Thermal and dynamic mechanical relaxation behavior of stereoregular poly(2-hydroxyethyl methacrylate)

Thermal, dynamic mechanical, and dielectric relaxation techniques were used to determine the relaxation behavior of isotactic and syndiotactic poly(2-hydroxyethyl methacrylate) (pHEMA). Activation energies E_a were determined for the dielectric γ relaxation and compared with those of poly(2-methoxyethyl methacrylate) (pMEMA) to determine the influence of hydrogen bonding on side-chain relaxation processes. No difference in E_a was observed between syndiotactic pHEMA and atactic (predominantly syndiotactic) pMEMA. Isotactic pHEMA, however, had E_a + 1 kcal/mole higher than that of syndiotactic pHEMA. This was attributed to improved side-chain packing in the isotactic polymer.     

The dependence of the glass transition temperature of poly(N-vinylcarbazole) upon steric microstructure and molecular weight

Poly(N-vinylcarbazole) (PVK) samples were found to exhibit up to three glass transition temperatures T_g, corresponding to the whole chain and the syndiotactic and isotactic stereoblocks. An increasing tendency to multiple T_gs, and hence to phase separation, was observed with increasing isotacticity. Limiting values at infinite molecular weight for syndiotactic and isotactic PVK were obtained from correlations of the T_gs corresponding to the syndiotactic and isotactic stereoblocks with their respective average stereoblock lengths derived from ¹³C NMR measurements. They were found to be 549 and 399 K, respectively. The conventional T_g for PVK was found to exhibit the following dependence upon the syndiotactic dyad mole fraction X_s: The molecular weight dependences of the conventional T_gs for several fractionated PVK samples obeyed a Fox–Flory-type relation with values of ?dT_g/d(1/M) varying between 7.6 × 10³ for isotactic PVK and 2.7 × 10⁵ for Luvican M 170.     

Effect of tacticity on conformation of p-tert-butylphenol acetaldehyde resins as studied by molecular simulation

p-tert-Butylphenol acetaldehyde resins can have isotactic, syndiotactic, and atactic sequences. Structural characteristics of the p-tert-butylphenol acetaldehyde resin with different tacticities were studied using molecular mechanics and molecular dynamics. Trimer–decamer isotactic and syndiotactic resins and 12 stereoisomers of a hexamer were calculated. In the p-tert-butylphenol acetaldehyde resin, the hydroxyl groups cluster in the center of the molecule through intramolecular hydrogen bonding and the tert-butyl groups are extended out. It has been found that the energy-minimized structures of the isotactic resin are more stable than those of the syndiotactic resin by 7–17 kcal/mol. From the results of molecular dynamics at 303, 373, 474, and 573 K for 300 ps, the isotactic resin was also found to be more stable than the syndiotactic resin. For atactic resins, the closer to isotactic their structures are, the more stable they are. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 1355–1361, 1998     

Syndiotactic and isotactic poly(tert-butylethylene oxide)

The rate of polymerization of t-BuEO by t-BuOK in DMSO is about one-tenth that of propylene oxide. The slow rate of propagation was accompanied by considerable chain transfer. In the absence of solvent, the polymer obtained was crystalline, different from the isotactic form and therefore must be syndiotactic. The NMR spectra indicate the isotactic polymer exists in solution preferentially in the skew₁ form, while syndiotactic is about 60% skew₁, 40% skew₂. Amorphous polymer accompanying isotactic exists about 50% in the trans conformer, by NMR data.     

