Copolymer synthesis of poly(L-lactide-b-DMS-L-lactide) via the ring opening polymerization of L-lactide in the presence of α,ω-hydroxylpropyl-terminated PDMS macroinitiator

Various amounts of hydroxy terminated PDMS were linked into PLLA chains via in-situ ring opening polymerization at a very low content of SnOt₂. The ¹H and FTIR spectra provided evidence for the incorporation of the PDMS in the PLLA chains. The molecular weights, T_g, T_m, crystallinity and the heats of fusion decreased as the feed mole ratio of PDMS/LLA in the block copolymer increased. The molecular weight distribution broadened as the content of the PDMS increased, due the occurrence of two initiation and propagation mechanisms. Linking PDMS into the PLLA chains improved its thermal stability. © 1996 John Wiley & Sons, Inc.     

2‐Ethyl‐6‐methylpyridin‐3‐ol and its salt bis(2‐ethyl‐3‐hydroxy‐6‐methylpyridinium) succinate

The title compounds, C₈H₁₁NO, (I), and 2C₈H₁₂NO⁺·C₄H₄O₄²⁻, (II), both crystallize in the monoclinic space group P2₁/c. In the crystal structure of (I), intermolecular O—H...N hydrogen bonds combine the molecules into polymeric chains extending along the c axis. The chains are linked by C—H...π interactions between the methylene H atoms and the pyridine rings into polymeric layers parallel to the ac plane. In the crystal structure of (II), the succinate anion lies on an inversion centre. Its carboxylate groups interact with the 2‐ethyl‐3‐hydroxy‐6‐methylpyridinium cations via intermolecular N—H...O hydrogen bonds with the pyridine ring H atoms and O—H...O hydrogen bonds with the hydroxy H atoms to form polymeric chains, which extend along the [01] direction and comprise R₄⁴(18) hydrogen‐bonded ring motifs. These chains are linked to form a three‐dimensional network through nonclassical C—H...O hydrogen bonds between the pyridine ring H atoms and the hydroxy‐group O atoms of neighbouring cations. π–π interactions between the pyridine rings and C—H...π interactions between the methylene H atoms of the succinate anion and the pyridine rings are also present in this network.     

Synthetic thermally reversible gel systems. VI

Forming and conditioning thermally reversible aqueous gels of polyacrylyglycinamide (PAG) at various temperatures has little effect on either the melting point (T_m) of the gels or the heat of crosslinking (ΔH_c) except at temperatures where partial hydrolysis can occur. This is added evidence that unlike with gelatin, crystallite formation does not play a role in gel formation. For unfractioned PAG, the linear relationship between the logarithm of molecular weight and 1/T_m predicted and observed for gelatin gels, does not hold. With mixed gelatin-PAG gels, a gelatin/PAG ratio of ≥4 completely inhibits the formation of a PAG gel network. At lower gelatin/PAG ratios, the PAG network forms, and if gelatin is considered as an inert diluent, normal values for the melting points and ΔH_c for PAG gels are observed. At a gelatin/PAG ratio of 4, the presence of PAG reduces the ΔH_c for the gelatin gel by inhibiting the formation of as large or as ordered crystallite crosslinks. To reconcile the problem of aggregation preceding gelation one can assume that M?_w of an aggregate is a linear function of C². If this is done, the same relationship which normally relates C with T_m is obtained. The equilibrium swelling of PAG films in water at 25°C is markedly molecular weight-de-pendent and can vary from below 5 to about 40 wt-% polymer at equilibrium. It has been found that long-term dark storage of dry samples of PAG under ambient temperature conditions results in pronounced decreases in the intrinsic viscosities of their aqueous solutions. It is speculated that this results from weak links, perhaps peroxy, in the polymer backbone. The possible relationship of this phenomenon to the slow stage of the viscosity deterioration of aqueous polyacrylamide solutions is pointed out. The higher viscosity of low DP PAG in 2M NaCNS compared to H₂O and the larger percentage increse of [η] with increasing temperature in the latter, verify the greater solvent power of 2M aqueous thiocyanate for PAG. At a concentration level of 3%, aqueous PAG solutions are almost Newtonian whereas at higher concentrations (5%), the viscosity decreases appreciably with increasing rates of shear. The copolymerization of AG with isopropylacrylamide has been studied and the somewhat unusual results discussed. Copolymers containing an AG mole fraction greater than 0.40 do not exhibit a cloud point up to 100°C. If the isopropylacrylamide mole fraction approaches 0.60, the solutions do not gel down to 0°C. This ability to prepare copolymers over a narrow composition range that neither gel or undergo phase separation in the temperature range 0–100°C is probably related to the random distribution of monomer units in the copolymer backbone.     

