Monodisperse Molecular Weight Distribution Research Articles

Supramolecular Structuring of Hyaluronan-Lactose-Modified Chitosan Matrix: Towards High-Performance Biopolymers with Excellent Biodegradation.

Non-covalent interactions in supramolecular chemistry provide useful systems to understand biological processes, and self-assembly systems are suitable assets to build-up innovative products for biomedical applications. In this field, polyelectrolyte complexes are interesting, especially when polysaccharides are involved, due to their non-toxicity and bio-absorbability. In this work, we investigated a polyelectrolyte formed by hyaluronic acid (HA), a negatively charged linear polysaccharide, with Chitlac (Ch), a positively charged lactose-modified chitosan. The aim of the study was the investigation of a novel Ch–HA polyelectrolyte complex, to understand the interaction between the two polysaccharides and the stability towards enzymatic activity. By means of gel permeation chromatography–triple detector array (GPC–TDA), nuclear magnetic resonance (NMR), dynamic viscosity, Zeta Potential and scanning electron microscopy (SEM), the polyelectrolyte complex properties were identified and compared to individual polysaccharides. The complex showed monodisperse molecular weight distribution, high viscosity, negative charge, and could be degraded by specific enzymes, such as hyaluronidase and lysozyme. The results suggest a close interaction between the two polysaccharides in the complex, which could be considered a self-assembly system.

Matrix-free hydrodynamic study on the size distribution and conformation of three technical lignins from wood and non-wood

Abstract Molecular weight (MW) and related conformational data of three commercially available technical lignins (Alcell L, kraft L, and soda L) have been studied by means of analytical ultracentrifugation, taking advantage of some recent developments in both sedimentation velocity and sedimentation equilibrium determinations. The lignins were dissolved in dimethyl sulphoxide (with ca. 90% solubility), and solutions were studied with regards to their oligomeric state, heterogeneity profiles (distribution of sedimentation coefficients), and molecular weight distributions (MWD). Alcell L and soda L have similar properties showing one major low MW component and two minor high MW components, whereas kraft L appears to be larger and more uniform, i.e., it shows a more monodisperse MWD. Weight average molecular weight (M w) data from sedimentation equilibrium obtained by the new SEDFIT-MSTAR procedure in conjunction with MULTISIG analysis were found to be ~18 kDa (Alcell L), 25 kDa (kraft L), and 15 kDa (soda L). Further analysis of the data by means of the routines MULTISIG and M_INVEQ confirmed the presence of additional components in Alcell L and soda L, and the larger size and high degree of monodispersity of kraft L. The intrinsic viscosity data of the three lignins were found to be very similar in the range of 22–24 ml g-1, and all data were consistent with an elongated plate shape molecular structure with an equivalent discoid aspect ratio ~30.

The arabinogalactan of dried mango exudate and its co-extraction during pectin recovery from mango peel

Hot-acid extraction of the peel of fully ripe ‘Nam Dokmai’ (NDM) and ‘Maha Chanok’ (MHC) mangoes yielded pectins (PNDM, PMHC) being comparatively poor in uronic acids (38–47 g hg−1) in favor of the neutral sugars (23 g hg−1), chiefly galactose. Beyond the high-molecular-weight fraction, they displayed a characteristic, almost monodisperse fraction with a peak molecular weight of 18,200–19,100 relative to dextrans. In order to examine the hypothesized co-extraction of non-pectin polymers, the pectin PNDM and the exudate of mature-green ‘Nam Dokmai’ fruit (SAPNDM) were characterized regarding composition and molecular properties both before and after fractionation. By fractionation of PNDM on a weak anion exchanger, the high-molecular-weight pectin was largely separated from this characteristic fraction, which was composed of arabinogalactans. Likewise, the almost monodisperse molecular weight distribution of the dried fruit exudate SAPNDM with a peak molecular weight of 22,400 was shown to be caused by arabinogalactans being the main component (77.6 g hg−1) with a molar ratio of 82/10/4/4 for the major carbohydrate residues galactose, arabinose, rhamnose, and glucuronic acid. Fractionation of SAPNDM by size exclusion chromatography (SEC) and rechromatography (HPSEC) confirmed the high molecular uniformity of these arabinogalactans. The occurrence of hydroxyproline-rich proteins (≤0.2 g hg−1) in these exudate fractions may indicate small percentages of arabinogalactan-proteins. Co-extraction of residual fruit exudate may thus be considerable in pectin recovery from mango peel. Options for obtaining both pure pectins (galacturonic acid, 94 mol hmol−1) and arabinogalactans from this source were discussed.

