Response of the mean-square optical anistropy of chain molecules to the imposition of excluded volume

The influence of the chain expansion produced by excluded volume on the mean-square optical anisotropy has been studied in six types of polymers. The mean-square optical anistropy for a specified configuration is calculated using the valence optical scheme. Realistic rotational isomeric state models are used for the configurational statistics of the unperturbed chains. Excluded volume is introduced by hard sphere interactions. Results obtained with chains of 100, 200, 300, and 400 bonds permit extrapolation to the behavior expected for much longer chains. The mean-square optical anisotropy of polyethylene is insensitive to excluded volume. A similar conclusion was obtained several years ago in a study of chains confined to a tetrahedral lattice and weighted in a manner appropriate for the short-range interactions in polyethylene.² Different behavior is seen in poly(vinyl chloride), poly(vinyl bromide), polystyrene, poly(p-chlorostyrene), and poly(p-bromostyrene). The mean-square optical anisotropy of these five vinyl polymers is sensitive to the imposition of excluded volume if the stereochemical composition is exclusively racemic. Much smaller effects are seen in meso chains and in chains with Bernoullian statistics and an equal probability for meso and racemic dyads.     

Initiator selectivity during free radical copolymerizations of styrene and methyl methacrylate initiated by anaerobic copper(II) oxidation of carbon‐13 labeled acetophenone

Nuclear magnetic resonance (NMR) analysis of the ¹³C‐labeled chain ends of polystyrene, polyMMA, and styrene‐MMA copolymers prepared by polymerizations initiated using ¹³C‐labeled‐phenacyl radicals were investigated. The phenacyl radicals were generated by anaerobic oxidation of acetophenone‐methyl‐¹³C using a Cu(II) octanoate‐pyridine complex in the presence of triethylamine and triphenylphosphine. NMR analysis of the ¹³C‐labeled chain ends of these polymers afforded insight into the initiation mechanism. In copolymerization experiments using ¹³C‐labeled acetophenone initiator, the NMR spectra provided evidence that the phenacyl radical reacts 2.7 times faster with styrene than with MMA. The resonances of the labeled phenacyl carbons also showed that the sequence and stereosequence distributions of monomer units at the chain ends are nearly the same as those that prevail along the polymer chains. Styrene–styrene, styrene–MMA, and MMA–styrene enchainments at the chain ends are equally likely to have meso (erythro) or racemic(threo) configurations but the ratio of meso to racemic MMA‐MMA enchainments is ～ 3/7. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 2347–2356, 2008     

Synthesis and properties of cyclolinear methylcyclotetrasiloxane polymers with vinyl groups

Atactic cyclolinear organosilicon polymers carrying vinyl groups in RSiO_1.5 or R₂SiO moieties or concurrently in two moieties of the polymer unit have been synthesized by the heterofunctional polycondensation of 2,6-dihydroxymethyl(vinyl)cyclotetrasiloxanes with 2,6-dichloromethyl(vinyl)cyclotetrasiloxanes. The structure of the polymers has been studied by ¹H and ²⁹Si NMR spectroscopy, IR measurements, and elemental analysis. According to DSC, X-ray diffraction, and polarization optical microscopy, the polymers are amorphous in the range from ?100 to +200°C; the interlayer spacing and the type of packing remain unchanged with an increase in the content of vinyl substituents in the repeating units of cyclolinear polymethylvinylsiloxanes.     

Stereochemistry of photoinitiated emulsion polymerization

The tacticity of poly(methyl methacrylate) (PMMA) samples made by emulsion polymerization (EP) has been determined by NMR spectroscopy at higher resolution than any previous study. When photoinitiation is employed at room temperature, the polymer obtained is highly syndiotactic and more stereoregular than that obtained in homogeneous radical polymerization under the same conditions. The percentage of racemic dyads and the length of racemic sequences vary significantly with the degree of conversion and the temperature, but not with variation of the hydrophobic or the hydrophilic part of the detergent molecules. A magnetic field of 5 kG or less accelerates the polymerization reaction and increases the polymer molecular weight when dibenzyl ketone is employed as oil-soluble photoinitiator, and has an indirect influence on the polymer tacticity. From measurements of the temperature dependence of the polymer steric composition, the difference of activation enthalphy and entropy of the meso and racemic additions have been calculated. The values do not follow the correlations from precedent literature for radical polymerization in solution if the reaction is considered a first-order Markov process. The persistence ratio p depends on the detergent and the temperature of the EP; in some cases its difference from unity exceeds the experimental error. These results are interpreted as a conformational effect of the locus of the polymerization in the first stage of the EP (when micelles are present) that favors the racemic addition more than is expected in a homologous solvent such as an alkane, e.g., at the same temperature.     

