Block copolymers with a polyvinyl and a polypeptide block: factors governing the folding of the polypeptide chains

AB block copolymers polystyrene-poly(γ-benzyl-l-glutamate) (SG) of various molecular weights and compositions were synthesized and studied by X-ray diffraction and infra-red spectroscopy. They exhibit lamellar mesophases in the solid state and in dioxane concentrated solution. Each sheet of the lamellar structure results from the superposition of two layers: one formed by the polyvinyl chains in a disordered conformation, the other formed by the polypeptide chains in an α-helix conformation arranged in a hexagonal array and generally folded. The comparison of the lamellar structure of copolymers polystyrene-poly(γ-benzyl-l-glutamate) (SG), polybutadiene-poly(γ-benzyl-l-glutamate) (BG) and polystyrene-poly(ε-carbobenzoxy-l-lysine) (SCK) showed that: (1) for the three types of copolymers (copolymers SG or BG or SCK) the number of folds of the polypeptide chains increases with the molecular weight of both the polyvinyl and the polypeptide blocks; (2) for copolymers of fixed molecular weight of the polyvinyl block and fixed degree of polymerization of the polypeptide block the number of folds of the polypeptide chains depends upon the nature of both the polyvinyl and the polypeptide blocks: a polypeptide chain is more folded when it is linked to a polystyrene chain than to a polybutadiene chain but a poly(ε-carbobenzoxy-l-lysine) chain is more rigid than a poly(γ-benzyl-l-glutamate) chain.     

Solid-state morphologies of linear and bottlebrush-shaped polystyrene-poly(Z-l-lysine) block copolymers

The solid-state structures of polystyrene-poly(Z-l-lysine) block copolymers were examined with respect to the polymer architecture and the secondary structure of the polypeptide using circular dichroism, quantitative small- and wide-angle X-ray scattering, and electron microscopy. Linear block copolymers exhibit a hexagonal-in-lamellar structure where folded and packed polypeptide α-helices form lamellae which extend over an exceptional broad range of the composition diagram. Star- or bottlebrush-shaped copolymers are able to stabilize a larger interface area than linear ones which promotes the formation of undulated lamellar mesophases. Depending on the secondary structure of polypeptide segments, plane lamellar, superundulated lamellar, or corrugated lamellar phases are formed. These results indicate the importance of a secondary structure and packing of polymer chains for the formation of new phases and ordering far from the ‘classical’ phase behavior.     

Use of freeze-fracture electron microscopy to study the refolding of crystallized chains in block copolymers

Block copolymers with an amorphous block or polystyrene (PS) of polybutadiene (PB) and a crystallizable block of poly(ethylene oxide) (PEO) or poly(?-caprolactone) (PCL) exhibit lamellar crystalline structures. In these structures the elementary sheet results from the superposition of two layers: the first contains the amorphous blocks and the second contains the crystallized and refolded PEO or PCL chains. Study of the lamellar structure by freeze-fracture and electron microscopy provides micrographs that show crystallization of the PEO and PCL chains in two superposed layers and confirm the model proposed to explain the results of earlier X-ray diffraction and differential scanning calorimetry studies.     

New block copolymers of α-amino acids

Triblock cepolypeptides of γ-benzyl-L-glutamate and L-leucine or L-valine of high molecular weight have been prepared. The solid state structure of the block copolymers cast from preferential solvents has been compared with that of random copolymers of similar composition as well as the appropriate homopolymers. Infrared and solid state circular dichroism measurements indicate that the γ-benzyl-L-glutamate and L-leucine blocks assume an α-helical conformation and L-valine blocks a β-sheet structure. The WAXD patterns of the block copolymers show overlapping reflections characteristic of the individual homopolymers. Further evidence for a phase separated morphology is provided by dynamic mechanical spectroscopy.     

Copoly(γ,dl-glutamate)s containing short and long linear alkyl side chains

Copoly(α-alkyl γ,dl-glutamate)s containing octadecyl and ethyl side groups were prepared from racemic poly(γ,dl-glutamic acid) of bacterial origin, and their structure in the solid state examined by DSC, polarized IR spectroscopy and X-ray diffraction methods. Copolymers with a blocky microstructure were prepared by transesterification of poly(α-ethyl γ,dl-glutamate) with octadecanol for a wide range of compositions. Random copolymers were prepared by sequential alkylation of the polyacid with ethyl bromide and octadecyl bromide in solution. All the multiblock copolymers were found to adopt the layered structure characteristic of comb-like polypeptides with the octadecyl side chains crystallized in extended conformation in a separate phase, and aligned normal to the main chains, which are arranged in helical conformation. Melting of paraffinic crystallites reversibly happened in the 50-55 °C range without significant alteration of the layered arrangement. Upon heating at 150 °C, the structure contracted but no indication on the existence of a second transition, as it is known to occur in comb-like poly(α-alkyl γ-glutamate) homopolymers, was observed by DSC. Random copolymers did show nor thermal transition neither evidence of any organized supramolecular structure.     

