Isotactic Chains Research Articles

Effect of Tacticity on Chain Entanglement in Poly(methyl methacrylate)

It is shown that the plateau modulus G N and entanglement molecular weight M e of poly(methyl methacrylate) are strongly dependent on tacticity, although they are independent of molecular weight and its distribution (in the absence of fractions with molecular weights below the entanglement value). Thje dependency on tacticity follows the geometric-mean (semilogarithmic) relationship with respect to diad fractions. The plateau modulus G 0 N and entanglement molecular weight M e vary from 2.4x10 6 dyn cm -2 and 16400 for pure isotactic chains to 5.3x10 6 dyn cm -2 and 7800 for pure syndiotactic chains

Quasiharmonic treatment of infrared and raman vibrational frequency shifts induced by tensile deformation of polymer chains. II. Application to the polyoxymethylene and isotactic polypropylene single chains and the three‐dimensional orthorhombic polyethylene crystal

AbstractQuasiharmonic equations are derived for stress‐induced vibrational frequency shifts in the infrared and Raman spectra of polymer chains subjected to a tensile stress. The expressions are applied to the helical chains of polyoxymethylene and isotactic polypropylene. Observed frequency shifts can be reproduced well by using reasonable anharmonic force constants. A semiquantitative interpretation is given for the close relationship between stress‐induced vibrational frequency shifts and the deformation mechanism of the polymer chains. Stress‐induced frequency shifts are also calculated for an orthorhombic polyethylene crystal subjected to uniaxial tension along the chain axis or to hydrostatic pressure. The results consistently and reasonably reproduce observed data, not only for the intramolecular vibrational modes but also for the external lattice modes. © 1992 John Wiley & Sons, Inc.

Conformational analysis of poly(2‐hydroxyethyl methacrylate)

AbstractThe poly(2‐hydroxyethyl methacrylate) (PHEMA) is a disubstituted vinyl chain in which the substituents CO2CH2CH2OH and CH3 differ in size and shape. In order to verify the various characteristics of the PHEMA chain, the conformational energy calculations for meso and racemic diads, which are the segments consisting of the stereoregular isotactic and syndiotactic chains, respectively, were carried out using ECEPP/2 potential. From these calculations, the averaged geometry and the statistical weights were obtained in a local minima. The characteristic ratio, C∞ = (〈r2〉o/nl2)∞, was determined from the statistical weights and geometries. The calculated C∞ for the isotactic and syndiotactic chain are 10.2 and 2.3, respectively. The characteristic ratio for isotactic chain is larger than that for syndiotactic chain. This shows that the syndiotactic chain is more folded than the isotactic chain is, and that the calculated tendency is in reasonably agreement with the experimental tendency of acrylate polymers.

X‐ray diffraction characterization of amorphous poly(propylene)

The differential radial distribution functions of a sample of amorphous (atactic) poly(propylene) and of an isotactic one were compared, showing the absence, in the former sample, of interatomic correlations beyond 8 Å (1 Å = 0,1 nm). The influence of the length of atactic and isotactic chain segments on the calculated X-ray interference functions was investigated. A model based on the arrangement of atactic chains in close-packed spheres fits well the experimental X-ray diffraction pattern of the amorphous sample.

Theoretical study of the dependence of the valence electronic levels on the tacticity and conformation of acrylonitrile model oligomers

With the aim of searching for signatures of conformation and tacticity in electrodeposited polyacrylonitrile valence XPS spectra, exploratory ab initio STO-3G calculations have been performed on four model structures of H—[CH2—CH(CN)]6—H: isotactic and syndiotactic all-trans planar chains and isotactic and syndiotactic (TG)3 helical chains. Contrary to what was assumed in earlier work it is found that the (TG)3 conformation is not a suitable model helix for polyacrylonitrile. Signatures of conformation and tacticity are noted in the ab initio simulated valence XPS spectra; their characteristic features (position on the energy scale, shape and intensity) are within the limits of experimental resolution.

