Syndiotactic Structure Research Articles

Insight into the release mechanisms of diflunisal and salicylic acid from poly(vinyl alcohol). The role of hydrogen bonding interactions.

Diflunisal (5-(2,4-Difluorophenyl)salicylic acid, DIF), salicylic acid (SAL) derivative, which, on the one hand, is active pharmaceutical ingredient, on the other hand, belongs to the compounds exhibiting excited-state intramolecular proton transfer (ESIPT) behaviour was used to study the drug interactions with poly(vinyl alcohol) (PVA) matrix. For clarifying the nature and mechanisms of the drug-matrix interactions the salicylic acid (SAL) molecule was selected as the model active ESIPT compound, whose physicochemical properties in different media are well understood. The solute–solvent interactions (non-specific (dipole–dipole) versus specific (hydrogen bonding)) of DIF and SAL with different neat solvents were investigated using the steady-state spectroscopic technique. The solvent effect on spectral behaviours of DIF and SAL was analyzed based on the parametric solvent scales. In order to identify functional groups in the PVA matrices, determine the structure present in the studied molecule-PVA system and thus obtain information about the potential interactions between PVA and the studied molecules, the Raman spectra of pure PVA, SAL-PVA and DIF-PVA systems were measured. It has been shown that the molecular structure of the active substance entrapped in the polymer matrix affects the structure of the polymer, i.e., isotactic (SAL-PVA) versus syndiotactic (DIF-PVA) structure. The analysis of drug release kinetics revealed that the DIF is more strongly bound to PVA in comparison to SAL, which confirms conclusions drawn from the analysis of the Raman spectra i.e., the isotactic structure of SAL-PVA material results in a faster initial release process of weakly bound, located on the surface of the polymer SAL molecules.

Single-molecule study on the force-induced melting in syndiotactic polypropylene single crystals

Highly stereoregular syndiotactic polypropylene (s-PP) with high crystallinity and melting temperature has unique physical properties such as high impact resistance and transparency, which is closely related to the chain conformation and crystal structure of s-PP. However, the molecular mechanism underlying the mechanical properties remains unclear, because the complex amorphous-crystalline network and the diverse crystal structure limit the classical characterizations. Here, we investigate the single-molecule force-induced melting in the melt-prepared and solution-prepared s-PP single crystal by using atomic force microscopy (AFM)-based single molecule force spectroscopy (SMFS). Our results show that the mechanical stability slightly increases with the increase of crystal thickness and the crystal thickness also has a significant impact on the melting pathway. The comparison of the melting pathways of s-PP and isotactic polypropylene (i-PP) suggests that the syndiotactic structure and lower tacticity of s-PP will increase the chain friction during slide and therefore suppresses the αc relaxation in the crystal region. Moreover, the difference of the chain folding trajectory in sectors (100) and (010) shows no effect on the mechanical stability. However, the mechanical stability of form I structure was higher than that of form II.

Effect of isomer and defect structure on the thermal stability of polyvinyl chloride: An experiment and molecular dynamics simulation

The relationship between molecular chain structure and thermal stability of polyvinyl chloride (PVC) was studied by experiment and molecular dynamics (MD) simulation. The experimental results of oven aging test, UV–Vis and 1H-NMR showed that the PVC samples generated fewer conjugated triene and total double bond during aging exhibited better thermal stability. The IR spectra of gas products indicated that thermal aging of PVC mainly generated hydrogen chloride (HCl) and low molecular weight hydrocarbon (CH2) between 50 and 500 °C. The PVC molecular chain model was constructed by MD simulation to study reaction pathways and products during PVC thermal aging, the results further confirmed that HCl and CH2 were the main products of PVC thermal aging. Besides, the thermal aging models of PVC with different isomers and defect structure were also established and calculated based on MD simulation. The results of MD simulation and 13C-NMR indicated that the CCl bond in the syndiotactic structure broke first and the 1,2-dichloride terminal was the most unstable defect structure. In a word, this work paves the way for the observation of the real-time structure evolution of PVC, especially during the aging process.

