A novel synthetic path to arborescent graft polystyrenes

A method for the preparation of arborescent polystyrenes based on acetyl coupling sites is described. The acetyl functionalities are randomly introduced on the grafting substrate by reacting polystyrene with acetyl chloride in the presence of anhydrous AlCl3 in nitrobenzene. Anionic polymerization of styrene with sec-butyllithium yields polystyryllithium serving as side chains; coupling of the side chains with the acetylated substrate generates the graft polymers. Direct coupling of polystyryllithium with acetylated polystyrene suffers from a low grafting yield (similar to 65% for a linear substrate), due to deactivation of the macroanions by side reactions. End-capping of polystyryllithium with a few isoprene or 2-vinylpyridine units, along with the addition of LiCl, increases the grafting yield to > 95%. Repetition of acetylation and anionic grafting cycles leads to generations of arborescent polymers with a low polydispersity index (M-w/M-n = 1.07-1.09). The polymers are characterized by a very compact structure and by a branching functionality and molecular weight increasing geometrically for successive generations. Tailor-designed arborescent architectures are attainable by this "graft-on-graft" procedure, by controlling the acetylation level and the molecular weight of the side chains used for each grafting reaction.