Stereospecific polymerization of 9-fluorenyl methacrylate: Tacticity effects on the thermal and photophysical properties of the polymers

Poly(9-fluoreneyl methacrylate) was obtained through anionic polymerization with t-BuLi and t-BuMgBr and through radical polymerization with α,α′-azobisisobutyronitrile. Anionic polymerization with t-BuLi in tetrahydrofuran and radical polymerization afforded syndiotactic polymers (rr ∼ 90%), whereas anionic polymerization with Li and Mg initiators in toluene and CH₂Cl₂ led to isotactic polymers. The thermal and photophysical properties of the polymers were examined. A syndiotactic polymer tended to show higher glass transition and decomposition temperatures than an isotactic polymer. However, polymers with different tacticities were not likely to assume specific, distinctive conformations such as a helix or a π-stacked conformation in solution. An isotactic polymer showed stronger interactions in a CH₂Cl₂ solution with 2,4,7-trinitro-9-fluorenylidenemalononitrile, an electron-acceptor molecule, than a syndiotactic polymer. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 4656–4665, 2004     

Stereoregularity of poly(methyl acrylate)

The stereoregularity of poly(methyl acrylate) and poly(methyl acrylate-αd) was determined from the NMR spectra. A method of quantitative determination of stereoregularity of poly(methyl acrylate) proposed in this paper is based on the fact that in the 100 Mc./sec. NMR spectrum the absorption peaks due to methylene protons in syndiotactic configurations overlap absorptions due to only one of two methylene protons in isotactic configurations. The stereostructure of poly(methy1 acrylates) polymerized with anionic catalysts such as Grignard reagents, n-butyllithium, and LiAlH₄ is generally richer in isotactic diads than in syndiotactic diads. For example, poly(methyl acrylate) polymerized with phenylmagnesium bromide as catalyst at ?20°C. consists of 99% isotactic and 1% syndiotactic diads. In radical polymerization, the isotacticity of poly(methyl acrylate) is independent of polymerization temperature. Poly(methyl acrylates) polymerized with a Ziegler-Natta catalyst consisting of Al(C₂H₅)₂Cl and VCl₄ have configurations similar to those polymerized by radical initiators. The stereoregularity of poly(methyl acrylate-α-d) resembled that of poly(methyl acrylate) polymerized under the same conditions.     

Magnetic circular dichroism studies of aromatic polymers

The magnetic circular dichroism (MCD) spectra of syndiotactic and isotactic polymers which contain aromatic chromophores have been found to be sensitive to configurational and conformational differences. For isotactic polymers it was determined that as the aromatic ring moved farther from the main chain the ration of B terms of the polymers to those of their model compounds reached a minimum but increased significantly when the aromatic ring was separated from the main chain by four atoms. This enhancement of MCD is believed to be caused by the alignment of the more flexible side chains which would allow the interaction of the aromatic rings with neighboring groups and could result in a favorable mixing of the ester electronic transition with the aromatic ¹A_1g?¹B_2u transition. This effect was not felt to any great extent by the syndiotactic polymers because the necessary nearest-neighbor interaction was sterically unfavorable. The ratio of the B terms of isotactic poly(phenyl methacrylate) to its model compound decreased as the polymer coil expanded, whereas it increased for the syndiotactic polymer. This effect reflects the different changes that the side chain interaction and orientations undergo in these polymers during coil expansion. The MCD ratios for iso- and syndiotactic poly(phenylethyl methacrylate) were not so sensitive during coil expansion. The ratio of the dipole strengths of the polymers and model compounds paralleled the MCD results, but the ultraviolet (UV) technique was less sensitive than MCD to subtle conformational differences. Poly(benzyl methacrylate) and benzyl pivalate were unsuitable systems for studying the MCD effect because the B terms of these materials approached zero.     

Dilute solution properties of tactic poly(2-hydroxyethyl methacrylate)

Isotactic and syndiotactic poly(2-hydroxyethyl methacrylate) (PHEMA) have been prepared. Intrinsic viscosity–molecular weight relationships were established for the isotactic and syndiotactic PHEMA in N,N-dimethylformamide (DMF) at 25°C by solution viscometry and light scattering. The unperturbed dimensions and interaction parameters were examined in DMF, water, methanol, ethanol, and water–methanol (1:7 by volume) mixture for isotactic PHEMA and in DMF, methanol, and water–methanol (1:7 by volume) mixture for syndiotactic PHEMA using the Stockmayer–Fixman representation. The results suggest that the compact random coil structure for isotactic PHEMA occurs in water solvent and the isotactic PHEMA is more highly extended in polar solvents.     