Elevation of the glass transition temperature in flexible‐chain semicrystalline polymers

In the idealized two‐phase model of a semicrystalline polymer, the amorphous intercrystalline layers are considered to have the same properties as the fully‐amorphous polymer. In reality, these thin intercrystalline layers can be substantially influenced by the presence of the crystals, as individual polymer molecules traverse both crystalline and amorphous phases. In polymers with rigid backbone units, such as poly(etheretherketone), PEEK, previous work has shown this coupling to be particularly severe; the glass transition temperature (T_g) can be elevated by tens of degrees celsius, with the magnitude of the elevation correlating directly with the thinness of the amorphous layer. However, this connection has not been explored for flexible‐chain polymers, such as those formed from vinyl‐type monomers. Here, we examine T_g in both isotactic polystyrene (iPS) and syndiotactic polystyrene (sPS), crystallized under conditions that produce a range of amorphous layer thicknesses. T_g is indeed shown to be elevated relative to fully‐amorphous iPS and sPS, by an amount that correlates with the thinness of the amorphous layer; the magnitude of the effect is severalfold less than that in PEEK, consistent with the minimum lengths of polymer chain required to make a fold in the different cases. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 1198–1204, 2007     

Polymer crystallization: Simulation with entropic barrier model and application to specific polymers

The growth of polymer single crystals has been simulated on the basis of a simple two-dimensional ‘entropic barrier’ model. The chain is described by a sequence of growth units. Their additions and removals are determined by rate constants obeying detailed balance. The crystallization is then simulated by a kinetic Monte Carlo algorithm. An application of the model to specific crystallizable polymers (polyethylene, isotactic polystyrene, isotactic polypropylene, polyhydroxybuterate and polypivalolactone) is presented. Input parameter values for the model are derived from the respective surface free energies, bulk enthalpies, melting points and crystallographic repeat lengths. The only free parameter is the length of a polymer growth unit. This is set to half the lamellar crystal thickness at large undercooling. The lamellar thicknesses calculated on this basis are in good agreement with experimental data. An analysis of the growth unit lengths of the different polymers indicates a scaling with the chain persistence length in the melt.     

The configurational stereochemistry of atactic vinyl homopolymers

Atactic vinyl homopolymers, synthesized from non-dissymmetric monomers, are produced racemic or enantiomerically pure depending on the length of the polymer chain. The optical inactivity of high molecular weight samples of these polymers arises by a mechanism unknown in small molecules: intermolecular compensation of diastereomers.     

Conformations et stereoregularite de poly 2 vinyl pyridines—II Etude theorique

The molecular optical anisotropy of isotactic and atactic (anionic) poly 2 vinyl pyridines has been interpreted according to the rotational isomeric state model and in terms of meso and racemic dyads. In the hypothesis of a Bernouillian propagation process, analysis of the results shows that isotactic and atactic compounds contain respectively 98–99% and 9–10% of meso dyads. Therefore the so-called atactic polymer is essentially syndiotactic.     

Polymer fibers drawn to the limit

The orientational drawing of polymers is known to be terminated because of sample rupture. The limiting draw ratio λ_lim reached may be different (either large or small) depending on the polymer and the actual drawing conditions. The purpose of the present work is to identify the change of supermolecular structure of polymer fibers which results in the termination of orientational drawing. Small-and wide-angle x-ray diffraction were used to study the variation of geometrical parameters of this structure with increasing draw ratio λ. The geometrical parameters discussed are the dispersions (fluctuation) of long periods and of longitudinal sizes of crystalline as well as amorphous regions. In this study we used fibers of poly(vinyl alcohol), poly(ε-caprolactam), polyoxymethylene, and poly(4,4′-diphenyloxide) pyromellitimide. It is found that the long period dispersion of these polymers, drawn under different conditions, increases to approximately the same value for different samples drawn to the limit, this relative standard deviation δ_L of long periods being 0.30-0.40. It is also found that the crystallite size dispersion does not increase with increasing λ; the increase of λ_L is due to increasing dispersion of the amorphous region lengths. For poly(vinyl alcohol) fibers drawn to the limit under different conditions and which have different λ_lim, the relative standard deviation of the sizes of amorphous regions δ_A turned out to be about the same (ca. 0.60). The latter evidence gives grounds to suggest that the rupture of polymers under drawing is associated with reaching a high degree of amorphous region size dispersion. In those regions which are considerably below the average size there probably will appear local overstress and molecular ruptures because the relative deformation of these regions is much larger than that of the adjacent regions in the cross section of the sample.     

Monomer distributions and number-average sequence lengths in hydrogenated butadiene–styrene copolymers from carbon-13 nuclear magnetic resonance

Monomer distributions and number-average sequence lengths are determined from ¹³C nuclear magnetic resonance (NMR) data for 1,4- and 1,2-butadiene additions and styrene additions in a series of four hydrogenated butadiene–styrene copolymers. The monomer distribution is expressed in terms of the six unique days from which it is possible to calculate the number-average sequence length of each monomer type. Carbon-13 NMR spectral assignments are given and the techniques for making the assignments are discussed. The method presented could, in principle, be applied to any copolymer or terpolymer. Limitations that are encountered in the analysis of hydrogenated butadiene–styrene copolymers high in 1,2 additions are discussed.     

