Stereoregularity of polyacrylonitrile determined by NMR

The NMR spectrum of polyacrylonitrile is interpreted and the tacticity of three polyacrylonitrile samples prepared at different temperatures in the range from ?78 to 120°C. determined. The ratio between isotactic and syndiotactic units is about 50:50 for all the three samples examined.     

Influence of the polymerization temperature on the stereoregularity of radical poly(phenyl methacrylate)

The dependence of the tacticity of radical poly(phenyl methacrylate) upon polymerization temperature has been examined by NMR spectroscopy. Bernouillian statistics were confirmed for polymerization temperatures between 30 and 80°, but slight deviation was found for elevated temperatures (100°). The calculated values of differential enthalpies and entropies of activation for isotactic and syndiotactic propagations revealed that poly(methyl methacrylate) radicals grow more easily in syndiotactic placement than poly(phenyl methacrylate) radicals.     

NMR spectrum of poly(vinyl chloride)

The 100-MHz proton NMR spectra of commercial and laboratory-prepared poly(vinyl chloride) (PVC) have been measured in various solvents at high temperature (80–150°C). Tacticity in PVC was determined by the analysis of the β-proton spectrum. The spectrum was calculated assuming that the PVC chain consists of tetrad sequences of monomer units and that their distribution in the chain is described by a simple Bernoulli-sequence statistics with a P_m (the probability of isotactic placement) of 0.45 for commercial PVC polymerized at 50°C. Tacticity calibration curves based on measurements made for the polymer in pentachloroethane and β-dichlorobenzene were established, and they provide a simple method for the measurement of tacticity in PVC directly from the observed spectra. Excluding samples prepared in butyraldehyde solution, the formation of syndiotactic structures in PVC (prepared by free-radical polymerization) was found to be favored by lowering the polymerization temperature. This preference is due to an increase in the activation enthalpy of 510 cal/mole which is required for forming an isotactic placement in the chain during the propagation step.     

Zur Taktizität anionisch hergestellter Poly-α-methylstyrole

α-Methylstyrene has been polymerized with lithium, sodium and potassium naphthalene in tetrahydropyran, tetrahydrofuran and 1.2-dimethoxyethane at ?30°. The tacticity of the polymers was determined by NMR. spectroscopy. Within the limits of reliability no influence of the cation on the tacticity was observed. With growing polarity of the solvent a slight but definite trend towards higher stereospecifity was noticed. The distribution of isotactic and syndiotactic links is purely statistical. The considerable dependence of tacticity upon the polymerization mechanism is ascribed to differences in the steric configuration of carbanions and carbonium ions due to different hybridization.     

Poly(alkyl α-chloroacrylates). V. Preparation and properties of methyl,ethyl, and isopropyl polymers of varied tacticity

Polymers of different tacticities, from highly isotactic to highly syndiotactic, were prepared from methyl, ethyl, and isopropyl α-chloroacrylates. These polymers were characterized for tacticity by infrared spectroscopy and 100 and 300 MHz nuclear magnetic resonance (NMR) and for thermal properties by differential scanning calorimetry (DSC). After corrections were made for molecular weight effects, the observed glass temperature-tacticity results were analyzed, and it was determined that the maximum differences in glass temperatures of the purely isotactic compared to the purely syndiotactic polymers should be 92°C for the methyl ester, 86°C for the ethyl ester, and 68°C for the isopropyl ester polymers. The highly isotactic polymers of all three esters were crystalline. Possible polymerization reaction mechanisms are discussed on the basis of the triad and tetrad tacticity values observed and the calculated propagation statistics.     

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.     

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.     

Stereoregularity of poly(isopropyl acrylate) and its racemization during hydrolysis

The diad tacticity of poly(isopropyl acrylate) was measured from the β-proton absorptions of poly(isopropyl acrylate-α,β-d₂) obtained with a 100 MHz NMR spectrometer, and temperature dependence of the tacticity of the polymers obtained by radical polymerization was determined. Enthalpy and entropy differences between isotactic and syndiotactic addition for poly(isopropyl acrylate) were calculated to give the following values: Δ(ΔS) = 0.7 eu; Δ(ΔH) = 0.51 kcal/mole. In the hydrolysis of poly(isopropyl acrylate-α,β-d₂), it was found that the rate of hydrolysis of poly(isopropyl acrylate) was dependent on the molecular weight rather than on the tacticity. As for the rate of racemization during hydrolysis, the rate for syndiotactic polymer was much faster than that for the isotactic polymer. The exchange reaction of deuterium at α-position with hydrogen occurred in all the polymers during hydrolysis reaction.     