Effect of solvent on the microstructure of polyvinyl alcohol

AbstractThe method for producing polyvinyl alcohol (PVA) by the solution polymerization of vinyl acetate first and then alcoholysis strategy is industrial. Where solvent plays a vital role in control over stereoregularity, molecular weight and its distribution, crystallinity and so on. In this report, when methyl acetate was used as a solvent in the polymerization of vinyl acetate instead of methanol, prepared PVA was characterized by Fourier‐transform infrared spectroscopy (FT‐IR), nuclear magnetic resonance (NMR), gel permeation chromatography (GPC), thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). The results showed that PVA obtained in methyl acetate has a higher content of syndiotactic structure, crystallinity melting temperature and enhanced thermal stability compared with PVA obtained in methanol due to the formation of Π ‐ Π complex between methyl acetate and vinyl acetate and the enhanced steric hindrance effect during polymerization. Additionally, methyl acetate has a smaller chain transfer constant (1.31 × 10−4), which benefits to decrease the branching degree and molecular weight distribution index of PVA. This research provides more ideas for the choice of solvents in the industrial production of polyvinyl alcohol.

An effective method to make polymers degrade readily: spatial isomerization.

The widespread application of hydrocarbon polymers has spurred an increasing interest in the study of their degradation mechanism. In general, the chemical inertness of polymers makes their degradation by low-energy processes a challenging problem. Herein, we report a method of spatial isomerization to make polymers degrade easily. The first-principles calculations show that the energy barrier required for degradation reaction is directly related to the spatial arrangement of the polymer, with the isotactic structure and most atactic structures being easier to degrade than the syndiotactic structure. Therefore, a new way to accelerate the degradation by achieving spatial isomerization of polymers has been proposed. Furthermore, the synthesis rates of these structures have also been calculated to support future experiments.

NMR Investigations of the Catalyst–Monomer Interaction and Stereochemistry of the Product in Catalytic Polymerization of 2-Hydroxyethyl Methacrylate

Nuclear magnetic resonance (NMR) spectroscopy has been used to study the structure of the product and possible intermediates in the polymerization of 2-hydroxyethyl methacrylate catalyzed by the oxovanadium (IV) complex. The assumption of the coordination nature of the polymerization is confirmed by noncovalent interactions between monomer and the catalyst, which has been examined by NMR on 1H, 13C, and 17O nuclei. The catalyst affects remarkably the stereospecificity of the reaction. The resulting polymer with high molecular weight Mn ≈ 200,000 has a predominantly syndiotactic structure; the chain growth is satisfactorily described by the first-order Markov model with the statistical parameters Pr/m = 0.156 and Pm/r = 0.857.

Syntheses of Molybdenum Oxo Alkylidene Complexes through Addition of Water to an Alkylidyne Complex.

Addition of one equiv of water to Mo(CAr)[OCMe(CF3)2]3(1,2-dimethoxyethane) (2, Ar = o-(OMe)C6H4) in the presence of PPhMe2 leads to formation of Mo(O)(CHAr)[OCMe(CF3)2]2(PPhMe2) (3(PPhMe2)) in 34% yield. Addition of one equiv of water alone to 2 produces the dimeric alkylidyne hydroxide complex, {Mo(CAr)[OCMe(CF3)2]2(μ-OH)}2(dme) (4(dme)) in which each bridging hydroxide proton points toward an oxygen atom in an arylmethoxy group. Addition of PMe3 to 4(dme) gives the alkylidene oxo complex, (3(PMe3)), an analogue of 3(PPhMe2) (95% conversion, 66% isolated). Treatment of 3(PMe3) with two equiv of HCl gave Mo(O)(CHAr)Cl2(PMe3) (5), which upon addition of LiO-2,6-(2,4,6-i-Pr3C6H2)2C6H3 (LiOHIPT) gave Mo(O)(CHAr)(OHIPT)Cl(PMe3) (6). Compound 6 in the presence of B(C6F5)3 will initiate the ring-opening metathesis polymerization of cyclooctene, 5,6-dicarbomethoxynorbornadiene (DCMNBD), and rac-5,6-dicarbomethoxynorbornene (DCMNBE), and the homocoupling of 1-decene to 9-octadecene. The poly(DCMNBD) has a cis,syndiotactic structure, whereas poly(DCMNBE) has a cis,syndiotactic,alt structure. X-ray structures were obtained for 3(PPhMe2), 4(dme), and 6.

Hollow poly(alpha-methylstyrene) shells for inertial confinement fusion targets

Using a low-temperature cationic polymerization technique, a number of poly(alpha-methylstyrene) samples with molecular masses of 300000–400000 were prepared, and their structural parameters were examined via nuclear magnetic resonance and X-ray diffraction techniques. It was found that all synthesized polymers display a stereoregular syndiotactic structure of macro chains and are amorphous. Using these polymers and the microencapsulation technique, hollow shells were obtained that were 2.4 mm in diameter and had a wall thickness of 10 to 60 µm. The quality of walls and surfaces of the shells was analyzed using scanning electron microscopy and optical profilometry. The hollow shells are intended for the preparation of inertial confinement fusion targets and for the laser plasma research.