Scaling behavior of proton mobility in sulfonated block copolymers

An in-depth analysis of proton mobilities in model ionic block copolymers has been carried out. The system of interest is a series of sulfonated poly(styrene-b-methylbutylene) (PSS-PMB) copolymers. Dilute solutions of PSS-PMB copolymers in methanol were examined where the PSS domains have an ability to conduct protons by offering sufficiently protonated conditions. A nearly monodisperse molecular weight distribution of PSS-PMB copolymers yields highly uniform spherical ionic micelles. In particular, on virtue of the self-assembly nature of block copolymers, the system revealed well-defined ionic PSS domains with different thicknesses ranging from 3.0 to 7.8 nm. The proton transport in PSS–PMB copolymers was found to be facilitated by the decrease in the ionic domain sizes with proton mobilities (μ) ranging from 1.96 × 10−4 to 8.48 × 10−4 cm2 V−1s−1. Notably, a unique scaling relationship between the μ values and the micelle radii (RH), μ ∝ RH−1.67, was described, which was rationalized by the different proximity of acid groups at the surfaces of ionic domains. The validity of the scaling behavior was verified by examining body-centered cubic forming concentrated solutions. Interestingly, when the same analysis was applied to the hydrated samples possessing different domain geometries, i.e., cylindrical ionic domains, the scaling behavior was also revealed, although an obtained exponent is significantly low as −0.35.

Dendritic physical gel: A liquid crystalline gel for application in light scattering displays

Dendrimers are fascinating nano-objects which have predictable three-dimensional architecture, monodisperse molecular weight distribution, and various remarkable properties, e.g. encapsulation, catalysis, low viscosity, and unique optical properties. Functionalized dendrimers, i.e. dendrimers incorporating active or reactive functionalities either in the core or at the periphery, are considered new materials with high potential for modern technology applications. The possibility of molecular design of organized supramolecular assemblies constructed from dendritic macromolecules has generated enthusiastic investigations at the frontiers of chemistry, physics, biology, and materials sciences. Along this line, we have reported dipeptide core poly(benzyl ether) dendrimers as a new class of functional dendrimers that form a physical gel in organic solvent with a significantly high gelation ability. Liquid crystals (LCs) play very important roles in modern life, and various types of LC displays have been proposed and put into practical use. A typical LC display has a twist nematic liquid crystal element which is composed of a pair of polarizing plates and a twisted LC layer. The presence of the polarizing plates results in decreased brightness, therefore, recently, several types of display media that do not contain polarizing plates have been proposed. LC gels have great potential for use as an element in the display medium and would not require polarizing plates. In this context, we report the first example of the application of dendritic physical LC gel in light scattering display materials.

Local v/a variations as a measure of structural packing frustration in bicontinuous mesophases, and geometric arguments for an alternating ${\rm Im}\overline{{\mathsf3}}{\rm m}$ (I-WP) phase in block-copolymers with polydispersity