Influence of Tacticity on Some Reactions of PVC

The influence of the tacticity on PVC reactivity is discussed on the basis of preliminary results obtained in ionic dehydrohalo, genation and chlorination reactions. From the reaction of an atactic PVC and a 70% syndiotactic PVC with LiCl in dimethyl-formamide and hexamethylphosphortriamide as solvents, it follows that both the reaction rate and the polyene sequence distribution depend markedly on the syndiotacticity content. This effect is accounted for by the fact that the isotactic parts are preferred in dimethylformamide and the syndiotactic ones in hexamethylphosphoramide. On the other hand, the chlorination of PVC appears to be easier through the heterotactic parts than through the syndiotactic sequences as shown by ¹³C-NMR.     

Stereoregular poly(methacrylic acids)

A convenient method is described for the preparation of isotactic and syndiotactic poly(trimethylsilyl methacrylates) by using the monomer trimethyl silyl methacrylate and butyllithium initiation in toluene and tetrahydrofuran, respectively. The structure of these polymers enables complete hydrolysis to the corresponding poly(methacrylic acids), which were characterized with respect to tacticity and molecular weight. The asymmetric induction in toluene produced 89% isotactic polymer, while that in tetrahydrofuran gave polymer <90% syndiotactic and heterotactic in terms of triads. A method of fractionation of the polyelectrolytes by gel-permeation chromatography on a preparative scale was shown to be applicable.     

Studies on tacticity of polyacrylonitrile. I. High-resolution nuclear magnetic resonance spectra of polyacrylonitrile

The nuclear magnetic resonance spectra of polyacrylonitriles prepared under various polymerization conditions were measured in NaCNS–D₂O solution. The methylenic proton resonance could be separated into two triplets: one is due to the protons of the syndiotactic methylene groups and the other is due to the isotactic ones. The tacticity of polyacrylonitrile could be determined by the ratio of the integrated intensities of the two triplets. Polyacrylonitriles prepared with radical catalysts are approximately 75% syndiotactic, and polyacrylonitriles prepared by the anionic polymerization at low temperature, γ-irradiation in the solid state, Mg molecular beam, and γ-irradiation on the urea canal complex are more isotactic but have no remarkable stereoregularity.     

Vibrational assignments for the CCL stretching region of the Raman spectrum of poly(vinyl chloride)

It is shown that, by assuming that each of the peaks resolved in the CCl stretching region of the Raman spectra of a series of poly(vinyl chloride) gels can be associated with a definite triad structure, most of the peaks can he assigned to syndiotactic, isotactic, or heterotactic triad configurations without reference to the spectra of model compounds or to vibrational calculations. The results are in general agreement with most previous assignments except for the assignment of the peak at 646 cm^–1. Previously this was assigned to syndiotactic material, but it is here assigned to a structure consisting of consecutive syndiotactic, isotactic, and syndiotactic placements in a conformation not very distorted from the planar zigzag conformation.     

Molecular motions of tactic poly(2-hydroxyethyl methacrylate) in solutions studied by 13C-NMR relaxation measurement

The spin-lattice relaxation time and the nuclear Overhauser enhancement were measured using Bruker AM 300 spectrometer operating at 75.5 MHz for ¹³C to investigate the molecular motional characteristics and its tacticity effect for tactic poly(2-hydroxyethyl methacrylate) (PHEMA) as a function of temperature in dimethyl sulfoxide and methanol solvents. The observed relaxation data have been analyzed for both backbone motion and methyl internal rotation according to the log-χ² distribution model and the diamond-lattice model. The correlation times thus obtained for the molecular motions show that isotactic PHEMA is more flexible than syndiotactic counterpart. The syndiotactic PHEMA seems to have intramolecular hydrogen bonding which restricts the motion of C-2 carbon at temperatures below 35°C, whereas the isotactic one indicated no hydrogen bonding at all temperatures examined in this study. The methyl group of isotactic PHEMA shows a greater degree of freedom for the internal rotation than that of syndiotactic one. From the temperature dependence of correlation times, the activation energies for both backbone segmental motion and methyl internal rotation are obtained. The activation energies, 20 kJ/mol for backbone motion and 19 kJ/mol for methyl internal rotation, for isotactic PHEMA are substantially lower than the corresponding activation energies of 30 and 32 kJ/mol obtained for syndiotactic one. An examination of these energies indicates that methyl side group and backbone motions in tactic PHEMA are linked together.     

