Chain Tacticity Research Articles

Hierarchical modelling of polystyrene surfaces

We present hierarchical simulations of amorphous polystyrene surfaces: first, we use a systematically coarse-grained description of atactic polystyrene that allows for efficient equilibration of polystyrene chain conformations and chain packing at the surface. This approach is followed by an inverse-mapping procedure that reintroduces the atomistic degrees of freedom in the coarse-grained structures and provides a detailed atomic-scale picture of the polymer surface characteristics and a direct comparison with experimental data, as the orientation of the rings at the surface. At the level of the coarse-grained modelling, several system specific properties, closely related to the molecular architecture such as the chain tacticity, can already be studied. We investigate how amorphous polystyrene surface properties depend on the molecular weight of the polymer, on the temperature and on the chain tacticity.

Orientation phenomena in chromophore DR1-containing polymer films and their non-linear optical response

The effectiveness of chromophore alignment in polymer films following corona poling can be assessed by the generated second harmonic signal. Optimization of the stability and strength of this nonlinear optical response may improve with a better understanding of the underlying principal order phenomena. Structural analysis by vibrational, optical, and 1H NMR spectroscopy reveals side chain tacticity, aggregation effects, and changes in orientation as a function of temperature. Co-polymers with the functionalized chromophore Disperse Red 1 methacrylate (MDR1) were prepared for three different methacrylate types. High side chain polarity and short side chain length increase generally chromophore aggregation in films, whereas the very long poly-ether side chains in PMEO based co-polymers are wrapped separately around the DR1 entities. Side chain tacticity depends on space requirements, but also on the capacity of side groups to form OH-bridges. Side chain tacticity might present an additional parameter for the assessment of chromophore aggregation and poling induced alignments. Stepwise heating of co-polymer films causes an increase in the number of random over ordered side chain arrangements. Cross-linking by anhydride formation is observed after heating the methacrylic acid based co-polymer.

SHG and orientation phenomena in chromophore DR1-containing polymer films

Corona poling of two different methacrylate-type co-polymers with the chromophore Disperse Red 1 methacrylate (MDR1) results in the orientation of molecular dipoles and gives a nonlinear optical response in the form of second harmonic generation (SHG). The intensity of the SHG signal was used to measure the effectiveness of poling-induced alignments, which depend also on fundamental order phenomena in the co-polymers. The two matrix polymers varied in length and polarity of their side chains, which affect strongly side chain tacticity and chromophore aggregation. In situ SHG measurements during and after poling showed that the relaxation rate in methacrylic acid (MAA) co-polymers could be lowered by heat-induced cross-linking of the polymer chains. Order parameters derived from SHG measurements were found to be greater than 4, even when no changes were recorded in the optical spectra after poling. When changes were observed in the optical spectra, it was found that these were due to a decrease in the molar extinction coefficient for hydroxy-propyl-methacrylate (HPMA) based co-polymers, and solely due to the poling-induced decrease in film thickness in methacrylic acid based co-polymers (MAA).

ChemInform Abstract: Stereospecific Living Radical Polymerization: Dual Control of Chain Length and Tacticity for Precision Polymer Synthesis

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Stereospecific Living Radical Polymerization: Dual Control of Chain Length and Tacticity for Precision Polymer Synthesis

Stereospecific Living Radical Polymerization: Dual Control of Chain Length and Tacticity for Precision Polymer Synthesis

Recursive neural networks prediction of glass transition temperature from monomer structure: an application to acrylic and methacrylic polymers

We propose a new method based on a Recursive Neural Network (RecNN) for predicting polymer properties from their structured molecular representations. RecNN allows for a completely novel approach to QSPR analysis by direct adaptive processing of molecular graphs. This model joins the representational power of structured domains with Neural Network ability to capture underlying complex relationships in the data by a process of training from examples. To this aim, a structured representation was designed for the modelling of polymer structures. The adopted representation can account also for average macromolecule characteristics, such as degree of polymerization, stereoregularity, comonomer distribution. To begin with, this model was applied to the prediction of the glass transition temperature of (meth)acrylic polymers with different degree of main chain tacticity. The results so far obtained indicate that the proposed representation of polymer structure can convey information on both the repeating unit structure and average polymer features. The ability of the proposed RecNN method of treating this structured representation makes this method more general and flexible with respect to standard literature methods. Moreover, the same model can handle at the same time the Tg of polymer samples present in only one tacticity form together with that of polymer with different stereoregularity.