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.     

Elution behavior of polyethylene and polypropylene standards on carbon sorbents

The elution behavior of linear polyethylene and isotactic, atactic and syndiotactic polypropylene was tested using three different carbon column packings: porous graphite (Hypercarb), porous zirconium oxide covered with carbon (ZirChrom-CARB), and activated carbon TA 95. Several polar solvents with boiling points above 150°C were selected as mobile phases: 2-ethyl-1-hexanol, n-decanol, cyclohexylacetate, hexylacetate, cyclohexanone, ethylene glycol monobutyl ether and one non-polar solvent, n-decane. Polyethylene standards were completely or partially adsorbed in all tested sorbent/solvent systems. Polypropylene standards were partially adsorbed on Hypercarb and carbon TA95, but did not adsorb on ZirChrom-CARB. ZirChrom-CARB retained polyethylene pronouncedly when 2-ethyl-1-hexanol, cyclohexylacetate or hexylacetate were used as mobile phases at temperature 150 or 160°C, while all three basic stereoisomers of polypropylene eluted in size exclusion mode in these sorbent/solvent pairs. This is very different from the system Hypercarb/1-decanol, which separated polypropylene according to its tacticity. The opposite elution behavior of polyethylene and polypropylene in system ZirChrom-CARB/2-ethyl-1-hexanol (polypropylene eluted, polyethylene fully adsorbed) enabled to realize separation of blends of polyethylene and polypropylene. Ethylene/1-hexene copolymers were separated according to their chemical composition using system Hypercarb/2-ethyl-1-hexanol/1,2,4-trichlorobenzene.     

Optoeletronic properties of poly(N‐alkenyl‐carbazole)s driven by polymer stereoregularity

Stereoregular polymers like isotactic poly(N‐butenyl‐carbazole) (i‐PBK), isotactic and syndiotactic poly(N‐pentenyl‐carbazole) (i‐PPK and s‐PPK), and poly(N‐hexenyl‐carbazole) (i‐PHK and s‐PHK) are synthesized using the stereospecific homogeneous “single site” Ziegler‐Natta (Z‐N) catalysts: rac‐dimethylsilylbis(1‐indenyl)zirconium dichloride ( 1 )/methylaluminoxane (MAO) and diphenylmethylidene(cyclopentadienyl)‐(9‐fluorenyl)zirconium dichloride ( 2 )/MAO. Catalytic activity is rationalized by density functional theory (DFT) calculations. All synthesized polymers are fully characterized by NMR, thermal, wide‐angle X‐ray diffraction, and fourier transform infrared spectroscopy analysis. Fluorescence measurements on isotactic and syndiotactic polymer films indicate that all polymers give rise to excimers, both “sandwich‐like” and “partially overlapping.” Excimer formation is essentially driven by the polymer tacticity. Isotactic polymers generate both sandwich‐like and partially overlapping excimers, while syndiotactic polymers give rise especially to partially overlapping ones. A theoretical combined molecular dynamics–time dependent DFT approach is also used to support the experimental results. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018 , 56, 242–251     

Influence of microstructure on the hydrodynamic behavior of poly-α-methylstyrene

The theta temperature of poly-α-methylstyrene in cyclohexane has been found to vary with the stereostructure of the polymer. The observed values range from 305.5°K. for highly syndiotactic material (0.95 syndiotactic diads) to 310.0°K. for anio ically polymerized samples (0.67 syndiotactic diads). Results indicate that the unperturbed dimensions of the polymer increase with increasing isotacticity of the chain, whereas the entropy parameter ψ₁, measured in cyclohexane, decreased as the structure became more isotactic. Measurements of the second virial coefficient in toluene showed an increasing interaction with the solvent as the polymer became more syndiotactic.     

Carbon-13 NMR spin-lattice relaxation times of isotactic and syndiotactic sequences in amorphous polypropylene

Spin-lattice relaxation times (T₁) for methyl, methylene, and methine carbons in an amorphous polypropylene have been measured as a function of temperature from 46 to 138°C. The carbons from isotactic sequences characteristically exhibited the longest T₁'s of those observed. The T₁ differences increased with temperature with the largest difference occuring for methine carbons where a 32% difference was observed. Activation energies were determined for the motional processes affecting T₁'s for isotactic and syndiotactic sequences with essentially no dependence upon configuration noted.     

