Melting Properties of Syndiotactic Polystyrenes and Effect of Hydrogen on Molecular Weight Distribution

Abstract

AbstractSummary: The melting properties of syndiotactic polystyrenes are significantly affected by the structural molecular properties of the polymers. The most important influences on the melting behavior are stereoregularity, molecular weight and molecular weight distribution of the polymers. The melting temperature is increased by an enhanced syndiotacticity at sufficiently high molecular weights and at narrow molecular weight distributions. The degree of syndiotacticity obtained primarily depends on the kind and structure of the cyclopentadienyl ligand of the transition metal catalyst, whereas the effect of the other ancillary ligands on stereoregularity is negligible at the same cyclopentadienyl ligand. At narrow molecular weight distributions with $\overline M _{\rm w} /\overline M _{\rm n}$ below 2.8 and at constant stereoregularities, the molecular weight has a remarkable effect on the melting behavior at weight‐average molecular weights lower than about 80 000 g · mol−1, resulting in a significant decrease of the melting temperature until below 230 °C. The presence of hydrogen during polymerization leads to a significant shift to lower molecular weights at comparably small amounts of hydrogen, but results in the occurrence of an additional peak in the molecular weight distribution at larger hydrogen concentrations giving evidence for the formation of a second active polymerization site producing lower molecular weight polymers. At constant stereoregularity, the broadening of the molecular weight distribution leads to decreased melting temperatures and to improved flow properties of the syndiotactic polystyrenes with increasing shear rates at moderate molecular weight distributions.Detailed molecular weight distributions of syndiotactic polystyrenes in dependence on the hydrogen concentration.magnified imageDetailed molecular weight distributions of syndiotactic polystyrenes in dependence on the hydrogen concentration.