Bio-inspired cationic polymerization of isoprene and analogues: state-of-the-art
Résumé
Natural rubber (NR), which is polyisoprene about 100% 1,4-cis of high molar mass, > 10(6) g mol(-1), is mainly produced in southeast Asia and represents around 40%-45% of total worldwide elastomer consumption. Whereas more than 2500 plant species are able to produce polyisoprenoids, the only established commercial source of NR is Hevea braziliensis. NR presents high performance properties that are so far not matched by synthetic rubbers. As a consequence, NR is irreplaceable in many applications (aircraft tires, surgery gloves etc.). Nature and synthetic polymer chemists start from different substrates to synthesize polyisoprenes, i.e. isopentenyl pyrophosphate (IPP), which is a universal building brick utilized by plants and animals, and isoprene monomer, respectively. Nevertheless, we proposed that the elementary processes involved in the biosynthesis of NR are very similar to those of cationic polymerization. In the course of a study on bio-inspired cationic polymerization of isoprene and IPP analogues, it appeared that cationic polymerization of isoprene proceeds readily and leads mainly to its 1,4-trans addition; such a process nevertheless remains difficult to control due to the occurrence of many side reactions (transfer, protic initiation, branching, cyclization). The present paper describes our understanding of the cationic polymerization of isoprene and its analogues catalyzed by different Lewis acids, in solution and aqueous dispersions.