Stereochemical regulation of polypropylenes prepared with various Ziegler-Natta catalysts

The 220-MHz proton magnetic resonance and infrared spectra of stereoregular polypropylenes polymerized with a number of Ziegler-Natta catalysts and isotactic polymers of low molecular weight obtained by thermal degradation of a highly isotactic polypropylene were measured in an attempt to obtain some information on the local regularity. The fraction of thermally degraded polymer soluble in diethyl ether shows stereorandomness (tactie sequence length is quite short), and the portion soluble in n-pentane has stereoblock character. The results so obtained provide strong evidence that racemic dyads of whole polymer consist of two models of racemic dyad isolated and racemic dyads in groups. The polymers prepared with vanadium catalyst systems show stereorandom character and these polymers have \documentclass{article}\pagestyle{empty}\begin{document}$\hbox{-\hskip-1pt-}\hskip-4pt({\rm CH}_2 \rlap{--} )$\end{document} groups formed by two propylene units in a tail-to-tail linkage. Syndiotactic polypropylene has head-to-head and tail-to-tail arrangements of two propylene units and this is the origin of randomness of syndiotacticity.     

Unusual influences of temperature and medium on the tacticity of radically polymerised poly(n-vinyl imidazole)

The tacticities of radical poly(N-vinyl imidazole)s prepared in several organic solvents, in water and at various values of pH and temperature have been investigated by ¹H and ¹³C-NMR. It seems most likely that high temperatures favour racemic placements over meso placements but that, at low pH, meso placements are favoured. Thus the changes in polymerisation rate with pH noted by others for this system are accompanied by changes in the stereochemistry of the resulting polymer.     

NMR studies of chlorinated poly(vinyl chloride) and polybutadiene

Molecular structures of chlorinated poly(vinyl chloride) and polybutadiene have been studied by high resolution NMR. The spectra of the chlorinated polymers give broad signals. New peaks appear in the lower fields of the ? CH₂? and ? CHCl? groups with increasing chlorine content. The chlorination of poly(vinyl chloride) takes place predominantly on ? CH₂? rather than on ? CHCl? , e.g., a 70% chlorinated polymer has about 10 mole-% of ? CCl₂? groups. Polybutadiene reacts first with chlorine by addition to give a head-to-head poly(vinyl chloride), and then the substitution of the hydrogen atom takes place. Chlorinated polybutadiene with 70% Cl has about 18 mole-% of ? CCl₂? . The multiplets characteristic of spin-spin couplings in the spectrum of the original poly(vinyl chloride) are still observed in that of the highly chlorinated product. This fact shows that a considerable number of poly(vinyl chloride) sequences of certain lengths persist in the highly chlorinated polymer.     

Electronic conduction and polarization in polyphthalocyanines

The polymeric hydrogen form of phthalocyanine is found to be more conductive than the metallic derivatives, in contrast to the behavior of the monomers. In addition, the polymers were found to be much more conductive than the corresponding monomers with the resistivity of the polymers ranging from 7 ohm-cm to about 3 × 10⁶ ohm-cm. The polymers were found to have moderately high dielectric constants ranging from 16 to 1300 at room temperature, depending upon the applied pressure. Based on the dependences of the conductivity and permittivity upon the electric field strength, the average molecular length of the conductive paths within the polymer molecule has been estimated to be 100–1000 Å. In view of these estimated lengths, together with the exponential dependence of the permittivity and conductivity upon the pressure and temperature, the dispersion of the dielectric constants in the range of 10–100 KHz, and the chemical architecture of these ribbonlike polymers, the electronic behavior of these polymers is concluded to be consonant with the model of essentially one-dimensional conduction within and along the chains by freed charges. Much as in a number of previously studied highly conjugated polymers, the present polyphthalocyanines are semiconducting and exhibit nomadic polarization, with dielectric constants ranging from 70 to 1300.     

