Bio‐inspired cationic polymerization of isoprene and analogues: state‐of‐the‐art

Abstract

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.

The Structure of Cationic Polyisoprene: Branching versus Cyclization

“…The reasons leading to the decrease of the content of double bonds in polymer chains of cationic polydienes are not clear up to date. According to some authors, this feature is connected with the occurrence of intramolecular cyclization with the formation of six‐membered cyclic structures. Others proposed another explanation of the observed reduced unsaturation, which is consistent with the formation of branched macromolecules due to the intensive chain transfer to polymer …”

“…In recent years, a great attention has been paid to the investigation of the cationic polymerization of conjugated dienes and, especially, isoprene . Polyisoprenes synthesized by cationic polymerization are usually characterized by broad multimodal molecular weight distribution (MWD), which is significantly changed through the polymerization.…”

“…The main feature of polydienes synthesized by cationic polymerization is the reduced unsaturation of synthesized polymers . The reasons leading to the decrease of the content of double bonds in polymer chains of cationic polydienes are not clear up to date.…”

Carbocationic polymerization of isoprene using cumyl initiators: progress in understanding side reactions

“…2,3 Up to the end of the 60s, Friedel-Crafts catalysts, Lewis acids and/or other organometallic compounds were mainly used as initiating systems for the cationic polymerization of isoprene, whereas 25 from the 70s protic promoters were preferred. Furthermore, many polymerization conditions were investigated, varying the solvent, the temperature and the monomer concentration.…”

A New Insight into the Mechanism of 1,3‐Dienes Cationic Polymerization I: Polymerization of 1,3‐Pentadiene with tBuCl/TiCl4 Initiating System: Kinetic and Mechanistic Study

“…In recent years, great attention has been paid to the investigation of the cationic polymerization of conjugated dienes such as isoprene, 1,3‐pentadiene, and even alloocimene, a naturally occurring triterpene. This interest was mainly stimulated by the recent proposition that natural rubber biosynthesis resembles a cationic step‐growth/chain‐growth reaction . However, despite intensive investigations of the cationic polymerization of isoprene directed to study the effect of initiator/Lewis acid nature or mechanisms of initiation (cationation) and branching, the trans ‐1,4‐oligoisoprenes with low number‐average molecular weight ( < 2000 g mol −1 ), broad molecular weight distribution (/ = 2–10), and significant loss of double bonds (double bonds content < 70%) were invariably obtained .…”