Effects of phenolic compounds on gelation behavior of gelatin gels

The effects of phenolic additives on the gelation behavior of gelatin gels were investigated using thermomechanical analysis (TMA) for study of gel‐melting temperature, dynamic mechanical analysis (DMA) for study of gel‐storage modulus and gel‐aging stability, viscometry for study of gelation time, and texture analyzer for study of gel strength and gel melting. Thermodynamically, the addition of 1,3‐benzenediol, 1,4‐benzenediol or 1,3,5‐benzenetriol favored the gelation process of gelatin solutions (increases in T_m and aging stability) due to the introduction of extra physical crosslinks among gelatin chains through hydrogen bonding, while the addition of 1,2‐benzenediol had a negative effect (decreases in T_m and aging stability) possibly due to intra‐hydrogen bonding of the additive molecule itself. All the phenolic compounds had little effect on gel moduli. Kinetically, the introduction of 1,2‐benzenediol or 1,4‐benzenediol slowed the gelation process, while introduction of catechin, a polyphenol, accelerated the first stage of the gelation process. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 39: 224–231, 2001     

Synthesis and thermal properties of soybean oil-based waterborne polyurethane coatings

Waterborne polyurethane coatings were prepared by a polyaddition reaction using toluene diisocyanate (TDI), 2,2-di(hydroxy-methyl) propionic acid, soy-based polyols with different hydroxyl values, plus 2-hydroxyethyl methacrylate (HEMA) as chain termination agent, triethylamine as neutralization agent, and DBTDL as catalyst. Six soybean oil-based polyols were used in this study with hydroxyl values of 100, 115, 128, 140, 155, and 164 mg KOH g⁻¹, respectively. The molar ratio of polyol hydroxyl to DMPA was varied from 1.6 to 2.6. The storage stability of the waterborne polyurethane coatings was greater than 6 months. The thermal properties of the coating films were investigated by TG and DCS. The results show that the soy-based polyurethane films decomposed in three stages. The glass transition temperatures, T _g, of the soy-based polyurethane films increased with the hydroxyl number of the soy-based polyol.     

Synthesis,Characterization, and Crystal Structure of Mononuclear and Dinuclear Dioxomolybdenum(VI) Complexes with Tridentate Schiff‐base Ligands. Part 2

Mononuclear [MoO₂LD], and dinuclear [MoO₂L]₂ or [MoO₂L]₂ · D dixomolybdenum(VI) complexes have been prepared by the reaction of tridentate Schiff‐base ligands L with [MoO₂(acac)₂]. The Schiff‐base ligands have been synthesized from salicylaldehyde ( 1 , 1a , 1c , 1d ), 2‐hydroxy‐1‐naphthaldehyde ( 2 , 2c ) and 2‐hydroxy‐3‐methoxybenzaldehyde ( 3a , 3b , 3c , 3d , 3e ) with 2‐amino‐p‐cresol. All prepared complexes consist of cis‐MoO₂²⁺core coordinated by Schiff‐base ligand through two deprotonated hydroxyl groups and one imino nitrogen atom. The usual octahedral coordination around the molybdenum atoms is completed by the neutral ligand D (methanol, ethanol, dimethyl sulfoxide, imidazole or 4, 4′‐bipyridine). All compounds were characterized by elemental analyses, IR spectroscopy and some of them by X‐ray crystallography ( 1a , 2c , 3a , 3b , 3c and 3e ).     

