Kinetics and morphology of metallocene catalyzed syndiospecific polymerization of styrene in homogeneous and heterogeneous reaction systems

Abstract

Syndiotactic polystyrene (sPS) is a semicrystalline thermoplastic polymer with many advantageous properties such as excellent heat resistance with a high melting point of 270-272oC, strong chemical resistance against acids, bases, oils and water, and low dielectric constant. The relatively fast crystallization rate makes sPS a promising material for a large number of applications in the automotive, electrical and packaging industries.

In this study, the kinetics of syndiospecific polymerization of styrene is investigated through experimentation and theoretical modeling using homogeneous and heterogeneous Cp*Ti(OCH3)3/MAO catalysts. During sPS slurry polymerization, the physical phase changes of reaction mixture occur. With an increase in total solid content, sPS slurry undergoes a series of physical changes from clear liquid to a wet cake or paste-like material. A detailed reaction kinetic model based on a two-site kinetic mechanism has been developed to predict the polymerization rate and polymer molecular weight distribution. The monomer partition effect is incorporated into kinetic models to account for the nonlinear dependence of polymerization rate on the bulk phase monomer concentration. Quite satisfactory agreement between the model simulation results and experimental data has been obtained.

The morphological development of nascent sPS particles during the polymerization has also been investigated. Most notably, it was found that sPS particles grow with the nanofibrillar morphology with either homogeneous or silica-supported metallocene catalyst. The analysis of nascent morphology of sPS using X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive X-ray (EDS/EDX) analysis, revealed that there is a strong correlation between the formation of sPS nanofibrillar structure and sPS crystallization. A mechanism for the growth of sPS particles is also proposed based on the experimental observations and analysis.

Ultrahigh molecular weight sPS has also been synthesized in silica nanotube reactors (SNTRs) and the morphological characteristics of sPS produced in the nanotube reactors have been analyzed. A new mechanism is proposed for the formation and growth of sPS nanofibrils extruding out from the nanotube reactors. Also, a kinetic analysis is presented to interpret the observed molecular weight enhancement effect that is believed to be caused by the constrained reaction environment inside the nanotubes.