Atactic Chains Research Articles

Compatibilizing effect of a graft copolymer on bacterial poly(3‐hydroxybutyrate)/poly(methyl methacrylate) blends

AbstractBlends of isotactic (natural) poly(3‐hydroxybutyrate) (PHB) and poly(methyl methacrylate) (PMMA) are partially miscible, and PHB in excess of 20 wt % segregates as a partially crystalline pure phase. Copolymers containing atactic PHB chains grafted onto a PMMA backbone are used to compatibilize phase‐separated PHB/PMMA blends. Two poly(methyl methacrylate‐g‐hydroxybutyrate) [P(MMA‐g‐HB)] copolymers with different grafting densities and the same length of the grafted chain have been investigated. The copolymer with higher grafting density, containing 67 mol % hydroxybutyrate units, has a beneficial effect on the mechanical properties of PHB/PMMA blends with 30–50% PHB content, which show a remarkable increase in ductility. The main effect of copolymer addition is the inhibition of PHB crystallization. No compatibilizing effect on PHB/PMMA blends with PHB contents higher than 50% is observed with various amounts of P(MMA‐g‐HB) copolymer. In these blends, the graft copolymer is not able to prevent PHB crystallization, and the ternary PHB/PMMA/P(MMA‐g‐HB) blends remain crystalline and brittle. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 1390–1399, 2002

Mesoscopic modeling of the polymerization, morphology, and crystallization of stereoblock and stereoregular polypropylenes

AbstractThermoplastic elastomers require domains that act as physical crosslinks, and these can be either glassy regions or crystallites. For crystallites in a polymer such as polypropylene (PP), the challenge is to develop polymerizations that form the long stereoregular chain sequences required to produce crystallites large enough to act as stable temporary crosslinks but not so many of them that the material becomes a significantly crystalline thermoplastic rather than an elastomer. For PP, the requirement is for long isotactic sequences within predominantly elastomeric atactic chains, and catalysts required to achieve such an unusual stereoblock structure are now available. This provided encouragement for the simulations described herein that illustrate some relationships between polymerization mechanisms and distributions of isotactic sequences (particularly those long enough to crystallize). A Windle‐type Monte Carlo algorithm was then applied to arrays of the representative PP chains in searches for matches of crystallizable sequences. This approach provided estimates of degrees of crystallinity, melting points, interfacial free energies, standard free energies of fusion, and Young's moduli at small extensions. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 840–853, 2002

Propene Polymerization with Catalyst Mixtures Containing Different ansa-Zirconocenes: Chain Transfer to Alkylaluminum Cocatalysts and Formation of Stereoblock Polymers

Substantial transfer of growing polypropyl chains to methylalumoxane and trimethylaluminum occurs with the highly substituted, isospecific zirconocene catalyst Me2Si(2-Me-4-tBu-C5H2)2ZrCl2/MAO, while little if any such chain transfer is observed for the more open isospecific catalyst Me2Si(2-MeInd)2ZrCl2/MAO, for aspecific H4C2(Flu)2ZrCl2/MAO, and for syndiospecific Ph2C(Cp)FluZrCl2/MAO. Propene polymerization with MAO-activated mixtures of Me2Si(2-MeInd)2ZrCl2 and H4C2(Flu)2ZrCl2 gives completely separable mixtures of the isotactic and atactic polymers characteristic for each of the individual catalysts. MAO-activated mixtures of Me2Si(2-Me-4-tBu-C5H2)2ZrCl2 and H4C2(Flu)2ZrCl2, however, give polypropene mixtures that contain, besides isotactic and atactic polymers, polymer fractions in which isotactic and atactic polypropene chain segments are inseparably linked. While clear evidence for isotactic−syndiotactic stereoblock formation was not obtained, some polymeryl exchange between isospecific and syndio...

Approximate methods of the calculation of effective energies in disordered chains: I. Comparative theoretical study of the energy properties of structural isomers of polypropene

The synthesis of conformational isomers of long oligomers built up from propene units can be controlled. Propene may polymerize to long isotactic, syndiotactic or atactic chains depending on the experimental conditions, especially the catalyst. The total energies of these conformational isomers, having the methyl groups either all on one side, or alternating sides or randomly distributed are different as a result of the different interactions. We have calculated the total energies and effective energies per propene unit of polypropene in different conformations with the help of the elongation method and the direct cluster approach, respectively. These results are compared with each other and in addition with ab initio Hartree–Fock crystal orbital calculations on infinite periodic polypropene chains.

