Synthesis and physical characterization of arborescent (dendrigraft) polymers

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Dendritic molecules are characterized by an architecture with multiple branching levels, similar to a tree. These can be subdivided into three distinct families. Dendrimers and hyperbranched polymers are both derived from ABn-type monomers (where functional group A can only react with group B; n = 2 or 3). The perfect structure and uniform size of dendrimers results from strictly controlled cycles of monomer protection, condensation, and deprotection reactions, but an imperfectly branched structure and a broad size distribution often arise when the condensation reaction is carried out without protecting groups. The arborescent (or dendrigraft) polymers, of interest in this proposal, are obtained through successive grafting reactions of linear polymer segments. The growth of these molecules is much faster than for dendrimers over successive generations (reaction cycles), but they display many of their characteristics; they are therefore an interesting alternative to dendrimers for many nanotechnology applications.*This proposal concerns the synthesis and the physical characterization of arborescent polymers (AP). These are novel materials, so there is little understanding of the relationships between their structure/composition and their properties. The information derived from the well-defined AP systems will also help improve the understanding of branched polymer systems in general, many of which are of commercial importance. *The first project involves core-shell AP structures with a hydrophobic polypeptide core and a shell of hydrophilic segments. The research will focus on the solubilization and release properties of these amphiphiles for different model hydrophobes as a function of the structure (branching density, size) and composition of the AP molecules. This will also provide information on the morphology of the AP molecules (extent of phase separation between the core and shell).*In the second project the formation of metallic nanoparticles by AP copolymers with core-shell (CS) and core-shell-corona (CSC) architectures, containing metal-binding poly(2-vinylpyridine) segments, will be examined. We recently observed the formation of intriguing nanomorphologies constrained within the individual AP molecules upon the introduction of metallic ions. These nanostructures likely arise from large differences in interaction (chi) parameter between the metal-rich and metal-poor regions, but their consequences on the physicochemical properties of these hybrid materials are hitherto unexplored. We will examine the formation of these nanostructured materials in a systematic fashion, and ultimately their catalytic properties. The CSC systems display many interesting properties, but their synthesis is very time-consuming and is marred by low yields. We just started investigating a new strategy for the synthesis of these materials, by self-assembly of an AP core-shell structure with a block copolymer where one of the blocks can bind with the AP substrate. This approach looks very promising, but again there is no clear understanding of the rules (thermodynamics) governing this self-assembly process. These will be explored through systematic variations in the structure (e.g. binding block and AP substrate size) of the components and the conditions (solvent, temperature) used.*Finally, to pursue our efforts in developing new methods for the synthesis of AP structures, we will combine a "click" grafting technique which we just developed with atom transfer radical polymerization (ATRP) to considerably expand the range of arborescent polymer compositions available. We will also explore methods for the synthesis or arborescent polybutadiene structures with long side chains, since these are of interest as model highly branched polymer systems.