Design of Aggregate Structures and Molecular Capture by Using Molecular‐Cluster‐Assembly Method

AbstractTechnology for designing functional polymers needs to incorporate biomimetic structures for a new functionality. A simulation method, referred to as “molecular‐cluster‐assembly,” is used to design aggregate structures comprised of oligomers in a bottom‐up manner and predict the capability of molecular capture. Acrylic acid oligomer forms an arched and syndiotactic structure with minimum energy. In accordance with the degree of aggregation, arched oligomers form a constricted structure with a unique channel width. The aggregate structures capture ethylene carbonates (ECs) inside the constricted channel and produce attractive energy between the ECs in host‐guest complexes. The optimum numbers of ECs stably captured are determined when the binding energy at which the guests are captured at the outside wall of the channel exceeds the attractive energy between the guests in the constricted channel.

Tungsten Oxo Alkylidene Complexes as Initiators for the Stereoregular Polymerization of 2,3-Dicarbomethoxynorbornadiene

We have employed 2,3-dicarbomethoxynorbornadiene (DCMNBD) as a monomer to explore new tungsten oxo alkylidene complexes as initiators for stereoregular ROMP (ring-opening metathesis polymerization). The initiators include MAP (monoaryloxide pyrrolide) oxo alkylidene complexes with the general formula W(O)(CHCMe2Ph)(Me2Pyr)(OAr) (Me2Pyr = 2,5-dimethylpyrrolide, OAr = an aryloxide) and W(O)(CHCMe2Ph)(OR)2 (OR = an aryloxide or OC(CF3)3), or PPh2Me or CH3CN adducts thereof. We have found that MAP initiators yield cis,syndiotactic-poly(DCMNBD) as a consequence of stereogenic metal control. In contrast, W(O)(CHCMe2Ph)(OR)2(L) initiators (where L = PPh2Me or acetonitrile) are strongly biased toward formation of cis,isotactic structures, while W(O)(CHCMe2Ph)(OR)2 initiators are strongly biased toward formation of cis,syndiotactic structures. Addition of B(C6F5)3 to W(O)(CHCMe2Ph)(Me2Pyr)(OR) species leads to a dramatic increase in the rate of polymerization and to an increase in the cis,syndiotacticity of the po...

Synthesis and ROMP Chemistry of Decafluoroterphenoxide Molybdenum Imido Alkylidene and Ethylene Complexes

The bisDFTO alkylidene complexes of molybdenum Mo(NR)(CHCMe2Ph)(DFTO)2 (R = 2,6-i-Pr2C6H3, 2,6-Me2C6H3, C6F5, 1-adamantyl; DFTO = 2,6-(C6F5)2C6H3O) and monoaryloxide monopyrrolide (MAP) complexes Mo(NR)(CHCMe2Ph)(Me2Pyr)(OAr) (Me2Pyr = 2,5-dimethylpyrrolide; R = C6F5, OAr = DFTO, 2,6-dimesitylphenoxide (HMTO); R = 2,6-Me2C6H3, OAr = DFTO) have been prepared in good yields. Addition of dicarbomethoxynorbornadiene (DCMNBD) to bisDFTO complexes yielded polymers that have a cis,isotactic structure. Polymerization of DCMNBD by Mo(NC6F5)(CHCMe2Ph)(Me2Pyr)(HMTO) gives a polymer that contains the expected cis,syndiotactic structure, but polymerization of DCMNBD by Mo(NR)(CHCMe2Ph)(Me2Pyr)(DFTO) (R = C6F5, 2,6-Me2C6H3) generates a polymer that has a cis,isotactic structure, the first observation of a cis,isotactic polymer prepared employing a MAP initiator. Norbornene is polymerized to give what is proposed to be highly tactic cis-polyNBE. Addition of ethylene to Mo(NC6F5)(CHCMe2Ph)(DFTO)2 leads to formation of Mo...