Study of macroinitiator efficiency and microstructure–thermal properties in the atom transfer radical polymerization of methyl methacrylate

The atom transfer radical polymerization (ATRP) of methyl methacrylate (MMA) with poly vinylacetate macroinitiator (PVAc-CCl3) and CuCl/PMDETA as catalyst was successfully carried out in bulk and solution. The apparent propagation rate constant ( $$k_{\text{p}}^{{\text{app}}} $$ ) and concentration of active species ([P°]) were higher in the bulk. In solution they increased with polarity of solvent. Two different molecular weights of macroinitiators were used in ATRP of MMA. The linear relation of Ln[M]0/[M] versus time was only confirmed for the low molecular weight macroinitiator. The ratio of $${{k_{\text{t}} } \mathord{\left/ {\vphantom {{k_{\text{t}} } {K_{{\text{eq}}} }}} \right. \kern-\nulldelimiterspace} {K_{{\text{eq}}} }} = 1.31 \times 10^{13} \;{\text{M}}^{ - 1} \,{\text{s}}^{ - 1} $$ was calculated in the bulk reaction with the low molecular weight macroinitiator, this ratio was 1.77 × 1014 M−1 s−1 for larger macroinitiator in solution. The MWD of block copolymers were sharper with lower molecular weight macroinitiator in the solution, but it appeared broader in the bulk polymerization. Our results indicated that smaller molecular weight macroinitiator was more efficient and formed a block copolymer with lower PDI. Thermal analysis and microstructure of the block copolymers are investigated by 1H NMR, FT-IR, TGA and DSC. The chain tacticity of the MMA units is found not to be sensitive to the kinetic of the reactions with two different molecular weights of macroinitiator. DSC measurement shows two different transitions at 39 and 108 °C assigned to PVAc and PMMA blocks. The TGA profile shows a three-step degradation. The initial small weight loss that occurs around 220 °C and two large weight loss around 238 and 310 °C are attributed to dechlorination step and decomposition of the PMMA and PVAc blocks.

Study of cryostructuring of polymer systems: 27. Physicochemical properties of poly(vinyl alcohol) cryogels and specific features of their macroporous morphology

Based on aqueous poly(vinyl alcohol) (PVA) solutions with different content of polymer having different molecular masses and chain tacticity, macroporous viscoelastic gels (PVA cryogels) are prepared in various regimes of freezing-storage in a frozen state-thawing. Shear modulus and fusion temperature of corresponding samples are measured; the structure of thin sections is studied by optical microscopy and the images are processed and analyzed. It is shown that the rigidity and heat endurance of cryogels rise with an increase in the concentration of initial PVA solution and a decrease in the rate of thawing. The influence of the temperature of cryogenic treatment and the PVA molecular mass has an extreme character. At the same time, the effect of the main parameters of cryotropic gelation on the macroporous morphology of PVA cryogels is manifested in the form of more complex dependences because of its multiple-factor character. Therefore, distinct structure-property correlations are not observed in many cases. Cluster analysis of the morphometric characteristics of cryogels in comparison with data on their rigidity makes it possible to classify these systems.

Unperturbed dimensions of atactic poly(dibenzyl itaconate)s

The unperturbed dimension and temperature character of poly(dibenzyl itaconate)s (PDBzI) are studied by a revised rotational isomeric state (RIS) method. The improved formulas of the mean-square radius of gyration, deduced by the pseudo-stereochemical equilibrium approach, may be used to investigate the configurational-conformational properties of atactic polymers with large side groups [poly(itaconates) for instance]. The calculated results showed that poly(itaconates) have larger dimension of the molecule than other vinyl polymers. Comparison of the dimension between considering and without considering side groups showed that the effect of large side groups on the unperturbed dimension for short-chain polymers is more obvious than that of long-chain polymers and, if the dimension of side groups increases, the effect also increases. The dimension differences of PDBzI between short-chain and long-chain polymers are investigated by the relation of characteristic ratios and temperature coefficients with temperature. Moreover, the dependence between the temperature coefficients and the tacticity of chains shows that the temperature characters of the isotactic, syndiotactic and atactic PDBzI chains have remarkable difference.

