Cationation of dimethylallyl alcohols by B(C6 F5 )3 as models of the (Re)initiation reaction in the bio-inspired cationic polymerization of isoprene

Abstract

The polymerization of isoprene initiated by dimethylallyl alcohols (DMAOH) in the presence of Lewis acids (LAs) as coinitiators has been selected as a potential model of the proposed cationic mechanisms involved in natural rubber (NR) biosynthesis. In view to investigate the activation/ionization mechanism of the allyl alcohol chain terminus (PIAllOH), which was shown to exist in NR, different isomers of DMAOH were used as simple models of polyisoprene chain-end structures in the presence of tris(pentafluorophenyl) borane (BLA) as a LA. It is shown that cationation of 3,3-DMAOH by BLA proceeds by direct OH abstraction. However, this process is strongly retarded in the presence of 2,6-di-tert-butylpyridine (d(t)BP), due to the formation of different BLA complexes, one with 3,3-DMAOH, active for cationation, and a dormant one involving its ionized form 3,3-DMAOH. The monomer generated in situ by ionization/proton elimination steps subsequently adds on the primary allylic carbocation form, then resulting in the formation of oligoisoprenes, whereas the tertiary carbocation form of the allylic carbocation yields exclusively to proton elimination and isoprene formation, a possible mechanism of chain-end termination. (C) 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 49: 4948-4954, 2011