Controlled oligomerisation of isoprene-towards the synthesis of squalene analogues

Abstract

Poly(isoprene) oligomers have been synthesised by the use of a range of controlled radical polymerisation techniques, including Catalytic Chain Transfer Polymerisation (CCTP), Reversible Addition Fragmentation Chain Transfer Polymerisation (RAFT), Atom Transfer Radical Polymerisation (ATRP) and Reverse Atom Transfer Radical Polymerisation (RATRP). The key molecular targets were (a) a low number average molecular weight equivalent to that of squalene and (b) a final backbone structure that contains a minimum of 75% of the 1,4 addition form of the isoprene monomer. These structural elements closely match those of the natural squalene which is the cosmetic/health care industry benchmark. CCTP and RAFT were successful in achieving the set targets via solution based polymerisation. The yields of oligomer from these techniques were approximately the same (∼42% from CCTP and ∼47% from RAFT), as were the M_ns, (750 and 850 ± 220 g mol⁻¹ respectively versus a GPC squalene target M_n of 570 g mol⁻¹) and polydispersities (1.28 and 1.21 respectively versus a GPC squalene target of 1.02). The backbone structure of the materials produced by these techniques were shown to exceed the 75% 1,4 addition target. Therefore, both of these techniques have the potential to produce the target structures on a larger/commercial scale. However, the choice of initiator and/or RAFT agent was found to have a strong influence on the molecular weight obtained. The ATRP and RATRP techniques proved unable to demonstrate sufficient control over the polymerisation to produce the target oligomers under the reactions conditions applied whilst achieving acceptable yields (>40%).