Synthesis and characterization of arborescent (dendrigraft) polymers

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Dendritic molecules have multiple branching levels, similarly to a tree. Dendrimers and hyperbranched polymers are derived from ABn-type monomers (where functional group A can only react with group B; n = 2 or 3). The perfect structure of dendrimers results from laborious syntheses, while less controlled conditions lead to the imperfect structure of hyperbranched systems. Arborescent polymers (AP), of interest in this proposal, are obtained through successive grafting reactions of linear polymer segments. They grow much faster than dendrimers but display many of their properties, so they can replace them in multiple nanotechnology applications. This proposal focuses on the synthesis and the physical characterization of APs. We recently developed a "click" grafting scheme for the synthesis of APs. The scope of this methodology can be expanded considerably if combined with atom transfer radical polymerization (ATRP) or reversible addition-fragmentation chain transfer (RAFT) polymerization to generate polymeric building blocks, and provide AP compositions hitherto inaccessible. Beyond the short-term goal of developing efficient techniques for AP synthesis, structure-property relations will be established for these novel materials that will improve the understanding of branched polymers in general, many of which are of commercial importance. More complex AP structures, with core-shell (CS) or core-shell-corona (CSC) architectures, are obtained by using polymers with different compositions in successive grafting reactions. CS AP micelles potentially useful as drug delivery systems (DDS) were thus obtained using hydrophobic polypeptide chains for the core of the molecules, and hydrophilic chains for the shell. Over the next five years, this research will focus on designing these molecules to control their drug loading capacity and release kinetics, but also their ability to cross cell membranes. The long term goal of this research is the development of efficient DDS targeting tumor sites in cancer therapy. We also investigated CSC APs containing metal-binding polymer segments within their inner shell as templates for the preparation of metallic nanoparticles, and observed intriguing nanomorphologies upon loading with metallic ions. This nm-scale phase separation should have important consequences on the physicochemical properties of CSC polymers, but the synthesis of these materials is marred by very low yields. We just developed a new synthetic approach based upon the self-assembly (SA) of a CS AP with a block copolymer where one of the blocks can bind with the AP substrate. SA is very promising, but reversible in some cases. Our short term goal is to develop a better understanding of the SA thermodynamics, and strategies to stabilize the CSC structures obtained. On a longer timescale, this will allow their synthesis in amounts sufficient to enable detailed properties studies, with applications in nanotechnology and catalysis.