Design, synthesis and polymerization of highly branched pseudodendrimers through tandem reactions and kinetics measurements of pseudodendrimeric polymerization

Date

2008

Authors

Wang, Junyan

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Abstract

Dendritic polymers exhibit some unique properties which clearly distinguish them from other polymers. Their fascinating structural features have made them one of the best candidates for nanotechnology applications. The synthesis of dendrimers involves multi-step procedures including protective group technology. This results in high cost and accessibility only in small quantities. Hyperbranched polymers can be prepared in one-pot polymerization of ABn monomers, however, a competitive reaction between the formations of linear chains and branching occurs, and consequently the structure is imperfect and the control over layers or generations vanishes. Pseudodendrimers are intermediate polymers between hyperbranched polymers and dendrimers. Similar to dendrimers, they are also fully branched structures. But they are not perfectly symmetric, like hyperbranched polymers. Pseudodendrimers may serve as potential replacements for dendrimers.

We have developed a single-reaction synthesis of pseudodendrimers. The overall goals of this project are to optimize the synthesis of pseudodendrimers, measure the kinetics of pseudodendrimer polymerization, show that pseudodendrimers have similar properties to dendrimers. The careful design of an ABB' monomer leads to higher branching by virtue of a tandem reaction that increases the reactivity of linear units during polymerization. Our work has shown that 6-amino-3-bromophthalide leads to a highly branched polymer via bromohydrin decomposition during polymerization, giving polymers of Mn of 3000 and a polydispersity index of 1.03. Our findings indicate a degree of branching of 0.84, suggesting the polymerization is a single-reaction synthesis of a pseudodendrimer. Polymerization of analogous polymers confirms our proposed intermediate.

The kinetics of the model reactions show that the reaction is first order in benzylamine. We determined that the energy of activation (E a) for the first addition reaction is 8.9 kcal mol-1 and Ea for the second addition reaction is 6.6 kcal mol-1. These data imply that dimer formation step is the rate limiting step of the two-to-one addition reaction. The reaction rate of linear units (product are dendritic units), kt, is about 30 times faster than the reaction rate of terminal units (product are linear units), kd. These differences of reactivity satisfy our designed requirements for the 3-bromophthalide-based ABB' monomer, will lead to enhanced DB, and consequently form pseudodendrimers on polymerization.

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Keywords

Enhanced Degree of Branching, Hyperbranched Polymer, Pseudodendrimer, Pseudodendrimeric Kinetics, Tandem Reactivity

Citation

Department

Chemistry