Star And Cyclic Shaped Macromolecular Architectures Prepared Using Copper-catalyzed Azide-alkyne Cycloaddition: Synthesis, Purification And Characterization
The use of advanced functional polymer materials has gained an enormous impact during the past decades. Due to the fact that the physical properties of macromolecules are inherently dependent on their structure and connectivity on the nanoscale, precisely control over polymer architecture has been a longstanding goal for polymer chemists. The recent development of copper catalyzed azide-alkyne click chemistry provides a nearly quantitatively tool for macromolecular coupling. Through the combination of living polymerization and click chemistry, novel complex polymer architectures can be readily constructed, including star polymers, brush polymers, cyclic polymer and ladder polymers. While amphiphilic block copolymers have demonstrated their utility for a range of practical applications, the behavior of block copolymers that contain cyclic topologies remains largely unexplored due to limited synthetic access. In order to investigate their micelle formation, biocompatible cyclic amphiphilic poly(ethylene glycol)-block-polycaprolactone, c-(PEG-b-PCL), and tadpole shaped PEG-PCL, were synthesized by a combination of ring opening polymerization (ROP) and click chemistry. In addition, exactly analogous linear block copolymers have been prepared as control samples to elucidate the role of polymer architecture in their self-assembly and acid-catalyzed degradation. High purity homo-arm and mikto-arm poly(ethylene glycol) (PEG) stars were successfully prepared by the combination of epoxide ring openings and azide-alkyne click reactions. First, monohydroxy-PEG was modified via epoxide chemistry to bear one hydroxyl and one azide functionality at the same polymer chain end. An alkyne functionalized PEG chain was then coupled to the azide. Subsequently, the remaining hydroxyl could be reactivated by epoxide chemistry again to an azide and alcohol group. This enabled a step-wise coupling and reactivation of the end group to add a series of well-defined polymer arms onto a star polymer. The use of efficient reactions for this iterative route provided star polymers with an exact number of arms, and a tailorable degree of polymerization for each arm. Detailed characterization confirmed the high purity of multi-arm polyethylene glycol products. Novel cyclic brush-shaped polymers can be successfully prepared by using the CuAAC click coupling reaction. First, cyclic-shaped polymer bearing a single hydroxyl group can be synthesized by CuAAC click cyclization. After a one-step modification of the hydroxyl group by esterification with an azido-carboxylic acid, a "clickable" polymer ring was obtained. A linear polymer backbone with an alkyne functional group on every repeat unit was prepared by ATRP of acetoxystyrene followed by reduction to poly(4-hydroxystyrene) and esterification with pentynoic acid. Finally, by coupling multiple equivalents of the cyclic precursor onto the linear backbone, a cyclic brush-shaped polymer was prepared. This provides a highly efficient approach to prepare novel polymer architectures containing multiple cyclic components.