Carbon‐13 nuclear magnetic resonance spectroscopy of polystyrene and poly‐α‐methylstyrene

Abstract

The carbon-13 high resolution nuclear magnetic resonance spectra with proton spin decoupling of polystyrene and poly-a-methylstyrene were measured. The spectra clearly split corresponding to triad, tetrad, and partially pentad placements, and these peaks were tentatively assigned. By comparison of the triad tacticity of poly-a-methylstyrene deteLmined from carbon-I3 spectra with that from proton spectra, it was confirmed that the nuclear OVERHAUSER effect due to proton decoupling has little effect on the relative intensities of the peaks assigned to chemically equivalent carbons except for stereochemical configuration.The stereospecific polymerization mechanisms are discussed on the basis of the microtacticity determined from carbon-13 spectra. As a result, it is concluded that poly-a-methylstyrene polymerized with cationic and anionic catalysts can be described by BERNOULLIAN statistics, while the atactic polystyrene does not obey BERNOULLIAK but first or second order MARKOVIAN statistics. ZUSAMMENFASSUNG:Hochaufgeloste 13C-NMR-Spektren von Polystyrol und Poly-a-methylstyrol werden unter Bedingnng der 1H-Spinentkopplung aufgenommen. Die Spektren zeigen klare Aufspaltungen, die den Triaden, Tetraden und teilweise den Pentaden entsprechen ; die verschiedenen Signale werden versuchsweise zugeordnet. Durch Vergleich der Triaden-Taktizitaten von Poly-a-methylstyrol auf Grund der 13C-NMR-Spektren mit den Protonea-Spektren konnte gezeigt werden, daB der OVERHAUSER-Effekt die relativen Intensitaten der Signale von chemisch aquivalenten Kohlenstoffatomen nur wenig beeinfluBt, mit Ausnahme bei der stereocbemischen Konfiguration.Die Mechanismen der stereospezifischen Polymerisation werden auf der Grundlage der durch W-NMR bestimmten Mikrotaktizitat dlskutiert. Die kationische und die anionische Polymerisation von a-Methylstyrol kann durch die BERNOULLI-Statistik beschrieben werden, wogegen das ataktische Polystyrol der MARKOV-Statistik gehorcht.

A Carbon-13 NMR Study of Molecular Motion of Polystyrene in Solution

“…15 The solvent peaks are greatly attenuated and pratically negligible compared to the polymer peaks, because the repetition times of the 90° pulses are much less than the spin-lattice relaxation times of the carbons in the solvent and because of splitting due to 13 C-2 D spin coupling.…”

“…peratures, 10 • 17 while T1 becomes longer and the NOE becomes larger with increasing temperatures, when 13 C-1 H dipole-dipole interaction is predominant. 10 • 17 If the dipolar mechanism is predominant and the extreme-narrowing condition is fulfilled, the NOE has a maximum value of 2.988.…”

“…Proton magnetic relaxation times .are difficult to analyze quantitatively in terms of molecular motion because of the difficulty of separating intramolecular and intermolecular contributions. The latter contribution to 13 C spin-lattice relaxation is generally negligible. 10 Furthermore, carbon-13 spin-lattice relaxation times of protonated carbons in large molecules may be analyzed simply if relaxation is overwhelmingly dominated by dipolar interactions with directly bonded protons.…”

“…It was proposed that rotation of the phenyl ring may be hindered by the a and /3 main-chain hydrogens, and that the transition at about 50°C may be due to a sudden change in the mobility of the phenyl ring. 9 However, the molecular motion has not been heretofore studied quantitatively; we have used 13 C NMR to elucidate the molecular process occurring in this transition.…”

“…IO ppm) and of splitting of peaks due to spin-spin coupling. Proton-decoupled natural abundance 13 C NMR spectra are easier to analyze because of a wide range of chemical shifts (ca. 250 ppm) and the absence of complex spin-spin coupling.…”