Thermoplastic Elastomers – Green Rubbers for the Future
von Judit E. Puskas, Joseph P. Kennedy
The readers of this magazine are well familiar with thermoplastic elastomers (TPEs) – rubbery materials that can be processed as plastics. In fact, TPEs are “claimed” to be both rubbers and plastics by the relevant research and business communities. Either way, they can be considered as “green” materials since articles and scraps can be reprocessed.
The saga commences in the late 1960s, when the utility of styrenic TPEs synthesized by anionic polymerization was demonstrated. Carbocationic polymerization is a relative latecomer to the world of TPEs. Nonetheless, within a short time this chemistry has enriched the science and technology of TPEs in many ways. After two decades that the first US Patent disclosing this technology has issued, TPEs manufactured by carbocationic polymerization became a commercial reality. An important motivation was the desire to replace the unsaturated rubbers, polybutadiene or polyisoprene used in styrenic TPEs, with the equally inexpensive but saturated (and consequently oxidatively and chemically resistant) polyisobutylene rubber as the elastomeric moiety.
Long before the discovery of living carbocationic polymerizations and that of the introduction of the sequential monomer addition technique for the preparation of triblock TPEs, efforts were made for the synthesis of glassy-rubbery-glassy block copolymers by cationic techniques. Motivation for this research came from the spectacular academic and commercial successes of the rapidly expanding field of styrenic TPEs (notably the Kraton family originally marketed by Shell) prepared by living anionic polymerizations, and from the realization that similar cationically prepared glassy-rubbery-glassy structures should have better overall properties than the anionically polymerized products.
The road toward high-quality styrenic TPEs by cationic routes was opened by three discoveries made in rapid succession: (1) The living carbocationic polymerization of isobutylene yielding high molecular weight narrow molecular weight distribution polyisobutylene; (2) The living carbocationic polymerization of styrene and styrene derivatives; and perhaps most importantly, (3) the finding that living polyisobutylene di- and trications PIB, PIB in the presence of certain additives readily induce the living polymerization of St and St derivatives. These findings set the stage for the current development of PIB-based styrenic TPEs. Another breakthrough was the discovery that good TPE properties can be achieved with a randomly branched polyisobutylene core and broad molecular weight distribution.
In 2004, the Food and Drug Administration approved the use of a polyisobutylene-based TPE as the drug eluting coating on the Taxus coronary stent, marketed by Boston Scientific Co. Reportedly, this is one of the most
successful medical devices, generating 1.4 billion dollars in revenue. To date more than five million patients have this polymer in their heart – in more than one way. There is no better feeling for researchers to be able to contribute to society this way. The saga continues with new polyisobutylene-based materials, including novel polyurethanes described in this issue (p. 227ff.). There is a bright future for all TPEs, but particularly polyisobutylene-based TPEs, especially in medical applications.
Judit E. Puskas
Joseph P. Kennedy
University of Akron, OH, USA