The valence electronic structure of isotactic polystyrene revisited

We present a comparison between the experimental UPS spectrum of polystyrene and a theoretically simulated spectrum derived from valence effective Hamiltonian (VEH) calculations. The calculations are carried out on a stereoregular helical isotactic polystyrene chain where the bonds joining the backbone to the phenyl groups are trans relative to the backbone (defining an isotactic polystyrene conformation denoted i-PS). This conformation is different from that considered in a previous work (Brédas and Street (1985)) where the backbone-phenyl bonds were gauche with respect to the backbone (i′'-PS conformation). The theoretical UPS spectra produced by the i-PS and i′'-PS conformations present subtle yet significant differences which, when comparison is made to experiment, all point in favor of the i-PS conformation considered in this work. This resolves some of the discrepancies between theory and experiment from previous work.

Stereoregularity of fractionally crystallized poly(propylene oxide) samples by 13C‐NMR spectroscopy

AbstractSeveral stereoregular poly(propylene oxide) (PPO) samples were synthesized employing either trimethyl aluminum hydrolyzates or Pruitt–Baggett type catalysts. Dilute isooctane solutions of these samples were fractionally crystallized by stepwise cooling from 333 K down to 273 K. The fractions which dissolve at 273 K were noncrystallizable and were further fractionated by GFC. The isolated fractions were characterized by 13C‐NMR, DSC, and viscometry techniques. Both structural (head‐to‐head and tail‐to‐tail linkages) and steric (syndiotactic dyads) irregularities were found in otherwise isotactic chains and their abundance apparently increased as the melting point and the chain length of the fractions decreased.

Study on the chain conformation of polymers.: II. Conformational characteristics of 1,2-polybutadiene and its characteristic ratio

In this work, improvement was made for conformational analysis of 1,2-polybutadiene by means of the molecular mechanics force field program (MM2). Thus the first and the second order interaction energies obtained are much more reasonable. The characteristic ratio of isotactic 1,2-polybutadiene was emphasized properly, which varied with energy E (E = Eη + Eω″). In the same time, the characteristic ratios for isotactic chain of polystyrene and so on were noticed, which varied with energy E according to the same way as that for isotactic 1,2-polybutadiene. It is shown that there is a general rule or a common feature for isotactic chains of poly -α-olefine and vinyl polymer.

General iterative calculation of moments and distribution functions of polymer chains. Application to the poly(methylphenylsiloxane)

Previous iterative equations for the calculation of high moments, based on the spherical harmonic representation of rotational operators and only valid for symmetric chains in their original version, have been generalized for any real polymer chain represented by the rotational isomeric state model. From these high moments, the end-to-end distance and end-to-end vector distribution functions can be inferred. The new generalized expressions have been applied to obtain the radial distribution of the end-to-end distance W(R) of short isotactic and syndiotactic poly(methylphenylsiloxane), PMPS, chains. Then, the remarkable influence of tacticity in W(R) has been analyzed. Furthermore, a significant role of the fluctuations of rotational angles has been clearly manifested in the conformational distribution of isotactic PMPS chains.

Conformations and dipole moments of poly(methyl) methacrylate molecules

Local conformations of stereospecific PMMA chains have been investigated. For these chains and for atactic PMMA we calculated the mean square dipole moment 〈μ 2〉 and the correlation factor g=( μ/nm 2) n→∞ . In the case of isotactic PMMA chains the share of tt conformations amounts to 75% of the monomer units, and in the syndio chains the amount of tt conformations is increased up to 90%. Vaues of g calculated for the isotactic and atactic chains over the entire interval of values permissible for the energy parameters that may be reconciled with experimental findings are obtainable only if it is assumed that there is a regular alternation of the directions of dipole moments of side polar groups (pendants), the rotation angle being x = 0 and 180°; in the syndio chains the probability of there being an alternation of the directions of dipole moments of the pendants 1 2 </ w x </ 3 4 .

