Polyisobutylene-Based Thermoplastic Elastomers: A Review

Abstract

Abstract This paper honours the 40th anniversary of the discovery of the living polymerization concept. Polymeric materials exhibiting both thermoplastic and elastomeric characteristics have a variety of unique properties which makes them valuable articles of commerce. Such thermoplastic elastomers or TPEs, schematically represented in Scheme 1, are block copolymers — ABA linear triblock, A(BA)n linear alternating block or (AB)n−X radial block, where A is a thermoplastic glassy block with a high glass transition temperature (Tg) while B is an elastomeric block with a low Tg. These TPEs behave like vulcanized rubbers at room temperature and like thermoplastics at elevated temperatures. Thus the materials can be melt extruded like plastics, while retaining their beneficial rubbery properties upon cooling. This ability is not only of advantage during processing, but allows the materials to be reprocessed, which is of importance from both the material savings and the environmental protection point of view. These block copolymers exhibit physical behavior similar to reinforced elastomers, or in other words, they behave as vulcanized rubbers but without the need of chemical vulcanization. Polymers having such dual nature have been commercialized for some time [Shell's ABA type Kraton®s; where A is polystyrene (PSt) and B is polybutadiene (PBD) or polyisoprene (PIP)]. In order to have good physical properties of these block-type TPEs, a high degree of microphase separation between the rubbery and glassy phases and the formation of “physical crosslinks” by the glassy segments are required. It is therefore of utmost importance for these blocks to have nearly uniform or narrow molecular weight distribution (MWD). The synthesis of nearly uniform polymers was made possible by the discovery of the living polymerization concept by M. Szwarc, which in turn made possible the synthesis and commercialization of the Kraton® series. While Kraton®s exhibit the above described fundamental duality of thermoplasticity and elasticity, they also possess certain undesirable characteristics such as poor oxidative stability due to the highly unsaturated nature of the elastomeric blocks. To overcome this problem the Kraton G® series (Shell) have been developed by hydrogenating the PBD rubbery segment. It should also be mentioned here that hydrogenation of solution and emulsion SBRs produced materials with TPE properties. In this case phase separation was achieved between the rubbery and glassy/crystalline phases leading to reasonably good physical strength (up to 10 MPa), in spite of the lack of uniformity in these polymers. At the same time, the synthesis of block TPEs with polyisobutylene (PIB) rubbery segments became a desirable objective because of the superior oxidative, chemical and thermal stability, and the outstanding barrier and damping properties of PIB. However, PIB can be made only by carbocationic polymerization. Just a decade ago living carbocationic polymerization was declared to be impossible due to the highly reactive nature of the active centers. Today, both the living carbocationic polymerization of isobutylene (IB) and other olefins, and the synthesis of PIB-based TPEs by living polymerization are a reality. This paper will review the most important milestones that led to this chemical and technological achievement, and discuss the characterization and some applications of these new materials.