Synthesis and Characterization of Polymer Brushes with Clickable Functionality

Architecture of brush-like polymers had attracted considerable attention not only due to the recognition of the importance of relationship between structure and properties, but also possible prospective applications, especially in drug delivery and other biomedical applications. In nature, this form of structure is not uncommon. Proteoglycans, for instance, have a structure which resembles a bottle brush with protein backbone and carbohydrate side chains. The biomolecule acts as water sponges in cartilage controlling its shock absorption and lubrication properties and also as mucociliary clearance of lung airways 1 . Molecular brushes can be homopolymer or copolymer, with a flexible or stiff side-chain. The side-chain can be loosely or densely grafted. By understanding the importance of these properties, specific strategy can be drawn. Two strategies have been identified and applied in this research which is the "grafting through" and "grafting from" methods. In "grafting from" method side-chains were grown from the macroinitiator backbone meanwhile; "grafting through" method involves the polymerization of macromonomers.These routes were used to construct the molecular brushes' backbone 1, 2. As shown in Figure 1, novel molecular brush using biocompatible materials as the core was prepared using 2-hydroxy ethyl methacrylate (HEMA) and ε-caprolactone (CL). For the "grafting from" method, HEMA was polymerized using Atom Transfer Radical Polymerization (ATRP) to form the graft polymer backbone and PCL chains were grown from it using Ring Opening Polymerization (ROP) 3,4 . The main advantage for this method is that specific length of the PHEMA backbone can be easily defined. HEMA- PCL macromonomers were alternatively synthesized in the "grafting through" technique with well defined side-chain length via ROP. Subsequently, a similar graft polymer was constructed by ATRP of the macromonomers. The results are shown in Figure 2. The next crucial step involved the transformation of hydroxyl groups in the resulting PCL brush to bromoisobutyryl ATRP initiating groups using 2-bromoisobutyryl bromide as the agent to create the bromo-ester initiating group. Protected acetylene monomer, trimethylsilyl propargyl methacrylate (TMS-PgMA), was grafted from the pendent bromo-ester groups using ATRP. Deprotection of the trimethylsilyl groups using tetra butyl ammonium fluoride (TBAF) revealed the acetylene functionality of the side chains. This consequently provided a basis for the click reaction to occurred. The final step involved the attachment of azide-functionalized linear polystyrene via a copper (I)- catalyzed cycloaddition reaction between the azide and acetylene groups.