Nouvelle méthode de détermination des taux de réativité des copolymères par résonance magnétique nucléaire: Application aux poly(méthacrylate de méthyle–chlorure de vinylidène)
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Abstract Constant‐composition copolymers of methyl methacrylate and vinylidene chloride produced by radical copolymerization are studied by 1 H‐NMR at 60 and 250 MHz. The different methods of the literature for the derivation of reactivity ratios from either the copolymer composition or the sequence average lengths, or even the diad distribution, are applied but lead to rather dispersed results. A new graphical method is proposed, based on the use of peculiar values of the triad distribution functions. It allows us to detect a penultimate effect for the vinylidene chloride‐rich region. In the same range, a change in tacticity of the diads and triads on the methylmethacrylate sequences, as compared with homopolymers, is observed; it suggests that the anomaly is caused by the competition of the depropagation reaction.Keywords:
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Reactivity
Diad tacticity of methyl methacrylate (MMA) sequences in the copolymers of MMA and various acrylates and their crosslinked polymers was characterized by pyrolysis–gas chromatography (Py-GC) based on the relative peak intensities of the diastereomeric MMA tetramers in the pyrograms. The diad tacticity in the copolymers synthesized at a given temperature proved to be almost consistent with that of corresponding MMA homopolymers (PMMA) prepared under the same conditions. Furthermore, the diad tacticity of PMMAs was also consistent with that of the corresponding crosslinked polymers. These results suggest that the tacticity of MMA sequences in the polymer chains would be dominated by polymerization temperature, independent of the copolymerization and crosslinking. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 78: 2140–2144, 2000
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Abstract Propylene is the only monomer which can be polymerized both isospecifically and syndiospecifically with the same family of catalysts, i.e., organometallic compounds of the Ziegler–Natta type. Whereas isotactic poly(propylene) can be manufactured by reasonable polymerization conditions to highly valued thermoplastics, the corresponding syndiotactic polypropylene has remained a laboratory product. However, theoretical considerations show that syndiotactic polypropylene may possess some ultimate properties which surpass those of isotactic polypropylene. Propylene was thus polymerized with various catalyst systems and the resulting polymers characterized by carbon‐13 nuclear magnetic resonance, infrared spectroscopy, X‐ray diffraction, gel permeation chromatography, differential scanning calorimetry, and stress/strain measurements [1,2]. Several polymers were fractionated. The combined evidence points toward stereoblocks in syndiotactic polypropylenes [1]. The highest syndiotacticities achieved are, however, still too low to permit the ultradrawing to highly oriented polymers, which would allow approaching the ultimate properties. This result is probably due to a stereostatistic peculiar to the catalyst system, i.e., a symmetric Markov statistic of the first order [3]. This statistic predicts the experimentally found linear dependence of triad fractions on diad fractions [4], a characteristic restricted diad range, and an unusual dependence of the heterotacticity index on the diad contents. The same formalism also describes the features of the so‐called heterotactic polymerizations.
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Abstract Samples of poly[( m ‐phenylene terephthalate)‐ co ‐oxybenzoate] with different compositions were analyzed by 13 C NMR spectroscopy. Diad and triad signals could be identified and assigned. Furthermore, the signals of acetoxy, carboxy and hydroxy end‐groups were assigned. The analysis of NMR substituent effects confirms the assignment. Comparison of the calculated statistical with the experimentally determined diad and triad contents shows a slight tendency for the formation of oxybenzoate blocks in the polymer chain.
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Abstract The DMSO solution copolymerization of acrylonitrile and vinylidene chloride can be adequately described by the terminal‐unit copolymerization model. The sequence distribution measured by 13 C‐NMR is in excellent agreement with the theoretically calculated diad, triad, and pentad distributions. The variation of glass transition temperatures with copolymer composition is explained by sequence distribution.
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Polyacrylonitrile
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The extent of colour development in an aqueous poly(vinyl alcohol) (PVA)–iodine complex solution increased with increasing syndiotacticity up to a certain syndiotactic content, then decreased with increasing syndiotacticity. The complex formation depended on the content of the syndiotactic pentad sequence length in the PVA molecules. The most suitable content of the syndiotactic pentad for forming complexes was around 12% in the case of reaction at 30°C. A 0·5% solution of syndiotactic PVA (S-PVA) with an syndiotactic diad (s-(diad)) content of 63·8% changed into a sol, whereas that with an s-(diad) content of 58% did not change after standing for 2h at 30°C, and the colour due to the iodine complex in the latter was deeper than in the former. The gel prepared from the 0·5% solution of S-PVA with an s-(diad) content of 63·8% was hardly coloured. In the solutions containing two kinds of s-PVA having different syndiotacticities, additivity for the extent of colour development did not hold, and the extent was lower than the sum of that of the components. The decrease in the extent of colour development at higher syndiotacticity was due to the formation of microgels. The absorption maximum shifted to a longer wavelength with increasing syndiotacticity. The length of polyiodine increased with increasing syndiotacticity. © of SCI.
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Abstract 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.
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Poly(N-isopropylacrylamide)
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Abstract Vinyl trifluoroacetate was polymerized by free radicals in various kinds of solvents at temperatures between +60 and ‐40°C. The syndiotactic and isotactic diad contents of the polymer were determined via the triad content, which was obtained by their NMR‐spectra. The differences of the activation enthalpies and those of the activation entropies between isotactic and syndiotactic arrangements were calculated from the temperature dependence of the ratio of isotactic to syndiotactic diads formed in the different solvents. A compensation effect exists between the two differences. The syndiotacticities of the polymers synthesized in alkanes in which the polymers were insoluble were higher than those in esters and ketones in which the polymers were soluble. The syndiotactic diad contents of the polymer synthesized in alkanes and ketones at −40°C amount to 60∼63% and 52∼56% respectively.
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