Self-assembly of amphiphilic liquid crystal block copolymers containing a cholesteryl mesogen: Effects of block ratio and solvent

We report on the self-assembly, in water and in bulk, of amphiphilic liquid crystal block copolymers consisting of a cholesterol-based smectic LC polymer block (PAChol) and poly(ethylene glycol) (PEG) block. Two series of block copolymers, PEG₄₅-b-PAChol and PEG₁₁₄-b-PAChol (45 and 114 are the degree of polymerization of PEG blocks) with different hydrophilic/hydrophobic weight ratios were synthesized and characterized in detail. Depending on the diblock composition, smectic polymer vesicles and/or nanofibers were formed by adding water into a dilute solution of copolymers in dioxane. If THF is used instead of dioxane as solvent, solid spherical aggregates were obtained upon water addition for PEG₄₅-b-PAChol series, while macroscopic precipitation occurred for PEG₁₁₄-b-PAChol series. The mesomorphic and microphase segregation structures of the block copolymers in bulk were studied by X-ray scattering, DSC and POM. The interdigital smectic A (SmA_d) phase with a lamellar period of 4.25 nm was detected in all block copolymers. For PEG₁₁₄-b-PAChol₅ (PEG/PAChol weight ratio = 66/34) and PEG₁₁₄-b-PAChol₁₂ (45/55), lamellar type of microphase segregation was observed.     

Effectiveness of block copolymers as stabilizers for aqueous titanium dioxide dispersions of a high solid content

Copolymers of 2-(dimethylamino)ethyl methacrylate (DMAEMA) and sodium methacrylate (MANa) have been synthesized and tested as dispersants for the aqueous dispersion of titanium dioxide (80 wt.% solid). The molecular composition of the block copolymers has an effect on the dispersion stability. Triblock copolymers consisting of PMANa outer blocks are unable to stabilize the dispersion. Triblock copolymers of the reverse structure form poorly stable dispersions due to at least partial particle bridging. In the case of non-blocky distribution of the DMAEMA and MANa comonomers in the chains, the stabilization capability is lost. A decrease in the anchoring strength of the diblocks has a deleterious effect on the dispersion stability. Finally, the surface coverage depends on the availability of free copolymer chains (unimers) coexisting with micelles. These trends have been confirmed in a fully formulated paint containing Cu-phthalocyanine pigment.     

Conformations of polysulfone‐block‐polydimethylsiloxane chains in solution and in the solid state

Polysulfone block copolymers containing polydimethylsiloxane segments were obtained in solution by the condensation reaction of chloro‐terminated polysulfone oligomers and α,ω‐dihydrogensilyl‐polydimethylsiloxane. The conformations of the copolymer chains were investigated both in solution and in the solid state. Viscometric and UV absorption measurements were carried out in dilute dichloroethane solutions over a wide region of concentrations, in the temperature range 20–75 °C. The results point to a conformational transition phenomenon, located at around 55 °C, which is attributed to the transition from a segregated to a pseudo‐Gaussian conformation through a compressed‐segregated conformation. Copyright © 2004 Society of Chemical Industry     

Effect of flow on solutions of rod-coil block co-polymers

An elongational flow field is imposed on a solution of block copolymers consisting of semirigid macromolecules with rigid, rodlike sequences of units in combination with random coil (flexible) units. The problem is formulated according to the lattice treatment of Matheson and Flory. In this formulation, the system consists of rigid blocks whose lengths and locations are fixed by the structure within each macromolecule. These blocks are separated by random coiled units. An excess free energy other than the equilibrium Gibbs free energy of the quiescent solution has to be considered due to the flow field that tends to align the rods. This excess free energy is calculated from the potential energy of rods when a steady-state, homogeneous and irrotational flow field is applied to the solution. The effects of composition, polymer-solvent interaction, size of the co-polymer and flow rates are investigated. Depending on the size and number of rods, some of the chains studied exhibit a biphasic region at equilibrium that shifts to lower concentrations with increasing flow. Longer chains with shorter rods that are isotropic at equilibrium, exhibit a biphasic region at finite values of flow. The degree of orientation increases sharply when the system is biphasic. For larger flows, the orientation function is very close to unity which is perfect orientation.     

