SBS/rosin films with rosin composition between 10 and 20 wt% rosin, prepared by slow evaporation of toluene during solvent casting, exhibited uniform near-surface morphology of lamellae oriented parallel to the surface. However, due to the limited solubility of rosin in the PS domains, the rosin started to phase-separate from the PS domains at 15 wt%, and formed fully separated micron-sized domains above 20 wt% rosin. The probe tack force of the SBS/rosin films increased steadily when the near-surface domain orientation changed from perpendicular cylinder to parallel lamellae on addition of rosin. Specifically, for a given lamellar morphology and surface orientation, macrophase separation of rosin plays a critical role in determining the tack properties of SBS/rosin films.     

Synthesis and properties of novel biodegradable triblock copolymers of poly(5-methyl-5-methoxycarbonyl-1,3-dioxan-2-one) and poly(ethylene glycol)

Novel biodegradable triblock copolymers of poly(5-methyl-5-methoxycarbonyl-1,3-dioxan-2-one) (PMMTC) with poly(ethylene glycol) (PEG), PMMTC-b-PEG-b-PMMTC, were synthesized by the ring-opening polymerization of MMTC in bulk, using the dihydroxyl PEG as initiator and Sn(Oct)₂ as catalyst. The triblock copolymers with different compositions were characterized by IR and ¹H NMR, their molecular weight was measured by gel permeation chromatography (GPC). The results showed that the molecular weight of triblock copolymers increased either with the increase of the molar ratio of MMTC in feed while the PEG chain length kept constant, or by lengthening the backbone chain of PEG block with the same ratio of MMTC in feed. The hydrophilicity of copolymers was greatly improved by incorporation of PEG block into polycarbonate. The in vitro hydrolytic/enzymatic degradation and controlled drug release properties of the triblock copolymers were also investigated.     

Synthesis, thermal properties, and specific interactions of high Tg increase in poly(2,6-dimethyl-1,4-phenylene oxide)-block-polystyrene copolymers

We have synthesized a series of block copolymers of poly(2,6-dimethyl-1,4-phenylene oxide) and polystyrene (PPO-b-PS copolymer) by atom transfer radical polymerization. The PS content in these copolymer systems was determined by using infrared spectroscopy, thermal gravimetric analysis, and solution and solid-state NMR spectroscopy; good correlations exist between these characterization methods. DSC analyses indicated that the PPO-b-PS copolymers have higher glass transition temperatures than do their corresponding PPO/PS blends. Our FTIR and solid-state NMR spectroscopic analyses suggest that the PPO-b-PS copolymers possess stronger specific interactions that are responsible for the observed relatively higher values of T_g. We found one single dynamic relaxation from the dynamic mechanical analysis, which implies dynamic homogeneity exists in the PPO-b-PS copolymer; this result is consistent with the one single proton spin-lattice relaxation time observed in the rotating frame [T_1ρ(H)] during solid state NMR spectroscopic analysis. In addition, the 2D FTIR spectroscopy reveals evidence for the stronger interactions between segments of PPO and PS through the formation of π-cation complexes.     

Disparate polymerization facilitates the synthesis of versatile block copolymers from poly(trimethylene carbonate)

A disparate polymerization technique is utilized for preparing versatile block copolymers from poly(trimethylene carbonate) (poly(TMC)). In this study, 4-(chloromethyl)benzyl alcohol (CBA) is used for the disparate polymerization. The hydroxyl group of CBA is involved in ring-opening polymerization and the benzyl chloride group is involved in incorporating dithiocarbamate for pseudo-living radical polymerization. First, TMC is polymerized from the hydroxyl group of CBA by using an organocatalyst. The benzyl chloride group in CBA is modified using a dithiocarbamate, and then vinyl and methacrylate monomers are polymerized by photo-driven pseudo-living radical polymerization. The resulting block copolymers are versatile and the molecular weight distribution is reasonably narrow. In the present study, N-isopropylacrylamide, acrylamide glycolic acid, and 2-hydroxyethyl methacrylate are used for the disparate polymerization. The resulting block copolymers could be well dissolved in water by incorporation of hydrophilic segment into hydrophobic poly(TMC). The solution property is characterized in terms of hydrophobic domain formation and phase transition under ambient conditions. Moreover, enzymatic degradation is evaluated by using a copolymer-coated substrate. The block copolymer synthesis technique is considerably versatile, and the resulting polymer function can be freely designed. The disparate polymerization technique is a promising approach that provides universal materials for integrating biodegradable polyesters and functional polymers.     

Polymer electrolyte membranes from fluorinated polyisoprene-block-sulfonated polystyrene: Membrane structure and transport properties

With a view to optimizing morphology and ultimately properties, membranes have been cast from relatively inexpensive block copolymer ionomers of fluorinated polyisoprene-block-sulfonated polystyrene (FISS) with various sulfonation levels, in both the acid form and the cesium neutralized form. The morphology of these membranes was characterized by transmission electron microscopy and ultra-small angle X-ray scattering, as well as water uptake, proton conductivity and methanol permeability within the temperature range from 20 to 60 °C. Random phase separated morphologies were obtained for all samples except the cesium sample with 50 mol% sulfonation. The transport properties increased with increasing degree of sulfonation and temperature for all samples. The acid form samples absorbed more water than the cesium samples with a maximum swelling of 595% recorded at 60 °C for the acid sample having 50 mol% sulfonation. Methanol permeability for the latter sample was more than an order of magnitude less than for Nafion 112 but so was the proton conductivity within the plane of the membrane at 20 °C. Across the plane of the membrane this sample had half the conductivity of Nafion 112 at 60 °C.     

Dynamic mechanical properties of annealed sulfonated poly(styrene-b-[ethylene/butylene]-b-styrene) block copolymers

Solution cast films of lightly sulfonated styrene-b-[ethylene-co-butylene]-b-styrene, (sSEBS) block copolymers were annealed for various times at 120 °C and thermal transitions are evaluated using dynamic mechanical analysis. Increased annealing time and increase in degree of sulfonation increases T_g for the PS phase while T_g for the EB phase is practically unchanged, and in some cases, there is suggestion of a relaxation due to EB-PS inter-phases. Annealing has a minor effect on the rubbery plateau storage modulus. Thus, annealing primarily alters the PS block phase. EB-PS phase separation appears to be refined with increasing SO₃H content. The region of rubber elasticity extends to higher temperatures with increased degree of sulfonation. A high temperature dynamic mechanical transition that is tentatively attributed to disruption of SO₃H—rich sub-domains within the PS block domains shifts to higher temperature with annealing.