This article explores global geometric features of bicontinuous space-partitions and their relevance to self-assembly of block-copolymers. Using a robust definition of `local channel radius', based on the concept of a medial surface [Schröder et al., Eur. Phys. J. B 35, 551 (2003)], we relate radius variations of the space-partition to polymolecular chain stretching in bicontinuous diblock- and terblock copolymer assemblies. We associate local surface patches with corresponding cellular volume elements, to define local volume-to-surface ratios. The distribution of these v/a ratios and of the channel radii are used to quantify the degree of packing frustration of molecular chains as a function of the specific bicontinuous geometry, modelled by triply-periodic minimal surfaces and related parallel interfaces. The Gyroid geometry emerges as the most nearly homogeneous bicontinuous form, with the smallest heterogeneity of channel radii, compared to the cubic Primitive and Diamond surfaces. We clarify a geometric feature of the Gyroid geometry: the three-coordinated nodes of the graph are not the widest points of the labyrinths; the widest points are at the midpoints of the edges. We also explore a more complex cubic triply-periodic surface, the I-WP surface, containing two geometrically distinct channel subdomains. One of the two channel systems is nearly as homogeneous in local channel diameters as the Gyroid, the other is more heterogeneous than the Primitive surface. Its hybrid nature suggests the possibility of an “alternating I-WP” phase in polydisperse linear ABC-terpolymer blends, with monodisperse molecular weight distributions (MWD) in the A and B blocks and a more polydisperse C block.

Chiral Fluorous Dialkoxy‐Diamino Zirconium Complexes: Synthesis and Use in Stereospecific Polymerization of 1‐Hexene

New catalysts for the isospecific polymerization of 1-hexene based on cationic zirconium complexes incorporating the tetradentate fluorous dialkoxy-diamino ligands [OC(CF(3))(2)CH(2)N(Me)(CH(2))(2)N(Me)CH(2)C(CF(3))(2)O](2-) [(ON(2)NO)(2-)] and [OC(CF(3))(2)CH(2)N(Me)(1R,2R-C(6)H(10))N(Me)CH(2)C(CF(3))(2)O](2-) [(ON(Cy)NO)(2-)] have been developed. The chiral fluorous diamino-diol [(ON(Cy)NO)H(2), 2] was prepared by ring-opening of the fluorinated oxirane (CF(3))(2)COCH(2) with (R,R)-N,N'-dimethyl-1,2-cyclohexanediamine. Proligand 2 reacts cleanly with [Zr(CH(2)Ph)(4)] and [Ti(OiPr)(4)] precursors to give the corresponding dialkoxy complexes [Zr(CH(2)Ph)(2)(ON(Cy)NO)] (3) and [Ti(OiPr)(2)(ON(Cy)NO)] (4), respectively. An X-ray diffraction study revealed that 3 crystallizes as a 1:1 mixture of two diastereomers (Lambda-3 and Delta-3), both of which adopt a distorted octahedral structure with trans-O, cis-N, and cis-CH(2)Ph ligands. The two diastereomers Lambda-3 and Delta-3 adopt a C(2)-symmetric structure in toluene solution, as established by NMR spectroscopy. Cationic complexes [Zr(CH(2)Ph)(ON(2)NO)(THF)(n)](+) (n=0, anion=[B(C(6)F(5))(4)](-), 5; n=1, anion=[PhCH(2)B(C(6)F(5))(3)](-), 6) and [Zr(CH(2)Ph)(ON(Cy)NO)(THF)](+)[PhCH(2)B(C(6)F(5))(3)](-) (7) were generated from the neutral parent precursors [Zr(CH(2)Ph)(2)(ON(2)NO)] (H) and [Zr(CH(2)Ph)(2)(ON(Cy)NO)] (3), and their possible structures were determined on the basis of (1)H, (19)F, and (13)C NMR spectroscopy and DFT methods. The neutral zirconium complexes H and 3 (Lambda-3/Delta-3 mixture), when activated with B(C(6)F(5))(3) or [Ph(3)C](+)[B(C(6)F(5))(4)](-), catalyze the polymerization of 1-hexene with overall activities of up to 4500 kg PH mol Zr(-1) h(-1), to yield isotactic-enriched (up to 74 % mmmm) polymers with low-to-moderate molecular weights (M(w)=4800-47 200) and monodisperse molecular-weight distributions (M(w)/M(n)=1.17-1.79).