Structural transitions in propylene-1-hexene copolymer films during low-temperature plasma treatment

The effect of a dc discharge on the structure of film samples of isotactic and syndiotactic polypropylene, as well as propylene-1-hexene copolymers with isotactic and syndiotactic sequences, was studied. It was found that the plasma treatment of homopolymer films did almost not change their phase composition. However, in the propylene copolymers with 1-hexene (0.9 and 1.7 mol % 1-hexene) synthesized in an isospecific system, the γ-to-α phase transition was observed. The structural transition from a maximally ordered crystalline modification of form I, which has a body-centered orthorhombic unit cell, to a maximally disordered form of I, which has a simple orthorhombic unit cell, also occurred in a copolymer (1.1 mol % 1-hexene) synthesized in a syndiospecific system. It was found that an electric field that resulted from the localization of plasma electrons in the surface layers of polymer samples was responsible for the structural transformations observed.     

Solid matrix polymerization and stereoisomeric complexes of poly(methyl methacrylate)

The polymerization of methyl methacrylate within solid matrices of stereoregular poly(methyl methacrylate) has been studied by proton NMR and wide angle X-ray diffraction. The semi-crystalline isotactic (i-) PMMA matrix was synthesized in the laboratory by anionic polymerization initiated by phenylmagnesium bromide, and the syndiotactic (s-) PMMA matrix was synthesized through a Ziegler–Natta reaction. Matrix polymerization of the monomer was initiated through the redox activation of benzoyl peroxide with N,N-dimethyl-p-toluidine. NMR measurements of triad distributions in matrix-polymerized chains suggest that the well-known stereospecific replica polymerization in PMMA (syndiotactic sequences promote isotactic sequences and vice versa) plays only a limited role in the systems studied. Experimental results indicate that chains grown within the i-PMMA or s-PMMA solid matrices have greater degrees of configurational disorder. The greater concentration of atactic triads in these chains could be the result of limited free volume or steric effects during polymerization in a highly condensed environment. X-ray diffraction studies of solution cast blends of isotactic PMMA and PMMA with conventional tacticity reveal some crystallinity with a structure characteristic of the stereocomplex formed by isotactic and syndiotactic PMMA from suitable solvents. Evidence was obtained for the presence of this complex in solidified mixtures of the i-PMMA solid matrix and liquid monomer. This observation is an example of special intermolecular structures that can form under conditions of in situ growth of chains within a pre-polymerized matrix.     

Influence of tacticity on Tβ, Tg,and TLL in poly(methyl methacrylate)s by the method of thermally stimulated current (TSC)

The thermally stimulated-current method (TSC) has been employed to determine the temperatures and intensities of T_β, T_g, and T > T_g for pure isotactic, pure syndiotactic, and five atactic specimens with syndiotactic triad content from 49.5 to 75%; T_g was found to increase linearly with syndiotactic triad content as T_g (°C) = 48.0 + 0.856 (% syn), with R² = 0.970 standard error 5.6°C; T_g for the syndiotactic specimen is 136.6°C measured, 133.6°C calculated. Several atactic specimens exhibit a second glass temperature 15 to 35 K above the regression line ascribed to some pure syndio content, and/or some isotactic–syndiotactic stereocomplexes. All specimens exhibited the liquid–liquid or T_LL transition (relaxation) which increases linearly with 100-% isotactic triad content. Isotactic PMMA shows a T′_LL relaxation 50 K above T_LL. The T_g and T_LL values obtained correlate extremely well with values from differential scanning calorimetry (DSC) determined in a separate study, as well as with most literature data. Intensities of T_g and T_LL by TSC are greatest for isotactic, next for syndiotactic, with a broad, low minimum for atactic materials. The intensity of a β relaxation increases slowly from isotactic to syndiotactic. The T_LL found by TSC compares well with literature values for isotactic PMMA obtained by several methods, and T_LL in the atactic region compares well with literature values for atactic material. The ratio T_LL/T_g ranges from 1.09 to 1.20 with no dependence on tacticity. T_g follows simple Arrhenius behavior with enthalpies of activation about one-half of the values normally calculated from dielectric and mechanical loss. The frequency dependences of T_LL and T′_LL follow a Vogel–WLF relationship with temperature. The origin of T_LL is discussed in terms of the Frenkel hypothesis of segment–segment interaction. Evidence for T_LL and T_LL from a variety of methods indicates that these two temperatures are not artifacts of the TSC method.     

