Anionically grown monofunctional polystyrene macromonomers are coupled through the hydrolysis and condensation of trimethoxysilyl end groups. Hydrolysis is initiated by adding acidified water to a tetrahydrofuran solution of the macromonomer. Condensation is facilitated by evaporating the solvent and heating the polymer under vacuum above the glass transition temperature. The macromonomers couple to form high molecular weight polymers and reach a finite size, beyond which further growth is inhibited. The final products are completely soluble star-shaped polymers, which are characterized by size-exclusion chromatography with molecular weight sensitive detectors. The molecular weight distributions of the stars are surprisingly narrow, although there are definitely mixtures of stars with different numbers of arms. The average number of arms in a star decreases as the molecular weight of the macromonomer increases. The final, limiting structures of the stars can be explained by the free energy changes associated with the number and length of arms. The results strongly suggest that the prevalent mode of growth at later stages of condensation becomes addition of macromonomer to stars, rather than addition of stars to stars. Studying the condensation coupling of the macromonomers provides understanding for more complicated networkforming systems; the technique also provides a unique method for synthesizing star-shaped polymers that has several advantages over other synthetic methods.
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