Precise Synthesis of ABA Triblock Copolymers Comprised of Poly(ethylene oxide) and Poly(β-benzyl-l-aspartate): A Hierarchical Structure Inducing Excellent Elasticity

ABA triblock copolymers comprised of poly(β-benzyl-l-aspartate) (PBLA) as an A segment and poly(ethylene oxide) (PEO) as the B segment were precisely synthesized by a ring-opening polymerization of the N-carboxyanhydride of BLA initiated from amines at both terminals of the PEO chain. The copolymers have a number-average molecular weight (Mn) of 1.5 × 104−3.6 × 104 and a monodisperse molecular weight distribution in the range 1.04−1.07. The copolymer films casted from a dichloromethane solution were flexible and elastic despite their low Mn. The film formed a hierarchical structure, i.e., the chains adopted an α-helix−72 helix−α-helix conformation and was highly crystallized, forming a long-range ordering structure which resulted in a rodlike construct arranged like ripples, as confirmed by Fourier transformed infrared spectroscopy, X-ray diffraction, and atomic force microscopy. Differential scanning calorimetry confirmed that the film showed a melting endotherm of the PEO segment at 313−326 K depending ...

Strain Induced Crystallization in Stereoregular Poly[Methyl(3,3,3-Trifluoropropyl) Siloxane] Networks

Abstract Elastomeric networks based on poly[methyl(3,3,3-trifluoropropyl)siloxane] having a minimum level of stereo-regularity crystallize and self-reinforce when stretched at room temperature. The effect is especially pronounced in networks where the cross-links are uniformly spaced. The polymers are made by modification of a recently described anionic ring opening copolymerization of prescribed mixtures of cis- and trans-1,3,5-trimethyl-1,3,5-tris (3′, 3′, 3′-trifluoropropyl) cyclotrisiloxane. This provides polymers enriched in stereoregularity with nearly monodisperse molecular weight distributions and reactive groups at the chain ends. The end-linked networks show markedly enhanced mechanical properties due to self-reinforcement from strain-induced crystallization. In addition to applied strain, self-reinforcement depends on temperature and the amount of stereoregularity introduced to the polymer. Strained specimens having high levels of stereoregularity and pre-existing crystallinity exhibit plastic flow and, upon release of the applied strain, stress-hardening. Wide angle X-ray scattering (WAXS) on strained specimens establishes that, although the relative degree of crystallinity varies, the basic crystal structure is unaltered by cross-linking, amount of strain, or amount of stereoregularity. Unit cell parameters from WAXS indicate a monoclinic crystal and suggest a 4*11 chain conformation.

Association behavior of living anionic lipophobic head‐groups in hydrocarbon mileau

AbstractThe commercial applications of anionic polymerization have become wide‐spread with regard to the preparation of linear and star shaped homopolymers and block copolymers (Diene‐35TM, KratonTM, VectorTM, SolpreneTM, K ResinTM and ShellVisTM). Furthermore, randomized styrene/butadiene linear copolymers (SolpreneTM) and low molecular weight, 10K or less, polybutadienes (LithenesTM) of various microstructures are commercially available. Recently Liquid KratonsTM with OH functional units in place on one or both chain ends have become items of commerce. The lithium based systems yield materials of uniform composition and molecular weights with virtually monodisperse molecular weight distributions. Both molecular weight and composition are tunable via the simple expedient of manipulating the monomer/initiator ratio. Although the synthetic aspects of these anionic systems have been well‐mastered and exploited over the years the mechanistic features have remained controversial; issues which revolve around active center aggregation behavior and the reactivity (or its absence) of these self‐assembled anionic lipophobic head‐groups. Thus a study of these systems was undertaken using modern scattering techniques and data analysis procedures.Small angle neutron scattering (SANS) in combination with dynamic and static light scattering, DLS and SLS, has been used to evaluate the association behavior of the styryl‐ and dienyllithium head‐groups in benzene and cyclohexane solutions. Both types of lipophobic active centers were found to aggregate as dimers which in turn can self‐assemble to yield large‐scale wormlike micelles (prolate ellipsoids). These systems thus emulate the behavior of diblock copolymers and surfactants in their capacity to form flexible cylindrical micelles. It was also found for all head‐groups that the combination of long chain lengths and high polymer concentrations favored the presence of the dimer structure. This commonality of aggregation state is in consonance with results given in 1964.