NMR study of poly(vinyl chloride)-β,β-d2

The high-resolution NMR spectra at 60 and 100 Mcps of poly(vinyl chloride)-β,β-d₂ in o-dichlorobenzene, pyridine, and C₂HCl₅ solutions are reported. The use of low molecular weight samples and of {D} spin-decoupling experiments, which yield higher resolution spectra, results in the observation of a number of additional resonances for the α-proton. These can be interpreted in terms of pentad configurational sequences of monomer units. It is found that, whereas the S syndiotactic pentads cannot be resolved, two components of the H heterotactic and all of the possible I isotactic pentads are clearly discernible. From the tacticity values of polymers prepared at +40, 0, and ?40°C, enthalpy and entropy of activation for isotactic and syndiotactic monomer placement are found to be 630 cal/mole and 1.5 eu, respectively.     

Isotactic sequence lengths by ozonation of poly(propylene oxide)

Ozonation followed by lithium aluminum hydride reduction cleaved high molecular weight isotactic poly(propylene oxide) to crystalline polyglycols. From the melting point and molecular weight of the latter, the molar freezing point depression produced by end groups is found to be ca. 18°C./mole, as compared to that estimated for poly(ethylene glycols), K_f = 12°C./mole, from earlier data. By assuming syndiotactic placements (or other irregularities) would produce the same molar depression, the melting point of isotactic poly(propylene oxides) produced by various catalysts has been used to estimate the isotactic sequence lengths.     

Poly(alkyl α-bromoacrylate)s. III. Tacticity analysis from 19 mHz 13C-NMR spectroscopy

Polymer tacticity was determined by 19 mHz ¹³C-NMR spectroscopy for isotactic, atactic, and syndiotactic samples of six poly(alkyl α-bromoacrylate)s. Included in this series were the methyl, ethyl, n-propyl, i-propyl, n-butyl, and n-pentyl esters. Complete assignments for the 10 pentad peaks of the carbonyl carbon resonance were achieved for all but the i-propyl ester while a complete analysis of tetrad tacticity from the backbone methylene carbon resonance was possible for all but the methyl ester. The tetrad values calculated from the experimentally determined pentad contents were found to agree with the experimental tetrad values. As a result of insufficient peak separation of the quaternary carbon resonance, complete pentad assignments were possible in only a few instances. The polymerization reaction mechanisms were discussed in terms of the propagation statistics calculated from the experimentally determined tetrads and pentads. Both the atactic and syndiotactic polymers that were synthesized by free radical techniques displayed Bernoullian or random statistics while the stereochemical statistics of the isotactic polymers synthesized by a modified Grignard complex were more consistent with nonrandom or first-order Markovian statistics.     

Synthesis of polyacrylamide samples of known steric structure

The esterification of atactic, syndiotactic and isotactic samples of poly(acrylic acid) by phenol and p-nitrophenol, carried out at 95° in the presence of POCl₃, led only to atactic poly(phenyl and p-nitrophenyl acrylates) respectively, as shown by ¹H-NMR (250 MHz). These polymers and isotactic poly(phenyl acrylate), prepared by anionic polymerization of phenyl acrylate with n-butyllithium, when reacted with ammonia led to bridged polyacrylimides or to linear atactic or isotactic polyacrylamides according to the reaction conditions. Anionic homopolymerization of p-nitrophenyl acrylate did not occur.