Crystallization of double crystalline block copolymer/crystalline homopolymer blends: 1. Crystalline morphology

The crystalline morphology formed in binary blends of poly(ε-caprolactone)- block-polyethylene (PCL-b-PE) copolymers and PCL homopolymers has been examined using synchrotron small-angle X-ray scattering (SR-SAXS) and differential scanning calorimetry (DSC) as a function of the homopolymer fraction in the blend. The PE block crystallized first on quenching from a lamellar microdomain structure to set a hard lamellar morphology (PE lamellar morphology) in the blend, followed by the crystallization of PCL chains (i.e., PCL homopolymers + PCL blocks). Two binary blends were studied by considering the miscible state of PCL homopolymers in the microdomain structure: when the PCL homopolymers were uniformly mixed with PCL blocks, they formed a mixed crystal. When the PCL homopolymers were localized between PCL blocks in the microdomain structure, DSC results suggested the possible formation of separate PCL crystals in the PE lamellar morphology. The effect of the advance crystallization of PE blocks on the subsequent crystallization of PCL chains was discussed as compared with the crystalline morphology formed in PCL-block-polybutadiene copolymer/PCL homopolymer blends, where the crystallization of PCL chains started directly from a microdomain structure without forming the hard lamellar morphology.     

两亲性嵌段共聚物的合成及其在离子液体中的自组装行为

采用阴离子开环聚合法合成了嵌段共聚物PCL—PEG—PCL（聚己内醋-聚乙二醇-聚己内酯）。用1HNMR和GPC等对产物的分子量和组成进行表征,将其在离子液体中配成胶束,通过透射电镜（TEM）观察胶束的微观结构。研究结果表明,当疏水链段长度固定时,胶束的自组装形状主要依赖于亲水链的长度。     

两亲性聚乙烯醇-b-聚苯乙烯嵌段共聚物的合成及表征

采用原子转移自由基聚合(ATRP)法合成了具有两亲性的聚乙烯醇-b-聚苯乙烯嵌段共聚物P(VA-b-St)。首先利用调聚反应制备了带三氯甲基端基的聚醋酸乙烯大分子引发剂。以联二吡啶作配体、氯化亚铜为催化剂,引发苯乙烯单体聚合,得到结构明确的P(VAc-b-St)嵌段共聚物,而后通过皂化反应将其水解,从而得到两亲性嵌段共聚物P(VA-b- St);产物采用FT-IR、1H NMR、GPC等方法进行结构表征。P(VA-b-St)在不同浓度溶剂中的自组装行为用TEM进行了观察,结果表明:P(VA-b-St)可在DMF溶剂中形成球状囊泡结构,其尺寸达到纳米级。     

Salt effects on the conformation of a ‘statistical’ copolymer of l-leucine and l-lysine

For studying the influence of water structure breaking and making anions on the conformation of poly(α-amino acids) containing hydrophobic and ionic side chains, c.d. measurements on ‘statistical copolymers of l-leucine and l-lysine were carried out in LiClO₄, NaClO₄, Li₂SO₄, Na₂SO₄, KF, NaF, and NaCl solutions and also in salt-free water. Copolymers with 50 mol% l-leucine do not form α-helices in pure water at pH 7 (20°C); however LiClO₄ and NaClO₄ at concentrations as low as 0.003 moll^?1 induce α-helix formation almost independent of cation. Surprisingly, not only ClO^?₄ but also SO^2?₄ has an α-helix inducing effect in this case, in contrast with basic homopoly (α-amino acids), where such an effect of SO^2?₄ was not observed. Therefore the presence of l-leucyl residues seems to be responsible for this α-helix inducing effect of sulphate anions. This agrees with the fact that sulphate stabilizes the ordered periodic conformation of native proteins as the increase of denaturing temperature shows. It can be assumed that the results of these measurements support the assumptions on the influence of water structure in making SO₄ anions on hydrophobic interactions, as well as of water structure breaking ClO₄ anions.     

Linear-dendritic block copolymers: The state of the art and exciting perspectives

Concurrent with the rapid development of both dendrimers and hyperbranched polymers, a novel class of block copolymer architectures has emerged from the combination of these dendritic architectures with linear chains, the “linear-dendritic block copolymers” (LDBCs). This review gives a comprehensive summary of the state of the art in this rapidly developing field from pioneering early work to promising recent approaches.The different strategies leading to these hybrid architectures with either perfect dendrimer/dendron building blocks or imperfect, yet more conveniently accessible hyperbranched segments, are reviewed and compared. The consequences of the unusual polymer topology for supramolecular structures both in solution and in the solid state are summarized, and important differences in comparison with classical linear block copolymer structures are highlighted. Current challenges in the area of block copolymers, nanotechnology and potential applications of linear-dendritic block copolymers are also considered.     