Syndiospecific copolymerization of styrene with styrene macromonomer bearing terminal styryl group by CpTiCl3–methylaluminoxane catalyst

Copolymerization of styrene with styrene-terminated polystyrene macromonomer (SSTM) by CpTiCl3–methylaluminoxane (MAO) catalyst was investigated. SSTM was prepared by a reaction of living polystyrene initiated with sec-butyllithium (s-BuLi) and p-chloromethyl styrene at −78°C. The synthesized SSTM has a high terminal functionality and a narrow molecular weight distribution. Graft copolymers of polystyrene consisted of syndiotactic main chain and atactic side chain were synthesized by CpTiCl3–MAO catalyst. The synthesized graft copolymer was confirmed to have highly syndiotactic sequence on the main chain.

Detection of the onset of demixing in simulations of polypropylene melts in which the chains differ only in stereochemical composition

Simulations have been performed at 480 K for pure melts of atactic, isotactic and syndiotactic polypropylene chains with a degree of polymerization of 50 and composition C150H302, and also for the three 50:50 mixtures of two of these species. The simulations are performed on a high coordination lattice, with incorporation of short range intramolecular interactions from a rotational isomeric state model of polypropylene, and incorporation of long range interactions defined by a Lennard-Jones potential energy function for the interaction of pairs of molecules of propane. Both the rotational isomeric state model and Lennard-Jones parameters were taken directly from the literature (Suter et al. and Prausnitz, respectively). The efficiency of the simulation on the sparsely occupied high coordination lattice facilitates the equilibration of the one- and two-component melts within accessible computer time. Onset of a tendency for demixing of isotactic and syndiotactic polypropylene is apparent in the intermolecular pair correlation functions. No such demixing occurs with isotactic and atactic polypropylene. Both of these predictions from the simulation are consistent with experimental results in the literature (Maier et al. and Lohse, respectively). The simulation produces an ambigious prediction for the melt of atactic and syndiotactic polypropylene. This melt has been reported (Maier et al.) to exhibit phase separation, but less strongly than the isotactic-syndiotactic system, for which the simulation makes an unambiguous (and correct) prediction. The physical origin of the tendency for demixing in the simulations is identified as the differences in the preferred local conformations of polypropylene chains with various stereochemical sequences. This driving force is an example of “conformational asymmetry” induced solely by differences in stereochemical sequence.

Configurational-conformational statistics of stereoirregular poly(methyl methacrylate)s

The mean-square radius of gyration of poly(methyl methacrylate) (PMMA) with Bemoullian statistics was theoretically investigated utilizing the method of periodic condition. The dependent curve of the characteristic ratio of mean-square end-to-end distance on periodic microstructure length increases monotonously with the length and goes gradually to its asymptotic value for atactic PMMA chains (P m = 0.5). Conformational energies E α ∼1.3 kcal mol−1 and E β ∼-0.6 kcal mol−1 of the two-state scheme are acceptable, from which the derived meansquare end-to-end distance and mean-square radius of gyration conform to the experimental data. The ratio ⟨S2⟨/χ of atactic PMMA (P m = 0.2) as a function of the degree of polymerization χ first increases with increasing chain length, then passes through a maximum at χ = 30, and finally decreases to its asymptotic value, which is in reasonable agreement with the experimental measurements reported elsewhere. The ratio ⟨S2⟨/χ as a function of stereochemical composition indicates that theoretical and experimental data are in accord for isotactic and atactic PMMA, but small-angle neutron scattering (SANS) measurements are higher than the calculated values for syndiotactic PMMA.