Mechanical properties and transition temperatures of crosslinked-oriented gelatin

 This second part of a systematic study of the properties of crosslinked-oriented gelatin involves the effects of orientation and water content on the glass transition temperature T _g and on the melting behavior. The samples were the same as those in the preceding study, and their transition temperatures were determined by both differential scanning calorimetry and dynamic mechanical thermal analysis. The crosslinked gelatin which had been room-conditioned showed two transition temperatures: the lower one was attributed to T _g of the water-plasticized gelatin, and the higher one was interpreted as T _g of dried gelatin superimposed by melting. A rather unusual situation arose because of the fact that the T _g and melting temperatures T _m (217 and 230 ^°C, respectively) are so similar. Using water as plasticizer not only decreases T _g but produces imperfect crystallites which melt below the T _g of the system. The presence of the amorphous phase in the glassy state would presumably make it essentially impossible to define a melting point or crystallization temperature in the normal manner, as an equilibrium between crystalline and amorphous phases. Received: 8 October 1996 Accepted: 2 November 1995     

Hydrogen‐bonding patterns of two dihydroxy­lactone derivatives

In the hydrogen‐bonding networks of 8‐hydroxy‐5‐hydroxy­methyl‐3,6‐dioxatricyclo­[6.3.1.0^1.5]dodecan‐2‐one and 5,7‐bis(hydroxy­methyl)‐3,6‐dioxatricyclo­[5.3.1.0^1.5]undecan‐2‐one, both C₁₁H₁₆O₅, layers and double strands, respectively, lead to the formation of chains connected by hydroxy‐to‐hydroxy contacts, where the hydroxy­methyl group, present in both structures, acts as a donor. The secondary structures differ in the hydrogen bonding of these chains via the second hydroxy group, which is involved in hydroxy‐to‐carbonyl and hydroxy‐to‐hydroxy bonds, respectively.     

Crystal transition and structure of poly (γ-n-alkyl glutamate)s

The nature of the crystal transition of the α-helical forms of poly (γ-n-alkyl glutamate)s (alkyl = ethyl, propyl, and butyl) is described. The transition is thermally reversible, and its temperature T₂ is much higher than the glasslike transition temperature T₁ associated with the side-chain motion. The main chains undergo large-scale motion (librational about the chain axis and translational along the axis) above T₃ ≈ 200°C. The structure observed below T₂ is anomalously disordered compared with that observed between T₂ and T₃. The crystal structure emerging above T₂ is analyzed for a typical sample of poly(γ-n-propyl L -glutamate). The trigonal unit cell contains three α-helices so that each helix is surrounded by other helices in the same fashion, but the helices are not interrelated by a crystallographic symmetry element. The side chains suffer no particular change at T₂. The main-chain motion gives rise to the T₂ transition by inducing attractive forces between interpenetrating side chains.     

Hydration heat capacities of hydrophobic solutes at 248–373 K

A new equation is suggested to define the temperature dependence of the Gibbs energy of hydration of hydrophobic substances: ΔG ⁰ = b ₀ + b ₁ T + b ₂lnT. According to this equation, the hydration heat capacity is in inverse proportion to temperature. Consistent values of hydration heat capacity of nonpolar solutes have been obtained for different temperatures using data on solubility and dissolution enthalpy. The contributions of the hydrocarbon radicals and OH group to the heat capacity of hydration of the compounds were found for the temperature range 248–373 K. The hydration heat capacity of the hydroxyl group has a weak dependence on temperature and increases by only 12 J/(mol·K) in the specified temperature interval. Changes in the hydration entropy of hydrophobic and OH groups are calculated for the temperature increasing from 248 K to 373 K.     

Three dimension Liesegang rings of calcium hydrophosphate in gelatin

Three dimensional Liesegang spherical layers of CaHPO₄ in gelatin ball were performed by employing CaCl₂ and Na₂HPO₄ as the inner and outer electrolyte, respectively. Effects of concentrations of inner and outer electrolyte as well as pH on the morphologies of Liesegang rings (LRs) were investigated. As a result, it was observed that the time law, spacing law and width law found in 1D/2D gel systems were obeyed in this 3D gelatin system. The interaction of Ca²⁺ and HPO₄ ^2? with gelatin matrix played a key role to the formation of LRs due to the existence of carboxylic groups on the gelatin chains. Using Ca²⁺ as the inner electrolyte, LRs were prepared. However, employing HPO₄ ^2? as inner electrolyte, LRs were not obtained. Moreover, pH of gelatin solution greatly impacted on the formation of LRs. The number of LRs increased with the decrease of pH, whereas the width inversely decreased. pH 4.40 was a turn point, from which the spacing coefficient abruptly increased as pH increased. All these results indicated that the network was created by the interaction of Ca²⁺ and –COO^? of gelatin chains, which dominated the formation of CaHPO₄ LRs in gelatin.     