Glass Transition Temperature and Chain Flexibility of Ethylene‐Norbornene Copolymers from Molecular Dynamics Simulations

AbstractSummary: Model chains of ethylene‐norbornene copolymers were built up using the results of 13C NMR spectral analysis of copolymer samples synthesized with metallocene‐based catalysts. Our models statistically reproduce the microstructure, composition, and tacticity of the copolymer chains of experimental samples. They were used to test if MD simulations are suitable to investigate the relationships between microstructure and macroscopic properties. In particular, MD simulations were applied to calculate the glass transition temperature and to study the chain flexibility by the analysis of ACF of specific virtual bonds. Plots of specific volume versus temperature computed for models of four copolymer samples having different microstructures and norbornene contents yield Tg values in good agreement with experiments. Moreover, comparison of the ACFs provides some qualitative indications about the relationship between chain stereochemistry and Tg.ACF functions of the virtual bonds with microstructures NENE (bottom) and ENNE (top).magnified imageACF functions of the virtual bonds with microstructures NENE (bottom) and ENNE (top).

Influence of Stereoregularity of the Polymer Chain on Interactions with Surfactants: Binding of Cetylpyridinium Chloride by Isotactic and Atactic Poly(methacrylic acid)

Association of a cationic surfactant cetylpyridinium chloride, CPC, with isotactic and atactic poly(methacrylic acid), i-PMA and a-PMA, respectively, in aqueous 0.01 M NaCl solutions was studied by pH and fluorescence measurements in conjunction with potentiometric studies using a surfactant-sensitive membrane electrode. pH measurements have demonstrated that the presence of an oppositely charged surfactant increases ionization of carboxyl groups on PMA at low degrees of neutralization. The increase is more pronounced in the case of i-PMA. The isotactic form of PMA is not soluble in water at zero degrees of neutralization but can be rendered soluble by the addition of CPC at the surfactant to a polyion molar ratio of around 0.4. In the solubilized complex, the positive charge of the CPC molecule is facing the polar solvent, whereas surfactant tails are oriented toward the i-PMA compact coil. Binding isotherms and cooperativity parameters show that chain tacticity has an important influence on the interaction of cetylpyridinium cation with polymethacrylate anion. At the onset of cooperative binding, the association is stronger with i-PMA than with the atactic form, as demonstrated by lower CAC values and higher values of the cooperativity parameters. In contrast, more surfactant is bound by a-PMA in the region where polyion becomes saturated with surfactant ions. Results are interpreted by taking into account local chain conformations as obtained from quantum mechanical semiempirical molecular orbital calculations. Greater hydrophobicity and possibly higher charge density of i-PMA on one hand and more flexibility of the a-PMA chain on the other are held responsible for these observations.

Polystyrene-grafted titanium oxide nanoparticles prepared through surface-initiated nitroxide-mediated radical polymerization and their application to polymer hybrid thin films.

Polystyrene (PS)-grafted-titanium oxide (TiO) nanoparticles with a diameter of 15 nm were prepared by the 'grafting-from' method using the surface initiator for nitroxide-mediated radical polymerization (NMRP). The graft densities were estimated to be 0.04-0.28 chains nm. Under this graft dense region it is assumed that surface polymer chains form a polymer brush structure in good solvents. Wide-angle X-ray diffraction suggested that the tacticity of the PS chains on the nanoparticle surface is mainly atactic. PS-grafted-TiO nanoparticles were stably dispersed in good solvents for PS chains due to the steric repulsion between the grafted chains derived from the osmotic pressure effect and their affinity for the solvents. When the PS-grafted-TiO particles were finely dispersed, the ultraviolet absorption due to TiO was strong and the difference in absorbance between the visible and ultraviolet regions was ten or more times greater than that for non-grafted TiO. By adding PS-grafted-TiO particles with PS matrix powder in CHCl, hybrid films were prepared by solvent casting. The PS-grafted-TiO-PS hybrid films showed high transmittance in the visible light region accompanied by ultraviolet absorption characteristic of TiO due to the fine dispersion of nanoparticles in the PS matrix. Furthermore, friction tests revealed that the wear resistance of PS thin films with PS-grafted-TiO was improved compared with PS film.