Polyelectrolytes of High Charge Density in Organic Solvents. Synthesis and Viscosimetric Behavior

An easy anion exchange between electrolyte monomers of the quaternary ammonium halide type such as [(2-methacryloyloxy)ethyl]trimethylammonium chloride or 1-vinyl-3-methylimidazolium iodide and various cyanocarbon salts such as 1,2,2-tricyanoethenolate, 1,1,2,3,3-pentacyanopropenide, α,α-dicyano-p-toluoylcyanide, or p-(tricyanovinyl)phenyldicyanomethide readily leads to a series of new hydrophobic electrolyte monomers with yields generally over 70%. [(2-Methacryloyloxy)ethyl]trimethylammonium 1,1,2,3,3-pentacyanopropenide is a most representative example. Its free radical polymerization in homogeneous solution (DMF, 60 °C) currently yields atactic chains (Bernoulli stereopropagation process, probability of meso placement Pm = 0.31) of fairly high degree of polymerization (DPw ∼ 1600) which show an unique spectrum of solubility in dipolar organic solvents (dipole moment μ > 2.6 D) covering a very broad range of dielectric constants (e ∼ 10−180). Viscosity measurements performed in 13 solvents allow us to a...

Crystallinity in poly(vinyl alcohol). 1. An X-ray diffraction study of atactic PVOH

The significant level of crystallinity in atactic poly(vinyl alcohol) results from the comparatively modest size of the OH group enabling it to assume d and l positions in an atactic chain within a single crystal lattice. The structure is highly hydrogen bonded. An X-ray diffraction investigation has been made of the crystal structure to determine the distribution of hydrogen bonds, to understand the crystalline thermal expansion and to determine the influence of water on the polymer crystals and crystallisation process. Thermal annealing treatments have been developed to obtain samples of the maximum level of crystallinity with minimum disorder. The well-resolved diffraction peaks from such samples are analysed to measure the thermal expansion coefficients of the crystal unit cell which are related to the hydrogen bonding network in the crystal structure. Finally, the effect on the crystals of absorbing water into polymer of various levels of crystallinity and also during the crystallisation process is investigated, leading to the hypothesis that water molecules may be incorporated within the random crystal structure resulting in a change in the relative intensities of the diffraction peaks.

Unperturbed mean-square radius of gyration of atactic polystyrene chain with side groups

A theoretical treatment of the unperturbed mean-square radius of gyration for the monosubstituted polymer chains is presented. The characteristic ratio and the temperature coefficient for an atactic polystyrene chain are evaluated to be 1.89 and − 1.3 × 10 −3 deg −1 by the derived formulas, respectively, which agree well with the experimental data.

Poly(methylphenyl) silane: Structural properties

The physical structure of poly(methylphenyl) silane (PMPS) has been investigated using wide-angle x-ray scattering at various temperatures and optical polarizing microscopy. The results obtained by these techniques clearly show the existence of an ordered phase in PMPS. The crystallinity of our sample was estimated to be about 10% at room temperature. Below 190°C, the atactic chains pack into a monoclinic crystalline lattice of near hexagonal symmetry, with two types of disorder existing in the packing. At about 190°C, a phase transition to a liquid crystalline columnar hexagonal packing (Dho) occurs. Finally, the sample melts into an isotropic amorphous phase. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35: 1727–1736, 1997

PMMA conformational changes on γ-alumina powder: influence of the polymer tacticity on the configuration of the adsorbed layer

Stereoregular PMMAs have been adsorbed from solution on γ-alumina and studied by diffuse reflectance infrared (DRIFT) and solid state 13C NMR. Infrared has shown the fraction of surface bonded carbonyl (( ( c= O) ab ) is higher for the adsorbed isotactic PMMA than for the syndiotactic form. The shift of the infrared ν a may indicate that the methoxy functional group also interacts with the surface. From NMR spectroscopy, the changes in methylene carbon chemical shift, due to a lowering in the γ-gauche effect, are more pronounced for isotactic PMMA than for the syndiotactic form. The results agree with the assumption of a flattened conformation of adsorbed isotactic chains while atactic and syndiotatic chains are adsorbed more loosely.

Chain flattening of spin-cast PMMMA on aluminum mirrors: Influence of polymer tacticity