Paradoxes and paradigms: influence of the power of z on the estimation of entropies of formation of aqueous anions using simple parameters

We continue our use of “simple” energetic patterns, where simple means the use of parameters derived only from the stoichiometry of these species in our studies of the entropy of formation (TΔ_f S ^o) of aqueous anions. Relationships between the entropy of formation and different parameters such as the number of oxygen atoms, the natural logarithm of the molecular weight and the total number of atoms are explored. The charge of the species, z− continues to be explicitly considered where we now explore various choices of p and use of z ^p as a parameter.     

Block/segmented polymers. A new method of synthesis of copoly(amide-ester)-ester polymer

A new method of preparation of segmented copolymer amide-ester type is described here starting from two oligomers, one hard crystallizable (A) having a glass transition temperature (T_g) above room temperature and the other soft, amorphous (B) having T_g well below room temperature. A, an oligo amide-ester terminated with hydroxyl groups has been synthesized from bis(hydroxy acylo amide) alkane, a reaction product of a lactone and diamine and dicarboxylic acid. B, an oligoester hydroxyl terminated was synthesized by the conventional method. The two oligomers A and B were transesterified removing diol as by-product to obtain segmented (amide-ester)-ester copolymer. The polymer showed mostly two T_gs one at ?40 to ?50°C and other at +40 to +50°C; and one melting temperature 200°C. Max^m inherent viscosity was recorded at 1.75 dL/gm. © 1993 John Wiley & Sons, Inc.     

Helical catena‐poly[[tris(1H‐benzimidazole‐κN3)cobalt(II)]‐μ‐maleato‐κ3O,O′:O′′]

The crystal structure of the title compound, [Co(C₄H₂O₄)(C₇H₆N₂)₃]_n, consists of polymeric chains of the Co^II complex. Two maleate dianions and three benz­imidazole ligands coordinate to the Co^II atom with a distorted octahedral geometry. The maleate dianions bridge neighbouring Co^II atoms via both terminal carboxylic acid groups, one of which is monodentate and the other bidentate, to form a helical structure of alternating maleate dianions and Co^II atoms, with a pitch height of 9.2667 (17) Å. The absolute structure has been determined, and the crystal contains only right‐handed helices. Intrahelical N—H⋯O hydrogen bonds stabilize the helical structure, while interhelical N—H⋯O hydrogen bonds link neighbouring helices to form the supramolecular structure.     

Gas-phase interaction of Me3Si+ ions withcis- andtrans-cyclohexanediols,their methyl ethers,and acetates

The effect of stereochemistry on the mechanism of gas-phase fragmentation of [M⁺SiMe₃]⁺ ions was studied usingcis- andtrans-1,2- and -1,4-cyclohexanediols, their methyl ethers, and acetates as model compounds. The higher stability of the [M⁺SiMe₃]⁺ions is characteristic of cis-isomers of all the compounds examined, which is associated with chelation in the case ofcis-cyclohexanediols andcis-methoxycyclohexanols and with the higher reactivity oftrans-isomers due to anchimeric assistance of the methoxy and acetoxy groups. Dehydration is characteristic of the [M⁺SiMe₃]⁺ ions formed from cyclohexanediols; both hydrogen atoms of the hydroxyl groups take part in the process, thus providing direct evidence of the chelation.Translated from Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 2025–2029, August, 1996.     

One‐dimensional Diamagnetic‐metal Nitronyl Nitroxide Radical Complexes with Dicyanoargentate(I) Bridges: M(NIT4Py)2[Ag(CN)2]2 (M= Zn,Cd)

Two diamagnetic‐metal nitronyl nitroxide radical complexes with dicyanoargentate(I) bridges M(NIT4Py)₂[Ag(CN)₂]₂ (M= Zn, Cd) were synthesized. X‐ray crystallography reveals that the two compounds are isomorphous, which crystallize in the triclinic space group. Their structures consist of infinite chains of M(NIT4Py)₂ units linked by [Ag(CN)₂]^? μ₂‐bridging ligands. The magnetic measurements showed that the χ_MT values are nearly constants at higher temperature for both complexes. The sharp decreasing of χ_MT values at lower temperature are related to intermolecular antiferromagnetic interactions, which result from the shortest interchain contacts of nitroxide groups in the crystals.     

