Syndiotactic polypropylene

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.