Polymer tacticity has already been shown to be an influential parameter governing the behavior of polymer solutions and polymer blends, but not for polymer adsorption. However, using PMMA having a racemic fraction (ƒ r) ranging from 0.1 to 0.9, spin-cast on aluminum mirror surfaces, makes it possible to work out configuration differences in the adsorbed layer. Reflectance infra-red spectroscopy has been used to evaluate the fraction of surface bonded carbonyl groups (ƒ ab CO) and local conformations of both main and side chain. The ƒ ab CO value is 0.1 for highly syndiotactic PMMA and reaches 0.7 for mainly isotactic PMMA. This latter very high value of ƒ ab CO can only be explained by chain flattening resulting from a distortion from its Isotropic state. This distortion may be due to both strong interactions that may exist between PMMA and the aluminium substrate, and also to the spin-cast process that may lead to chain straightening and flattening. The assumption of conformational energy changes of both backbone and side chain is confirmed by changes in the FT-IR Av 2(1245 cm −1) Av 1(1265–1275 cm −1 ) ratio. Ellipsometric spectroscopy has provided information on the global conformation of adsorbed chains by precise thickness measurements. Thus, thickness ( h) increases for diluted solutions linearly from 10 to 70 A as the average syndiotactic sequence length ( n s ) increases. However, IR and ellipsometric results tend to prove that isotactic PMMA may adsorb on the aluminum surface in a more flattened way with longer chains than atactic and syndiotactic chains. This can be explained by the ability of chain segmental rearrangements ( meso tg state) that may allow isotactic PMMA to favor the establishment of interfacial acid-base bonds while the syndiotactic PMMA local stiffness of the tt state does not. The influence of the chain entropy in solution is also discussed as an important parameter.

Theoretical Study of the Conformational Behaviour of Poly(oxypropylene) Chains

The present paper describes the calculation of the conformational behaviour of isotactic, syndiotactic and atactic poly(oxypropylene) (POP) chains, using rotational-isomeric-state (R-I-S) theory in the form of BIOSYM's Polymer computer-software. It is found that the conformational behaviour of the POP chain, in the region of an R methyl-substituted skeletal carbon atom, differs from that in the region of the corresponding S carbon, due to the differences in the steric interactions between the methyl side-group and the methylene group two skeletal atoms away in the POP backbone. Thus, the isotactic and syndiotactic chains have markedly different values of Cn, the characteristic ratio. Accordingly, Cn was calculated for isotactic, syndiotactic and atactic POP chains of various numbers of skeletal bonds, n. The results showed that the isotactic chains occupy more extended conformations (with the highest values of Cn), compared with the highly-coiled syndiotactic chains (lower values of Cn). For the atactic POP chains, Cn was found to lie between the two extremes of the isotactic and syndiotactic forms. The value of C* for the infinite atactic POP chain was found to be consistent with values determined from experimental measurements.

Crystallization kinetics of isotactic poly(1-hexadecene)

The crystallization kinetics of the isotactic content of three poly(1-hexadecene) (PHD) samples with different tacticity has been analysed by differential scanning calorimetry (d.s.c.), using both isothermal and non-isothermal treatments. Two d.s.c. peaks are found in both the melting and cooling of the three PHDs. The high- and low-temperature peaks are attributed to the crystallites formed by the isotactic and atactic chains, respectively. Additional small-angle X-ray diffraction (SAXD) experiments, using synchrotron radiation, have been performed, indicating that the isotactic crystals exhibit appreciably smaller spacings than the atactic ones. Because of overlapping problems, only the isothermal crystallization kinetics of the isotactic content can be studied. The corresponding analysis in the range of crystallization temperatures from 36 to 45°C showed that the process can be described by the Avrami equation up to relatively high conversions. The rate constant k of this equation increases very rapidly with undercooling (about four orders of magnitude in only 8 degrees), indicating that the process is controlled by the nucleation. The value of the Avrami exponent is approximately 4, suggesting a homogeneous nucleation followed by three-dimensional growth. The melting temperature of the isotactic content is independent of the crystallization temperature and was found to be 58°C. The study of the non-isothermal crystallization of PHDs shows that it can be analysed in terms of an Avrami-type equation and similar mechanisms of nucleation and growth apply to all temperatures and cooling rates in the ranges studied. Moreover, the Avrami exponent in this analysis is of the same order as in the case of isothermal treatment.

Molecular modeling of polymers. 10. Characterization of conformational transitions of poly(acrylic acid) and poly(methacrylic acid) using dyad structures

AbstractThe conformational profiles of nearest side‐chain neighbors, methylene‐dyad structures, of poly(acrylic acid), PAA, and poly(methacrylic acid), PMA, were determined as a function of tacticity, extent of ionization, and presence of counterion. The dominant backbone conformer states are quite similar for both isotactic and syndiotactic diads in a common charge state. Thus, the overall dimensional properties of isotactic syndiotactic and atactic chains of PAA or PMA, based upon dyad interactions, are predicted to be alike for a given charge state. Significant deviations from precise t, g+, and g− states are found for the dyad minimum energy conformations. The rod‐to‐coil and coil‐to‐rod transitions observed in PAA and PMA, respectively, as a function of increasing counterion concentration can be explained, to a large extent, by the conformational profiles of the corresponding dyad model structures. © 1994 John Wiley & Sons, Inc.