Nano- and micro-fabrications of polystyrene having atactic and syndiotactic structures using focused ion beams lithography

Micro- and nano-fabrications of polystyrene (PS) having the atactic and syndiotactic structures were carried out with direct maskless etching using focused ion beam (FIB). Micro- and nano-scale structures were obtained with selective beam and material conditions avoiding the beam-heating and charge-up effects. The etching rates were different between atactic and syndiotactic PS. The rate of fabrication for syndiotactic PS shows higher than that of atactic one. Moreover, the direct etching was influenced by the molecular weight. The etching rate for the lower molecular weight became the faster. The FIB direct etching proceeds through the two steps (decomposition and desorption (outgas)). Both decomposition and desorption are influenced by both conformation (morphology) and configuration (stereoregularity). Furthermore, the beam profiles and fluence play the important roles to perform the nano-fabrication of PS.

Atom transfer radical copolymerization of glycidyl methacrylate and methyl methacrylate

AbstractGlycidyl methacrylate (GMA) and methyl methacrylate (MMA) copolymers were synthesized by atom transfer radical polymerization (ATRP). The effect of different molar fractions of GMA, ranging from 0.28 to 1.0, on the polymer polydispersity index (weight‐average molecular weight/number‐average molecular weight) as the indicator of a controlled process was investigated at 70°C, with ethyl 2‐bromoisobutyrate as an initiator and 4,4′‐dinonyl‐2,2′‐bipyridyne (dNbpy)/CuBr as a catalyst system in anisole. The monomer reactivity ratios (r values) were obtained by the application of the conventional linearization Fineman–Ross method (rGMA = 1.24 ± 0.02 and rMMA = 0.85 ± 0.03) and by the Mayo–Lewis method (rGMA = 1.19 ± 0.04 and rMMA = 0.86 ± 0.03). The molecular weights and polydispersities of the copolymers exhibited a linear increase with GMA content. The copolymer compositions were determined by 1H‐NMR and showed a domination of syndiotactic structures. The glass‐transition temperatures (Tg) of the copolymers analyzed by differential scanning calorimetry (DSC) decreased in the range 105–65°C with increasing GMA units. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012

Synthesis of stereoregular ROMP polymers using molybdenum and tungsten imido alkylidene initiators

Three of the four possible structures for polymers formed from an achiral monomer through a single ROMP polymerization step have been prepared for a small collection of monomers. Trans,syndiotactic structures have been prepared through chain end control, cis,isotactic polymers have been prepared through enantiomorphic site control, and cis,syndiotactic polymers have been prepared through stereogenic metal control. Stereogenic metal control at the metal center as a means of forming syndiotactic polymers is virtually unknown. Synthesis of ROMP polymers with a regular structure that contain alternating enantiomers from a racemic mixture of monomers is a natural consequence of stereogenic metal control. Ruthenium catalysts do not display ROMP specificities analogous to those described here, perhaps since alkylidene isomers have not been observed for Ru catalysts and the barrier to rotation of the carbene in a generic NHC dichloride Ru catalyst has been calculated to be relatively low.

Copolymerization in N-vinylcaprolactam-N-vinylpyrrolidone and N,N-diethylacrylamide-N,N-dimethylacrylamide systems: The effect of composition and spatial structure of copolymers on their thermal sensitivity

Factors that affect the temperature-responsive properties of water-soluble polymers were revealed by studying the copolymerization of two pairs of monomers: N-vinylcaprolactam-N-vinylpyrrolidone and N,N-diethylacrylamide-N,N-dimethylacrylamide. In each pair, the first monomer forms a temperature-responsive polymer and the second gives a polymer soluble in water up to the boiling point. It was found that in all cases, the addition of the second (more hydrophilic) monomer resulted in a monotonous increase in the phase separation temperature in an aqueous copolymer solution, with the temperature rise being comparatively slow in the initial stage and sharply accelerating after the addition of more than 40–50 mol % second monomer. The phase-separation temperature versus copolymer composition curves for N,N-diethylacrylamide-N,N-dimethylacrylamide copolymers of iso-and heterotactic structure synthesized via anionic polymerization are rather similar. At the same time, the copolymers of both types prepared via radical polymerization are characterized by steeper curves, a pattern that may be due to a high content of the syndiotactic structure, however, changes in the copolymer spatial structure have a lesser effect on the phase separation temperature than the presence of units of a more hydrophilic monomer. The addition of a relatively low amount (20–25 mol %) of a less hydrophilic monomer imparts temperature sensitivity to polymers, such as polyvinylpyrrolidone or polydimethylacrylamide, which do not possess this property in the pure form.