On Bridging and Mesogels

Copolymers bridging dissimilar domains are known to strengthen polymeric interfaces and to promote adhesion. A related effect gives rise to thermotropic elastomers formed by ABA triblock copolymers. Mesophases of ABA copolymers form physical networks due to bridging of different, glassy, A domains by B blocks. Shear alignment of the ABA melt produces “single crystal” mesonetworks with lamellar, cylindrical or micellar structure. The swelling of such mesonetworks by a selective solvent produces mesogels which differ from conventional gels because of their anisotropy and the high functionality of the cross links. Theoretical considerations concerning the bridging fraction, the swelling equilibrium and other issues are briefly reviewed for three types of systems: (1) Mesogels consisting of neutral, flexible ABA triblock copolymers. (2) Mesogels formed by ABA triblocks with a polyelectrolyte B block. (3) Mesogels produced by ABA triblock copolymers with a main chain liquid crystalline B block.     

Statistical thermodynamics of microdomain structures in block copolymer systems

A statistical thermodynamic theory of the microdomain structure of block copolymers is presented in qualitative physical terms. Special emphasis is placed on a presentation of the final algebraic formula which may be used to determine lamellar domain sizes as a function of block length and physical parameters. Calculations on lamellar domains and preliminary results for spherical domains are compared with experiments. Other topics discussed are: width of the interphases, results of a more exact theory, and thermodynamic., stability of the domain structure.     

PEO-PPO-PEO嵌段共聚物在水溶液中的自组装行为及其应用

聚氧乙烯-聚氧丙烯-聚氧乙烯(PEO-PPO-PEO)嵌段共聚物是一类重要的非离子表面活性剂,在选择性溶剂中可以自组装成多种形貌的介观结构。对PEO-PPO-PEO嵌段共聚物在水溶液中自组装行为进行了综述,介绍了其自组装行为的实验研究技术;阐明了嵌段共聚物构型、分子量、温度、浓度、添加剂等因素对PEO-PPO-PEO嵌段共聚物聚集行为的调控和作用机理;介绍了嵌段共聚物自组装特性的热力学模型、分子模拟及计算机预报等研究方法和研究进展;重点介绍了PEO-PPO-PEO嵌段共聚物在介孔材料制备、药物载体、生物大分子分离、嵌段共聚物修饰等方面的应用。     

Association behavior of thermo-responsive block copolymers based on poly(vinyl ethers)

Thermo-sensitive nanosized structures have been prepared in water from poly(methyl vinyl ether)-block-poly(isobutyl vinyl ether) (PMVE-b-PIBVE) block copolymers. The composition and the architecture (diblock and triblock architectures) of the PMVE-b-PIBVE copolymers have been varied. The investigated copolymers had an asymmetric composition with a major PMVE block. While the PIBVE blocks are hydrophobic, the PMVE blocks are hydrophilic at room temperature and become hydrophobic above their demixing temperature (around 36 °C) as a result of the lower critical solution temperature (LCST) behavior. At room temperature, the amphiphilic copolymers aggregate in water above a critical micelle concentration, which has been experimentally measured by hydrophobic dye solubilization. The hydrodynamic diameter of the structures formed above the cmc has been measured by dynamic light scattering (DLS) while their morphology has been studied by transmission electron microscopy (TEM). ¹H NMR measurements in D₂O at room temperature reveal that the aggregates contain PIBVE insoluble regions surrounded by solvated PMVE chains. These investigations have shown that polydisperse spherical micelles are formed for asymmetric PMVE-b-PIBVE copolymers containing at least 9 IBVE units. For copolymers containing less IBVE units, loose aggregates are formed.Finally, the thermo-responsive, reversible properties of these structures have been investigated. Above the cloud point of the copolymers, the loose aggregates precipitate while the micelles form large spherical structures.     

Rubber toughening of poly(lactide) by blending and block copolymerization

Copolymers of L-lactide with 15 or more mole % D-lactide are amorphous, noncrystallizable hydrolytically degradable materials. These glassy materials are brittle in tension and bending. To make these materials suitable for use as load-bearing devices in biomedical applications, toughness has to be enhanced. This is effectively accomplished by introducing a separate degradable rubber phase in the amorphous matrix. Several approaches have been explored: solution blending and coprecipitation of trimethylene carbonate and ?-caprolactone rubbers and poly(lactide), preparation of ABA triblock copolymers and blending of ABA block copolymers with the amorphous poly(lactide) matrix. In all cases very tough materials could be prepared. These materials are easily processable by compression molding at relatively low temperatures.     

Organized structures in amorphous styrene/cis-1,4-isoprene block copolymers: low angle X-ray scattering and electron microscopy

It is shown that amorphous styrene/cis-1,4-isoprene block copolymers form organized structures in concentrated solutions as well as in the solid state. The effects of the nature and the concentration of the solvent, the composition of the copolymer and the temperature on the parameters determined by low angle X-ray scattering and electron microscopy are discussed. It is observed that, independent of the nature or concentration of the solvent, a given block copolymer exhibits only one ordered structure which may be lamellar or hexagonal.