The role of intermolecular interactions in determining the mode of packing of crystalline polymers. Energy calculations on isotactic polypropylene

Packing energy calculations for isotactic polypropylene chains have been performed with the purpose: (a) to predict the most stable kind of monoclinic lattice for the α-modification; (b) to contribute to the understanding of the structural disorder which is commonly found in the α-modification; (c) to evaluate the order of magnitude of the differences in energies among the three modifications (α, β, γ) experimentally observed. The results for the α-form show that the best packing of the chains is achievable for the ordered P2 1/c space group, while more or less disordered structures may exist, which only slightly differ in energy from the most stable structure. In connection with point (c), our results are in agreement with the possibility of polymorphism in isotactic polypropylene.

Impurity Effect of the Dielectric Properties of Isotactic Polypropylene

Two dielectric relaxation peaks, the crystalline and the primary relaxations, were observed in the temperature-dielectric loss curves in isotactic polypropylene. Among them the primary relaxation ß was studied in detail using different kinds of polypropylene films and their films purified with acetone. It was found that ß-relaxation consists of two peaks, denoted ß1 and ß2, being consistent with the dynamic mechanical characteristics. The lower temperature peak ß1 is assigned to the amorphous phase, which is rich in atactic chains, and the upper temperature peak ß2 is ascribed to an antioxidant contained in the amorphous phase which is rich in isotactic chains. The antioxidant also causes an increase in dielectric loss in the higher temperature region. The extraction of impurities from polypropylene films with acetone reduces the magnitude of the primary relaxation peaks.

Proton spin–lattice and spin–spin relaxation times in isotactic polypropylene. I. Effects of crystallinity and atactic fraction

Spin–lattice relaxation time T1 and spin–spin relaxation time T2 were measured at 40°C on the isotactic polypropylene films of varying preparations and thermal history. T1 increases with increasing crystallinity and two T1's appear for the samples annealed at elevated temperatures (>120°C). These variations in T1 are well interpreted in terms of the spin diffusion and decoupling of the mobile protons with immobile ones. A free induction decay following a 90° pulse is the superposition of three different decay curves, one of which is exponential and other two are nonexponential. There is an increase in T2a with increasing crystallinity, which is indicative of the enhancement of the chain mobility in the amorphous region. There are differences between the crystallinity calculated from density and the fraction of crystalline region, Fc, obtained by the NMR method, which can be explained by the existence of the microparacrystals and the stress imposed on the amorphous chains on rapid cooling. On the other hand, there is a gradual lowering in T1 and a considerable increase in T2a as an atactic fraction is increased. The increase in atactic fraction also results in a decrease in the amount of the isotactic amorphous chains in the amorphous region.

Association of stereoregular poly(methylmethacrylates): 5. Stereo-complex formation in dilute solutions

Further study was carried out on the stereocomplex formation in dilute solution using viscometry, osmometry, light scattering and gel permeation chromatography. It appears that the process can be divided in two steps. The primary step involves the formation of very compact particles by association of several isotactic and syndiotactic PMMA chains in a base-molar ratio of 1 2 . The radius of gyration of these particles remains about the same as the additive value of the original components, whereas their molecular weight increases. The secondary step involves the aggregation of the particles which is reflected by an increase of the radius of gyration. It is proposed that this aggregation is caused by interparticle stereo-association of noncomplexed chain parts.

The influence of ester groups on the stereocomplex formation between different polyalkylmethacrylates

Isotactic poly(ethyl methacrylate) (it-PEMA) and syndiotactic poly(isobutyl-methacrylate) (st-PiBMA) were synthesized to investigate the stereocomplex formation with syndiotactic and isotactic poly(methyl methacrylate) (st- and it-PMMA) respectively. A d.s.c. study pointed out that it-PEMA and st-PMMA form incompatible mixtures whereas st-PiBMA and it-PMMA form compatible mixtures in which stereocomplex formation can occur as indicated by viscometry and X-ray diffraction. These results confirm the idea that the methyl ester groups of the isotactic chain and the α-methyl groups of the syndiotactic chain are responsible for stereocomplex formation.

