Synthesis of Arborescent Polybutadiene
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Arborescent polymers are characterized by a tree-like architecture and a high branching functionality. This type of polymer can be synthesized by different techniques, but the ‘grafting onto’ method is attractive because it provides good control over the molecular weight of the graft polymer and the side-chains used as building blocks. This method was applied to the synthesis of arborescent polybutadiene, using cycles of epoxidation and anionic grafting reactions. The research focused on optimization of the grafting yield for the synthesis of the G0 polymers, obtained by grafting side-chains onto a linear epoxidized substrate, with the ultimate goal of synthesizing successive generations of graft polymers using these optimized conditions. Two additives potentially useful as reactivity modifiers, N,N,N’,N’-tetramethylethylenediamine (TMEDA) and lithium bromide (LiBr), were investigated to increase the grafting yield. The influence of solvent polarity was also examined, and the reaction time was varied from one day to one week while monitoring the grafting yield. Optimal results (with grafting yields reaching up to 85% in one week) were obtained in cyclohexane-tetrahydrofuran mixtures, in the presence of LiBr, with only small (2-3%) yield increases observed after 24 h of reaction. These optimal conditions, when applied to the synthesis of G1 and G2 polymers, led to grafting yields of 78-80% when using a 1:1 ratio of epoxide groups to living ends. The influence of excess substrate was also examined individually for each generation, and likewise led to small (2-4%) increases in grafting yield. The results obtained showed that the grafting reaction was successful on the basis of 1H NMR spectroscopy and size exclusion chromatography analysis, and was sensitive to parameters such as the substitution level of the epoxidized substrate, the solvent composition, and the presence of additives.