Synthesis and Characterization of Triblock Copolymers Containing Amphiphilic Dendron

Abstract

Dendrimers/dendrons are regularly branched macromolecules which have received a great amount of attention due to their structural and physical uniqueness. Recently, introduction of microphase-separation concept has been proved to lead to a self-assembling system which shares the morphological behavior of dendrimers/dendrons and linear block copolymers. However, dendron and linear polymer fused system has not been extended to complicated chain architectures. For example, triblock copolymers, they have potential to selforganize into complex and exciting nanostructures such as a ring and a knitting pattern which can not be expected in conventional diblock copolymer systems. Therefore, it would be an interesting challenge to study the influence of dendron architecture on self-assembled nanostructures in triblock copolymer systems. To this end, we have designed and synthesized a novel triblock copolymer which consists of a hydrophobic crystalline peripheries (docosyl chains), a hydrophilic part [dendritic core plus poly(ethylene oxide) (PEO)], and a hydrophobic amorphous coil [polyisoprene (PI)]. As shown in Scheme 1, the synthesis began with anionic polymerization of isroprene. In the first step isoprene was polymerized in cyclohexane with sec-butyllithium as an initiator. To convert the living PI end into hydroxy group, ethylene oxide was added and then subsequently oxyanion of the resulting PI chain was protonated with degassed methanolic HCl. In the next step potassium naphthalenide was used to remove the proton on the hydroxyl group resulting in PI-O−K+. Polymerization of ethylene oxide at the end of PI was performed in tetrahydrofuran in the presence of a five-fold excess of KCl with respect to PI-OH chain ends. The polymerization was terminated with degassed methanolic HCl. Then, the PI-b-PEO was functionalized with carboxylic acid by the reaction with succinic anhydride in the presence of 4-dimethylaminopyridine (DMAP). The structural analysis from gel permeation chromatography (GPC) and nuclear magnetic resonance (NMR) spectroscopy determines that the molecular weights of PEO and PI are 2170 and 3000 g/mol, respectively. Meanwhile, synthesis of an amphiphilic dendron (denoted as 4G-OH) was performed using a convergent synthetic method. The synthetic procedure consists of two organic reactions such as a Williamson etherification and hydroboration/oxidation reactions. Several iteration of these steps led to the monodisperse dendron, the synthetic details of which were reported previously. The triblock block copolymer was achieved through a simple esterification coupling reaction using N,N'-diisopropylcarbodiimide (DIPC) and 4-(dimethylamino)pyridiniump-toluene sulfonate (DPTS). In this reaction, a two fold excess of 4G-OH with respect to PI-b-PEO was used to