Preparation and structure of chlorinated poly(vinyl flouride)

The low-temperature chlorination of poly(vinyl fluoride) (PVF) proceeds readily in CCl₄ suspension. The rate of chlorination is high initially, but the reaction slows down considerably when the chlorine content of the polymer reaches 40–50%. At long reaction times, polymers containing 62% chlorine (1.88 chlorine atoms per monomer unit) can be obtained. As the degree of chlorination increases, the solubility of PVF in organic solvents increases. Polymer crystallinity and polymer softening point decrease with chlorination. Polymers containing 40% chlorine appear to be completely amorphous by x-ray analysis. In this respect, PVF differs from poly(vinyl chloride) (PVC), where chlorination increases the softening point, and it resembles polyethylene where both crystallinity and softening point decrease with chlorination. ¹⁹F NMR analysis of the polymers indicates that up to a degree of chlorination of 1 chlorine atom per monomer unit, 50% of the substitution occurs on the α-carbon of the PVF molecule. This result is very different from the predominant β-chlorination of PVC reported by several workers. The chemical selectivity observed in the chlorination of PVF is in quantitative agreement with the results of free-radical chlorination of organic compounds and can be rationalized by considering the size and the electronic properties of the fluorine atom. The results of ¹H NMR analysis are also in support of a polymer structure where the chlorine atoms are distributed between α- and β-carbons. Based on a comparison of the ¹⁹F and ¹H NMR data, the average composition of chlorinated PVF at the 1 chlorine atom per monomer unit level can be represented as: C₂₀₀H₂₀₀F₁₀₀Cl₁₀₀ = (CH₂)₆₃(CHF)₅₀(CHCl)₂₄(CClF)₅₀-(CCl₂)₁₃.     

Conformational analysis of poly(2-hydroxyethyl methacrylate)

The 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.     

Synthesis and thermal behaviours of side-chain liquid-crystalline poly[N-(4-methoxyazobenzene-4′-oxyalkyl)ethyleneimine]

A series of side-chain liquid-crystalline polymers, poly[N-(4-methoxyazobenzene- 4′-oxyalkyl)ethyleneimine](PEnZO), has been synthesised in which the number of methylene units in spacers varies from two to six. The structures of the synthesised monomers and polymers were confirmed by infrared (IR) and ¹H nuclear magnetic resonance (¹H NMR) spectroscopy. The thermal properties of these polymers have been investigated using differential scanning calorimetry (DSC), polarising optical macroscopic (POM) X-ray diffraction and thermogravimetric analysis (TGA). The test results indicated that the obtained polymers exhibited thermotropic liquid-crystalline mesomorphism of nematic type with schlieren textures. It was observed that the thermal behaviours of the polymers were strongly dependent on the degree of substitution and the length of spacers. Polymers containing less than 57% of mesogenic groups did not exhibit mesogenic phase and resembled amorphous polymer. A more pronounced odd–even effect in the melting points and their enthalpy changes was observed on increasing the spacer length in which the odd members displayed lower values, which were also slightly dependent on the substitution degree of polymers. The mesomorphic temperature ranges of odd members were wider than those of even members. The decomposition temperatures of copolymers were near 230°C.     

Characterization of novel modified amorphous poly (ether sulphone)s

New high-temperature amorphous polymers with chlorine, amine, and maleimide chain-ends have been synthesized by nucleophilic polycondensation and fully characterized by ¹³C-NMR, ¹H-NMR, and potentiometric titration. From chain-end determination, number average molecular masses were calculated. It was confirmed that transetherification during the synthesis led to a randomized polymer of the monomer residues. For nominally amine-ended polymers obtained by addition of m-aminophenol at the end of the synthesis, a small amount of hydroxyl chain-ends was observed. This is ascribed also to transetherification. Complete reaction of the amine chain-ends with maleic anhydride was demonstrated. Reaction of hydroxyl chain-ends with acetic anhydride was also observed. The thermal stability of these different polymers was investigated; lower thermal stability was observed for amine and maleimide-ended polymers. By two different methods, a T_g around 270d°C was determined for these novel amorphous aromatic polymers. © 1994 John Wiley & Sons, Inc.     

Stereochemistry in cationic polymerization of alkenyl ethers. II. Formation of poly(ß-substituted vinyl ether)s with erythro-meso structures

The formation of polymers with erythro-meso structures, which could not be obtained from propenyl ethers with BF₃O(C₂H₅)₂, was studied by ¹³C-NMR spectroscopy on poly(ß-substituted vinyl ether)s obtained under a variety of conditions of polymerization. It was established that poly(cis-ethyl propenyl ether) obtained with Al₂(SO₄)₃–H₂SO₄ complex in toluene at 0°C was a highly stereoregular polymer with an erythro-meso structure. Cis-2-chlorovinyl ethyl ether and cis-methyl and ethyl butenyl ethers also yielded polymers with erythro-meso structures under the same conditions. In addition, with BF₃O(C₂H₅)₂ at ?78°C these three cis isomers produced amorphous polymers with threo-meso, racemic, and, in a few cases, erythro-meso structures, whereas cis-ethyl propenyl ether produced polymers with only threo-meso and racemic structures by the same catalyst. On the other hand, all trans isomers produced stereoregular polymers with threo-meso structures with BF₃O(C₂H₅)₂ at ?78°C, regardless of their ß-substituents; no erythro-meso structures were found in the polymers obtained.