Synthesis of alkoxysilyl-terminated polyisoprenes by means of "living" anionic polymerization, 1. Modeling of the termination step by studying the reaction of butyllithium with various alkoxysilane reagents

Abstract

In order to optimize the preparation of trimethoxysilyl- or triethoxysilyl-terminated 1,4-polyisoprene and to facilitate their characterization, the termination reaction of the anionic polymerization of isoprene was investigated by means of a model reaction. Butyllithium (n-Buli) was used as a model molecule of the “living” ω-carbanionic polymer chain, and its reaction with various alkoxysilyl reagents (3-chloropropyltrimethoxysilane, tetramethoxysilane and tetraethoxysilane) was investigated. Studies were carried out by varying the molar ratio r = [n-BuLi]/[alkoxysilane] (0.5 ≤ r ≤ 3). The corresponding rate of substitution depends on r and on the nature of the coupling reagent. It was shown that alkoxy groups at the silicon center are easily n-Bu-substituted. Whatever, r, higher n-Bu-substituted derivatives are always simultaneously formed with the n-Bu-monosubstituted compound. With tetraalkoxysilane reagents, the formation of butyltrialkoxysilane is always favoured, compared to that of the di- and trisubstituted homologs. This was interpreted in terms of reactivity differences existing between the reacting alkoxysilanes present in the mixture. Attempts realized with 3-chloropropyltrimethoxysilane in order to obtain selective formation of alkyltrimethoxysilane derivative showed that chlorine substitution was impossible because nucleophilic attack of butyl carbanion occurs exclusively on the silicon atom.