Novel polypropene materials derived from vinylidene‐terminated oligopropenes

AbstractMetallocene‐based homogeneous Ziegler–Natta catalysts produce mono‐olefin‐terminated oligopropenes with narrow molecular weight distributions, controlled stereoregularities, and molecular weights ranging from 100 to 30,000 g/mole in high yield slurry and solution processes. Steric and molecular weight control are influenced by metallocene structures, and by polymerization conditions such as temperature and propene concentration. Predominantly mono‐vinylidene‐terminated oligopropenes are attractive intermediates, and feedstock for the synthesis of a variety of polypropylene materials, including blends, block and graft copolymers. The key step is the chain end functionalization of the vinylideneterminated oligopropenes via double bond conversion reactions, followed by the controlled synthesis of polypropylene block and graft copolymers. In melt and solution processes the olefinic end groups have been converted into a variety of polar functional groups, e.g. hydroxy, carboxy, succinic anhydride, thiol and acrylic groups. The thiol‐terminated oligopropenes are chain transfer agents in radical methylmethacrylate polymerization with chain transfer constant measured to be 0.2. Acrylic monomers and styrene are grown onto the thiol end group via a chain transfer reaction, thus producing a family of block copolymers, e.g. poly(propene‐b‐methylmethacrylate) and poly(propene‐b‐styrene). As demonstrated by SEM fracture surface analysis, the poly(propene‐b‐styrene) block copolymers are efficient dispersing agents for compatibilizing polystyrene/polypropene (70/30) blends. Homo‐ and copolymerization of acrylic oligopropene macromonomers yield novel classes of graft copolymers with pendant isotactic or atactic oligopropene chains. Hydroxy‐terminated oligopropenes are useful initiators in caprolactone polymerization to form poly(propene‐b‐caprolactone) block copolymers. IR spectroscopic studies demonstrate that succinic anhydride‐terminated oligopropenes, obtained by ene‐type addition of maleic anhydride to the olefinic oligopropene end group, react with oligomeric diamine‐terminated polyamide‐6,6 in the melt to yield polypropene‐b‐polyamide‐6,6‐b‐polypropene triblock copolymers.

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.

Living enantiosymmetric and enantioasymmetric polymerization of methylthiirane in homogeneous phase

AbstractThe polymerization of racemic methylthiirane in homogeneous phase, initiated by bis(isopropyl‐S‐cysteinato) cadmium is a living process. The resulting polymers are isotactic and optically active at partial conversion. The optical purity of the residual monomer may reach 27% at half conversion. The propagation occurs mainly on one valency of Cd, however oligomers grow slowly on the second valency. The stereoregularity of the polymer chain appears only when the length of the oligomer becomes high enough, making possible a bicoordination of the Cd counterions. The stereoregularity of the polymer is characterized by the molar fraction σ of isotactic diads which varies from 0.5 for atactic chains—formed at the beginning—to about one for isotactic segments formed for longer chains. The stereospecifictity also depends on temperature of propagation and on initiator concentration. The kinetics observed (zero order in monomer and one‐half in Cd) are explained by monomer coordination before insertion and dimeric association of the thiolate end groups. The enantioasymmetric process observed results from an unbalance in the number of the two different types of active sites and possibly from a difference in their reactivities. Enantioasymmetry has been found to decrease significantly when the dielectric constant ε of the medium increases.

13C NMR study of the structure of the hydrogel polyester of sucrose methacrylate (Sucro-Gel H-70)

A 13C NMR study of four samples of poly(sucrose methacrylate) differing in molecular weight and degree of crosslinking was performed to determine their structure. It was concluded that Sucro-Gel H-70 polymer consists mainly of monomer units of isomeric monoesters principally esterified in C6 C6′ C4 position and forming atactic helical chains with some crosslinks due to the presence of substituted monomer units.