Depletion effect on partial organization of atactic polymer chain segments in microcells.

Glass transition for atactic poly(methyl methacrylate) (a-PMMA) prepared in nano-cells by microemulsion polymerization was measured at a faster heating rate after slow cooling of the sample from a temperature above Tg. An additional enthalpy relaxation and glass transition were observed at higher temperatures for the a-PMMA sample due to the partial organization of the chain segments which occurred during microemulsion polymerization. The re-precipitated a-PMMA did not show any self-organization under the same thermal conditions, although there are no changes in molecular weight or tacticity of the polymer chains. A depletion-interaction phenomenon was understood to provide entropic force for the self-organization of polymer chains inside the walls of the microemulsion cells.

Solid-State 13C NMR Study of Poly(vinyl alcohol) Gels

Poly(vinyl alcohol) (PVA) with 55% and 61% syndiotacticity, and their related dry and hydrated gels obtained by two different freeze–thawing cycles have been investigated using the solid-state 13C CP-MAS NMR technique. From a comparative analysis of the spectra, evidence was obtained that the gelation process largely disrupts the intramolecular hydrogen-bonded network of the PVA. The addition of water to the dry gels favours their swelling, destroying intra-chain hydrogen bonds between hydroxyl groups as a function of the degree of tacticity and the gelation procedure, and promotes the formation of new networks of interchain hydrogen bonds. Information on the dynamics of the polymeric domains in the kilohertz range has been obtained from the analysis of the spin relaxation times T1ρ(1H) and T1ρ(13C), indicating that homogeneous arrangements of the amorphous or swollen polymeric chains exist, independent of the preparation method or the tacticity of the PVA chains.

Autocatalytic hydrolysis of amorphous-made polylactides: effects of l-lactide content, tacticity, and enantiomeric polymer blending

Poly( dl-lactide), i.e., poly( dl-lactic acid) (PDLLA), poly( l-lactide), i.e. poly( l-lactic acid) (PLLA), and poly( d-lactide), i.e., poly( d-lactic acid) (PDLA) were synthesized to have similar molecular weights. The non-blended PDLLA, PLLA, and PDLA films and PLLA/PDLA(1/1) blend film were prepared to be amorphous state, and the effects of l-lactide unit content, tacticity, and enantiomeric polymer blending on their autocatalytic hydrolysis were investigated in phosphate-buffered solution (pH7.4) at 37 °C for up to 24 months. The results of gravimetry, gel permeation chromatography (GPC), and tensile testing showed that the autocatalytic hydrolyzabilities of polylactides, i.e. poly(lactic acid)s (PLAs) in the amorphous state decreased in the following order: nonblended PDLLA>nonblended PLLA, nonblended PDLA>PLLA/PDLA(1/1) blend. The high hydrolyzability of the nonblended PDLLA film compared with those of the nonblended PLLA and PDLA films was ascribed to the lower tacticity of PDLLA chains, which decreases their intramolecular interaction and therefore the PDLLA chains are susceptible to the attack from water molecules. In contrast, the retarded hydrolysis of PLLA/PDLA(1/1) blend film compared with those of the nonblended PLLA and PDLA films was attributable to the peculiar strong interaction between PLLA and PDLA chains in the blend film, resulting in the disturbed interaction of PLLA or PDLA chains and water molecules. The X-ray diffractometry and differential scanning calorimetry (DSC) elucidated that all the initially amorphous PLA films remained amorphous even after the autocatalytic hydrolysis for 16 (PDLLA film) and 24 [nonblended PLLA and PDLA films, PLLA/PDLA(1/1) blend film] months and that the melting peaks observed at around 170 and 220 °C for the PLLA/PDLA(1/1) blend film after the hydrolysis for 24 months were ascribed to those of homo- and stereocomplex crystallites, respectively, formed during heating at around 100 and 200 °C but not during the autocatalytic hydrolysis.