Conformations in partly ionized poly(methacrylic acid): 1. Ionic hydrogen bonds in syndiotactic chains

Comparisons of u.v. spectrophotometric titrations of some dicarboxylic acids, ‘conventional’ poly(methacrylic acid) and poly(acrylic acid) suggest that ionic or acid-salt hydrogen bonds form between adjacent carboxyl groups in racemic dyads in partly ionized forms of poly(methacrylic acid). The ionic hydrogen bonds form in tt conformations of the dyads. The maximum concentration of ionic hydrogen bonds occurs at 50% ionization in pure syndiotactic chains in the presence of electrolyte to give a chain having four-bond repeating units. Bonds 1 and 2 are presumed to be fixed in trans-trans conformations while bonds 3 and 4 retain some conformational freedom although this is somewhat reduced as a consequence of side group rotations necessary for hydrogen bonding. While ionic hydrogen bonds may form across meso dyads in tḡ conformations, isotactic chains incorporating ionic hydrogen bonds are predicted to be less stable than the corresponding syndiotactic chains.

Conformation of polystyrene with special emphasis to the near all‐trans extended‐chain model relevant in polystyrene gels

AbstractIn light of novel and problematic diffraction patterns arising through the thermoreversible gelation of isotactic polystyrene, the conformational possibilities were examined with particular reference to other conformational works published recently on this polymer. The specific objective was to ascertain whether highly extended, helical backbone geometries such as are required by the new x‐ray patterns are possible or not with a purely isotactic arrangement of side groups. The first realization was that the recently announced low‐energy, near‐trans conformations are not relevant to the present issue as they do not provide the required extended geometry. Further, specific sampling of the conformational energy map revealed that the required high chain extension may be energetically feasible, contrary to traditional conceptions and to recent conformational calculations elsewhere. The divergences as regards the latter were traced to the specific choice of nonbonded atomic radii—all within generally accepted limits—which accordingly has a crucial effect even to the extent whether specific conformation types are forbidden or energetically stable. The implications this has for conformational analysis in general are being pointed out. While the novel crystal structure has not been solved, the present study at least eliminates the previously believed categoric objections to it being associated with isotactic chains and, beyond this, opens up new perspectives as regards conformational possibilities in polyolefins. The more general problems concerning the nature and origin of the gelation phenomenon nevertheless remain.

The conformations and dipole moments of polymethacrylate molecules

The local conformations of stereoregular polymethacrylate (PMA) chains have been examined. The mean square dipole moment <μ 2> and the correlation parameter g=< μ 2>/ nm 2 for n→ α ( n is the degree of polymerization; m-dipole moment of the monomer unit) has been calculated for these and the stereoirregular chains. The isotactic PMA chains are mainly in the tg (or gt) conformation (60–80% of the monomer units), but a noticeable proportion (40-20%) have a tt conformation which serves as contact between them. The monomer units of the syndiotactic PMA chains have mainly the tt conformation (75–80%) with a participation of (about 20%) gg conformations. Up to 7% of the monomer units take on a gt and tg conformation. Values of g are greater than 1 over the whole range of admissible energy parameters where the chains are stereo-regular. The value is near the experimental in the case of all other chains and is obtained only by assuming strong hindrance to rotation of the pendant side group.

Temperature dependence of stereospecificity of active sites in heterogeneous catalysts for propylene polymerization

Temperature dependences of the stereoregularity parameters of the most stereospecific active centres of α-TiCl 3-AlEt 3 catalytic system from −25 to 120 and of VCl 3-AlEt 3 catalytic system from −15° to 90° have been measured. It was found that this temperature dependence could be represented by a curve with a minimum at 20–50. The results could be explained by a two-step mechanism of isotactic chain growth (propagation) with preliminary co-ordination of monomer on the active centres.

The structure of amorphous polystyrene by X-ray scattering and simple conformational analysis

The X-ray diffraction patterns of amorphous (glassy) polystyrene are interpreted by means of intensity functions calculated from model structures of isotactic polystyrene, incorporating only intra-chain interactions. The range of chain conformations or possible complexions is explained by means of the agreement between theoretical and experimental functions and in relation to published data for the conformational energy of polystyrene in solution. The calculations indicate that in the glassy state, isotactic chains adopt conformations characterized by a large phenyl group separation and a degree of rotational disorder defined about skeletal bonds.