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

The 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.     

Stereospecific polymerization of some methyl aryloxymethacrylates

By exposure to ultraviolet and gamma radiation and by the usual methods of thermal polymerization, the stereospecific polymerization of methyl aryloxymethacrylates was carried out at different temperatures in several solvents. Triad tacticity values of the polymers obtained by free-radical and ionic routes were computed from nuclear magnetic resonance spectral data. Results of this investigation support our earlier observation

1 See: K. Saunders, T. Balakrishnan, R. W. Lenz, and K. Hatada, Macromolecules, 12 , 392 (1979).

that, under the conditions used, heterotactic content is the maximum in most of these polymers, thereby justifying the high steric effect of the bulky and polar aryloxy side chain which offers equal isotactic and syndiotactic placements.     

Influence of tacticity on orientation and relaxation in uniaxially stretched polymethylmethacrylates

Orientation and relaxation behavior in uniaxially stretched stereoregular polymethyl-methacrylate (PMMA) was investigated. When compared at a reference temperature T = T_g + constant, isotactic PMMA orients more readily and relaxes slightly faster than the conventional or syndiotactic polymers. Orientation relaxation of the different PMMAs can be reduced to a unique master curve, whatever the tacticity, when the results are compared at same monomeric friction coefficient. © 1996 John Wiley & Sons, Inc.     

Functional Monomers and Polymers. XLV. Activation Parameters for Polymerization of N-[btilde]-Methacryloyloxyethyl Type Monomers Containing Nucleic Acid Bases

In order to determine the effect of functional structure of ester groups on the stereoregularity of polymers, polymerization of N-[btilde]-methacryloyloxyethyl type monomers containing nucleic acid bases was made by using a free-radical initiator. From the NMR spectrometric determination of the stereoregularity of the polymers obtained, it was found that for polymerization of the monomer with adenine as the side group in dimethyl sulfoxide solution, syndiotactic placement appears to be favored by the additional enthalpy of activation required for isotactic placement.     

Structure of chlorinated poly(vinyl chloride). II. Distribution of Cl atoms in the polymer chain

The distribution of chlorine atoms in the chain of solution-chlorinated PVC (CPVC) composed of ? CH₂? CHCl? (1), ? CHCl? CHCl? (2), and ? CCl₂? CHCl? (3) monomeric units is described by first-order Markoffian statistics based on the assumption that the chlorination mechanism is independent of tacticity. In this case, all the statistical parameters can be obtained from experimentally determined concentrations of CH₂, CHCl and CCl₂ units. Populations of the sequences (11) and (111) calculated from the statistical parameters agree with experimental values determined from infrared and 220 MHz NMR spectra. From infrared spectra, suspension-chlorinated CPVC contains blocks of intact PVC.     

Etude de la chloration du polychlorure de vinyle en fonction de sa tacticite

Polyvinyl chloride samples of various tacticity have been prepared and photochlorinated in CCl₄ for 4 hr. The chlorination degree correlates directly with the degree of syndiotacticity. Initial PVC with syndiotacticity about 56 per cent, which can be regarded as alternate sequences of syndiotactic and isotactic diads, is quickly chlorinated. The mechanism of chlorine attack on the macromolecula is directly connected with the stereoregularity.     

Liquid-crystalline stereoregular polyketone prepared from a mesogenic vinylarene and carbon monoxide

Isotactic and syndiotactic polyketones were synthesized by the alternating copolymerizations of a mesogenic vinylarene, 4-cyano-4′-[8-(4-vinylphenyl)octyl-1-oxy]biphenyl, with carbon monoxide, and this revealed the influence of backbone tacticity on their thermal properties. The isotactic polyketone was an amorphous polymer that did not show any mesophase, whereas the syndiotactic polyketone exhibited liquid crystallinity. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 3556–3563, 2003