An Analysis of Raman Spectra of Syndiotactic Polypropylenes. 2. Configurational Defects

Raman spectra of syndiotactic polypropylenes of different stereoregularity are reported. Spectra for ordered and disordered structures of these polymers were simulated using a small number of parameters. The features that occur in the 300 and 400 cm -1 regions can be used to assess chain tacticity. A quantitative determination of the racemic content of polypropylene can be obtained from the Raman spectrum in the range of tacticity studied.

Tacticity induced molecular microstructure dependence of the configurational properties of metallocene-synthesized polypropylenes

Stereoblock polypropylene (PP) produced using metallocene catalysts consists of alternating stereoregular and stereoirregular sequences. Through a control of the isotactic and atactic sequence lengths, the Bernoullian replication probability and the rate of isomerization of the metallocene catalysts, polymeric chains of various molecular microstructures could be generated. The dependence of the microstructure on the various polymerization parameters is examined in terms of the isotactic pentad content, [mmmm]. The dependence of the physical properties of PP on the molecular microstructure induced by the chain tacticity was also investigated both experimentally and theoretically using Metropolis Monte Carlo techniques. It was shown that the intrinsic viscosity and the characteristic ratio of the polymer are greatly influenced by the stereoregularity of the polymeric chains, which was attributed to a possible steric hindrance between the neighbouring methyl side groups. Other configuration-dependent physical properties such as the radius of gyration, the persistence length and the molar stiffness function were also investigated for PP of different stereoregular microstructures. It was shown by examining the distribution of the dihedral angles that the pentane effect resulting from the steric hindrance of the side groups is the underlying principle for the microstructure dependence of the configurational properties of stereoblock PP.

MOLECULAR MODELING OF POLYMER-CHAIN TACTICITY

Classical molecular simulation is used to study the polymer-chain tacticity through two approaches. The first purpose is to use it as a characterization technique: The chain tacticity of a commercial polymer is determined by comparing the experimental infrared absorption spectrum with those ensued from the simulated stereoisomers. The best agreement between the simulated and the measured spectra actually reveals the experimental polymer-chain configuration. The second is to employ molecular simulation as a probe technique. From the accurate simulation of the difference in the glass transition temperatures between the two PMMA stereoisomers, energetic and microscopic analyses are performed. Based on glass transition theories, the reasons for such a difference are then discussed.

Influence of Stereoregularity on the Wall Slip Phenomenon in Polypropylene

Thus, although a lot is now known about themechanisms of the interfacial slip, it is still not clearas to how the subtleties of molecular architectureinfluence the slip behavior. Thus, the effects of molec-ular structural parameters such as molecular weight,molecular weight distribution, tacticity, short- and long-chain branching, and the branching distribution are stillnot completely understood. In this work we show for thefirst time that the tacticity of polymer chains cansignificantly influence the slip behavior. We show thatsyndiotactic polypropylene shows sharkskin distortionson the extrudate surface, which is an indication of wallslip, while an isotactic polypropylene of similar molec-ular weight and molecular weight distribution does notshow any wall slip.

Methacrylic polymers containing permanent dipole azobenzene chromophores spaced from the main chain.13C NMR spectra and photochromic properties

Dynamics in solution and photochromic properties of radical copolymers of 4-(4-oxy-4′-cyanoazobenzene)but-1-yl methacrylate, 6-(4-oxy-4′-cyanoazobenzene)hex-1-yl methacrylate, and 8-(4-oxy-4′-cyanoazobenzene)oct-1-yl methacrylate with (–)-menthyl methacrylate were investigated. 13C NMR and 2D HETCOR spectroscopy allowed assignment of 13C NMR signals and evaluation of main chain tacticity. 13C T1 relaxation times evidenced a rather limited mobility of the azobenzene chromophores when inserted in polymer macromolecules. Mobility increased with increasing the length of the polymethylene spacer that acted as a flexible joint between the aromatic chromophore and polymer backbone. Both the trans to cis photoisomerization and the cis to trans thermal isomerization processes showed a small dependence on monomer structure and chemical composition of the investigated photochromic polymers. These data seem to suggest an appreciable contribution of the in-plane inversion mechanism to both isomerization processes of the azobenzene chromophores. The absence of appreciable dichroic bands in the copolymer CD spectra and the negative results obtained in preliminary second harmonic generation measurements are discussed in terms of the polymer structural features.