Effect of size on the strength of polymeric fibers

AbstractUsing a molecular model introduced previously, we study the effects of chain‐end segregation on the relationship between strength (σ) and diameter (d) in polymeric fibers. For segregated structures with a monodisperse molecular weight distribution, our results show a scaling law σ ∼ d−α, with α in the range [0.4–0.5], in agreement with experimental observation. A weaker dependence is found for polydisperse systems. Further investigation also reveals that macroscopic cracks have little influence on the fiber strength/diameter relationship, unless the crack width shows a faster than linear increase with fiber diameter. Finally, our model results also indicate a very weak dependence of fiber strength on its length, in good agreement with experimental observation. ©1995 John Wiley & Sons, Inc.

Ester interchange reactions in polyester: A Monte Carlo simulation approach

The effect of ester interchange reactions on the molecular weight distribution (MWD) has been analysed by the Monte Carlo simulation method. An interchange reaction is accepted or rejected according to the Metropolis rule, and the probability of a reaction is calculated from the MWD of the system. The initial biases from thermodynamic equilibrium, such as a monodisperse MWD and a fully extended chain configuration, are rapidly relaxed as the reaction progresses. The simulated MWD is in good agreement with the most probable MWD proposed by Flory. It is also observed that the end-to-end distance of molecules after ester interchange reactions follows Gaussian statistics.

Living polymerization of isobutyl vinyl ether by HI/I2 initiator in polar solvents

AbstractThis study has shown a nearly perfect living polymerization of isobutyl vinyl ether (IBVE) to proceed not only in nonpolar media (e.g., n‐hexane) but in relatively polar CH2Cl2 solvent, provided that the HI concentration is sufficiently high. The produced polymers had a nearly monodisperse molecular weight distribution (Mw/Mn ≤ 1.1); the number‐average molecular weight (Mn) increased in direct proportion to IBVE conversion and its increase continued on the addition of a fresh feed of the monomer at the end of the polymerization. The use of a more polar medium (PhNO2/CH2Cl2) or a lower HI concentration leads to chain transfer reactions, by promoting the ionic dissociation of the “nondissociated” living propagating species. The successful living polymerization by HI/I2 in CH2Cl2 indicates a very strong interaction between the iodide anion and the growing end.

Living Cationic Copolymerization of Isobutyl Vinyl Ether with P-Methoxystyrene by Hydrogen Iodide/Iodine Initiating System

Living cationic copolymerization of isobutyl vinyl ether with p-methoxystyrene was effected by an equimolar mixture of hydrogen iodide and iodine (HI/I2 initiator) at −15°C in nonpolar solvents (carbon tetrachloride and toluene). Isobutyl vinyl ether reacted faster than p-methoxystyrene. The products were random copolymers with a nearly monodisperse molecular weight distribution (MWD) (Mw/Mn=1.1–1.2). Their number-average molecular weight (Mn) increased in proportion to total monomer conversion. The iodine-initiated copolymerization under similar conditions led to long-lived (but not perfectly living) copolymers with a narrow MWD (Mw/Mn=1.2–1.3). In contrast, boron trifluoride etherate induced a conventional transfer-dominant copolymerization. The Mn of the copolymers obtained with the HI/I2 initiator was almost independent of the monomer feed ratio, but in the BF3OEt2-initiated non-living process, the polymer molecular weight decreased sharply with increasing isobutyl vinyl ether content in the feed. These findings show the living copolymerization by HI/I2 to be free from the cross-transfer usually involved in the conventional cationic copolymerization of vinyl monomers.

