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

摘要

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" omega-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 less than or equal to r less than or equal to 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. [References: 26]