Synthesis of alkoxysilyl-terminated polyisoprenes by means of 'living' anionic polymerization, 2. Synthesis of trialkoxysilyl-terminated 1,4-polyisoprenes by reaction of polyisoprenyllithium with various functional trialkoxysilanes selected as end-capping reagents

Abstract

In order to prepare trimethoxysilyl (or triethoxysilyl)-terminated 1,4-polyisoprene with good yields, the deactivation of polyisoprenyllithium (Mn = 800) was investigated by using various functional trialkoxysilanes (RO)3Si—G (G = 3-chloropropyl, methoxy or ethoxy) as silane end-capping agents. The studies were carried out in presence of THF by varying the molar ratio r = [1,4-polyisoprenyllithium chains]/[alkoxysilane]. The resulting products were characterized (qualitatively and quantitatively) by means of 1H and 29Si NMR spectroscopies (one or two dimensions), and also by supercritical fluid chromatography. The corresponding rate of substitution depends on r and on the nature of the starting alkoxysilane reagent. It was shown that alkoxy groups at the silicon center are easily substituted by the polyisoprenyl carbanions. Whatever the alkoxysilane used, two families of products are formed: (i) polymer derivatives resulting from nucleophilic substitution reactions of alkoxy groups from alkoxysilane reagent by ω-carbanionic polyisoprene chains, and (ii) substituted side-products issued from substitutions involving n-BuLi. The presence of these side products is explained by the fact that the butyllithium initially introduced to initiate the isoprene anionic polymerization is not totally consumed. The comparison of the results with those previously obtained in hexane with n-BuLi used as model molecule of ω-carbanionic polyisoprene shows some differences concerning the order of reactivity of the various reactive substituted species formed during the substitution reaction, as well as the selectivity of the substitution reaction in relation with the alkoxysilane reagent. These differences were attributed to the chelating properties of THF. On the other hand, results are the same concerning the attempts realized with 3-chloropropyltrimethoxysilane which have also shown that nucleophilic attack of polyisoprenyl carbanion occurs exclusively on the silicon atom and not on the carbon bearing the chlorine.