Synthesis and structural characterization of syndiotactic trans‐1,2 and cis‐1,2 polyhexadienes

AbstractThe (E) isomer in mixtures of (E) and (Z) 1,3‐hexadiene was polymerized with the system CoCl2(PiPrPh2)2‐MAO, a highly active and stereospecific catalyst for the preparation of 1,2 syndiotactic polybutadiene. A new crystalline polymer with a melting point of 109 °C was obtained. The polymer was characterized by IR, NMR (13C, 1H in solution and 13C in the solid‐state), X‐ray diffraction, DSC, GPC and it was found to have a trans‐1,2 syndiotactic structure with a 5.18 ± 0.04 Å fiber periodicity. Since only the (E) isomer was polymerized, at the end of the reaction we were able to separate the (Z) isomer, which was ultimately polymerized with CpTiCl3‐MAO at low temperature, obtaining a low molecular weight, stereoregular polymer that, characterized by IR and NMR methods, was found to exhibit a cis‐1,2 syndiotactic structure, never reported before. Molecular mechanics calculations were carried out on the trans‐1,2 syndiotactic polymer and structural models consistent with the X‐ray diffraction data are proposed. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 5339–5353, 2007

Radical polymerization of methyl methacrylate in the presence of N,N‐dimethyl‐N‐(methylferrocenyl)amine

AbstractThe specific features of free‐radical polymerization of methyl methacrylate in the presence of a new initiating system, benzoyl peroxide‐N,N‐dimethyl‐N‐(methylferrocenyl)amine, are studied. Mutual influence of ferrocenyl‐ and amine groups on kinetic parameters of polymerization and on the microstructure of the resulting polymers is found. It is shown that the polymer obtained in the presence of this initiating system has predominantly syndiotactic structure. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 724–727, 2007

N,N-dimethyl-N-(methylferrocenyl)amine as a bifunctional component of initiating systems for free-radical polymerization of methyl methacrylate

The specific features of free-radical polymerization of methyl methacrylate in the presence of a new initiating system, benzoyl peroxide-N,N-dimethyl-N-(methylferrocenyl)amine, are studied. Mutual influence of the ferrocene and amino groups on the kinetic parameters of polymerization and on the microstructure of the resulting polymers is revealed. The polymer formed in the presence of this initiating system is shown to have a predominantly syndiotactic structure.

Interfacial adhesion and mechanical properties of PMMA-coated CaCO 3 nanoparticle reinforced PVC composites

Polymethyl methacrylate (PMMA)-coated nano-CaCO 3 particles were prepared by in-situ emulsion polymerization. The mechanical properties of nano-CaCO 3 particles-reinforced PVC were investigated using an AG-2000A universal testing machine and an XJU-2.75 izod impact tester; interfacial adhesion between CaCO 3 nanoparticles and PVC matrix by SEM, and structure of PMMA coated on the surface of CaCO 3 by FTIR and 1H-NMR. The results indicate that the PMMA coated on the nano CaCO 3 particles consists mainly of syndiotactic structure, and their three tacticity contents were rr 52.8%, mm 7.3% and mr 39.9%, respectively. The interfacial adhesion between CaCO 3 nanoparticles and PVC matrix was significantly improved when the CaCO 3 nanoparticles were coated with PMMA, which led to increased Young's moduli and tensile strengths of the PMMA-coated CaCO 3/PVC composites. The izod impact strengths of the composites were strongly affected by the PMMA coating thickness and increased significantly with increasing the volume fraction of CaCO 3 filler in the composites.

Synthesis, Characterization and Molecular Conformation of Syndiotactic 1,2 Polypentadiene: The Cis Polymer

(Z)-1,3-pentadiene has been polymerized with the CpTiCl3−MAO system at −78 °C to a polymer consisting exclusively of cis-1,2 units and having a syndiotactic structure. The polymer has been characterized by NMR (13C and 1H in solution and 13C in the solid state), IR, DSC, GPC, X-ray, and molecular modeling techniques, comparing its conformational and structural properties with those of other 1,2-syndiotactic polymers. Diffraction and NMR data are consistent with the idea that a single crystalline polymorph prevails in the studied samples. The solid-state NMR results allow us to exclude the all-trans (T4) conformation for cis-1,2 syndiotactic polypentadiene and indicate a nonuniform helix of the T2G2 type as most plausible, although other possibilities such as the (T6G2T2G2) conformation cannot be completely ruled out. These conclusions follow from the fact that two conformationally distinct methylene C1 carbons are observed, which experience, respectively, zero and two γ-gauche interactions. Isolated chain...