Molecular geometry and chain entanglement: parameters for the tube model

The tube diameter in the reptation model is the distance between a given chain segment and its nearest segment in adjacent chains. This dimention is thus related to the cross-sectional area of polymer chains and the nearest approach among chains, without effects of thermal fluctuation and steric repulsion. Prior calculated tube diameters are much larger, about 5 times, than the actual chain cross-sectional areas. This is ascribed to the local freedom required for mutual rearrangement among neighboring chain segments. This tube diameter concept seems to us to infer a relationship to the corresponding entanglement spacing. Indeed, we report here that the critical molecular weight, M_c, for the onset of entanglements is found to be M_c = 28 A/(〈R²〉₀/M), where A is the chain cross-sectional area and 〈R²〉₀ the mean-square end-to-end distance of a freely jointed chain of molecular weight M. The new, computed relationship between the critical number of backbone atoms for entanglement and the chain cross-sectional area of polymers, N_c = A^0,44, is concordant with the cross-sectional area of polymer chains being the parameter controlling the critical entanglement number of backbone atoms of flexible polymers.     

Der Einfluss der O-Acetylierung auf das konformative Verhalten des Kollagen-Modellpeptides (L-Pro-L-Hyp-Gly)10 und von Gelatine

The Influence of O-Acetylation upon the Conformational Behaviour of the Collagen Model Peptide (L -pro-L -Hyp-Gly)₁₀ and of Gelatin (L -Pro-L -Hyp-Gly)₁₀ which can be considered as a model peptide for collagen structure studies has been synthesized by the Merrifield technique. Subsequently, the hydroxyprolin residues have been acetylated by acetic acid anhydride in trifluoroacetic acid. In the same way, the hydroxyl groups of commercial bovine gelatin have been selectively acetylated. The influence of blocking the hydroxyl groups upon the thermal stability of the tripel helix formed by (L -Pro-L -Hyp-Gly)₁₀ and upon the transition temperature and the gel stability of the gelatin has been investigated by the measurement of optical rotation, circular dichroism, molecular weights and gel melting points. The results show that O-acetylation reduces the thermal stability of the collagen-like tripel helices formed in solution by the synthetic peptide and during the gelation process of gelatin. Our experiments support data previously published by various authors indicating that the hydroxyl group of hydroxyprolin plays an important role in stabilizing the collagen tripel helix. The possibility to use O-acetylated gelatin with its reduced gel forming capability for the preparation of plasma substitute solutions is discussed.     

Two packing motifs based upon chains of edge‐sharing PbI6 octa­hedra

The compounds catena‐poly[p‐phenyl­enediammonium [[diiodo­lead(II)]‐di‐μ‐iodo] dihydrate], {(C₆H₁₀N₂)[PbI₄]·2H₂O}_n, (I), and catena‐poly[bis­(3,5‐dimethyl­anilinium) [[diiodo­lead(II)]‐di‐μ‐iodo]], {(C₈H₁₂N)₂[PbI₄]}_n, (II), crystallize as organic–inorganic hybrids. As such, the structures consist of chains of [PbI₂]⁻ units extending along the c axis in (I) and along the b axis in (II). The asymmetric unit in (I) contains one Pb atom on a site of 2/m symmetry, two I atoms and a water molecule on mirror planes, and a p‐phenyl­enediammonium mol­ecule that sits around a site of 2/m symmetry with the C and N atoms on a mirror plane. In (II), the Pb atom is on a twofold axis and the two I atoms are on general positions. Each Pb atom is octa­hedrally coordinated to six I atoms, arranged as chains of edge‐sharing octa­hedra. Both compounds undergo hydrogen‐bonding inter­actions between the ammonium groups and the I atoms. In addition, there are hydrogen bonds between the water mol­ecules and the ammonium groups and halides in (I), and between the ammonium groups and the ring systems in (II).