|M.Sc Student||Moshe Moshonov|
|Subject||Preparation of Olefin Thermoplastic Elastomer|
|Department||Department of Chemistry||Supervisor||Full Professor Eisen Moris|
|Full Thesis text|
In this thesis I will focus on thermoplastic elastomers, (TPEs), which are a family of polymers that can be repeatedly stretched without permanently deforming the shape of the part. Unlike rubber-like elastomers, they do not require curing or vulcanization, as they are true thermoplastics. Thermoplastic elastomers (TPEs) may be processed by conventional thermoplastic techniques such as injection molding, extrusion and blow molding. They often provide design freedom at a lower system cost for designers and manufacturers, and are considered as a cleaner, less cumbersome technology. Furthermore, they offer the benefits of thermoset rubbers such as flexibility and elasticity, but can be re-melted by heating above a certain temperature. This property not only means that processing thermoplastic elastomers is as simple as typical thermoplastics, it also means that this family of materials is easily recycled.
Herein, we report an approach to prepare a thermoplastic elastomer, (TPE) by using two different synthetic routes. The first is by using the process of chain shuttling polymerization, where a chain shuttling agent, (CSA) is used to pass a growing polymer chain between two different catalysts. This process gives rise to polymers that contain block segments with the microstructural signature of each catalyst and allows microblock polyolefin materials to be produced on a large scale. This catalytic system produces olefin block copolymers with alternating "soft" elastomeric segment and "hard" polymeric segments. In the segmented copolymers, the hard blocks account for the mechanical stability of the material, since they give rise to reversible crosslinks, which are embedded in an amorphous phase. This phase is mainly composed of the soft segment, and gives the material its flexibility.
In the second route we introduce a novel synthetic procedure by a special pressure modulated reactor system which is designed and prepared in order to attain high and low propylene partial pressures with very fast cycling, thereby isotactic/atactic multiblock chain propagation occurs. The original idea of this study was promoted by past research at our group, which revealed that the stereoselectivity in propylene polymerization at a given temperature decreases from isotactic to atactic with the decreasing propylene concentration. It was found possible only with the polymerization catalyzed by the bis(benzamidinate) Zr(IV) complex, activated with methylaluminoxane, (MAO). This was attributed to a competition between isotactic chain propagation at high pressure of propylene, and an intramolecular racemization of the last-inserted monomeric unit at atmospheric pressure. This method, when applied, allows the synthesis of an elastomeric polypropylene, (el-PP).