Polymer synthesis by cationic polymerization: Living and functionalized polymers

Nearly perfect living polymers have been obtained by the polymerization of vinyl ethers (VE) initiated with a mixture of hydrogen iodide and iodine (HI/I2 initiator) at low temperature in nonpolar solvents. The polymers had molecular weights directly proportional to monomer conversion and a nearly monodisperse molecular weight distribution (M̄w/M̄n < 1.1). A very similar living polymerization took place by the use of a 1:1 adduct between VE and HI (CH3CHI (OR)) (1) as an initiator in conjunction with iodine. From these facts, a new mechanism has been proposed for the living polymerization in which the CHI bond of the initially formed adduct 1 is activated by an electrophilic interaction of iodine and VE monomer inserts into the activated CHI linkage. The resultant polymer again has a CHI terminal that is in turn activated by iodine to allow further insertion of the monomer. Similarly, bifunctional living poly(VE) was obtained with a diiodo compound (CH3CHIO(CH2)-4OCHICH3), prepared from HI and 1,4-vinyloxybutane, as an initiator. These mono- and bifunctional living polymers were applied to synthesize AB and ABA block copolymers and function-alized poly(VE) having terminal and in-chain amino groups.

Living cationic polymerization of isobutyl propenyl ether as β‐substituted vinyl ether

AbstractIsobutyl propenyl ether [IBPE; CH3CH=CHOCH2CH(CH3)2] was polymerized with a mixture of hydrogen iodide and iodine (HI/I2 initiator) in n‐hexane at −40°C to yield living polymers with a nearly monodisperse molecular weight distribution (MWD) (M̄w/M̄n ≈ 1.1). The number‐average molecular weight (M̄n) of the polymers increased proportionally to IBPE conversion and further increased when a new monomer feed was added to a completely polymerized solution. The M̄n was controlled by the initial concentration of hydrogen iodide if the acid was charged in excess over iodine. In polymerization by iodine alone the M̄n of the polymers obtained in nonpolar solvents (n‐hexane and toluene) also increased with conversion, but their MWD was broader (M̄w/M̄n = 1.3–1.4) than in the HI/I2‐initiated systems under similar conditions. The iodine‐initiated polymerization in polar CH2Cl2 solvent, in contrast, led to nonliving polymers with a broad MWD (M̄n/M̄n = 1.6–1.8) and M̄n, independent of conversion. The living polymerization of IBPE was also compared with that of the corresponding isobutyl vinyl ether, to determine the effect of the β‐methyl group in IBPE.

Living Cationic Polymerization of 2-Chloroethyl Vinyl Ether with Iodine and Hydrogen Iodide/Iodine Initiators

This paper is concerned with the living cationic polymerization of 2-chloroethyl vinyl ether (CEVE). Living poly(CEVE) of a nearly monodisperse molecular weight distribution (MWD) (Mw/Mn\\lesssim1.1) was obtained in toluene at −40°C with a mixture of hydrogen iodide (HI) and iodine as initiating system. The number-average molecular weight (Mn) of the polymer increased proportionally to CEVE conversion, while the MWD remained nearly monodisperse throughout the reaction. An excess amount of HI over iodine was needed to achieve this living process. The polymerization at −15°C by the HI/I2 initiator in toluene or CCl4 also led to living polymers with Mn directly proportional to conversion, but their MWD was slightly broader (Mw/Mn=1.2—1.3). Iodine alone as initiator resulted in long-lived (but not perfectly living) polymers at −15°C in toluene. Although the Mn of the polymers increased proportionally to conversion, their MWD was much broader Mw/Mn=1.4—1.6 than with the HI/I2 initiator, indicating a slower initiation by this initiator.

Block Copolymerization. II. Polymerization of Tetrahydrofuran by Bifunctional Cationic Initiator

Polymerization of tetrahydrofuran (THF) by 2-methyl-1,3-dioxolenium perchlorate clarified the living nature of the polymerization. Although the initiation reaction is slower than the propagation reaction, the bonding mechanism of the initiation was shown from NMR and IR, and the formation of one polymer chain from one catalyst molecule was substantiated. Poly-THF di-cation of Mn 173000 to 322000 was prepared in high vacuum system by using a bifunctional initiator, 2,2′-octamethylene bis-1,3-dioxolenium perchlorate. Chain scission by sodium and n-butyl alcohol proved the presence of the living di-cation. The deviation from monodisperse molecular weight distribution is explained by the rate of the initiation reaction.