Branched renewable polymers via metathesis and thiol-ene additions

Highly branched macromolecules, from monodisperse dendrimers to less defined hyperbranched polymers, find multiple applications in different areas of polymer chemistry [1]. Due to the high number of chain ends present within these macromolecules, their properties are highly influenced by the chain-end functionalities. Therefore, hydrophobic branched polymers can, for instance, be rendered water-soluble by introduction of suitable hydrophilic groups at the chain-ends. Polymers with such properties could be able to encapsulate hydrophobic drugs while maintaining their water solubility, being thus potentially useful as drug delivery vehicles. However, this application field requires not only efficient delivery systems, but also ones being biodegradable and biocompatible. In this way, plant oil derived platform chemicals are useful building blocks for the synthesis of branched polymers presenting the promise of biodegradability and biocompatibility.
The objective of this work is the synthesis of plant oil based branched macromolecules. For this purpose, olefin metathesis and thiol-ene click chemistry are useful and versatile chemical transformations. Olefin metathesis has developed into one of the most powerful tools for the synthetic chemist [2] and thiol-ene additions, which are undergoing an exciting revival, offer a straightforward way to functionalize alkenes [3]. Therefore, unsaturated fatty acid derivatives are perfect starting materials for such conversions In this work, two different approaches to branched macromolecules are presented. First, the step-wise synthesis of a fully renewable dendrimer is described by alternating thiol-ene and condensation chemistry. Second, the synthesis of a first generation dendrimer is attempted taking advantage of the selectivity of cross-metathesis between acrylates and terminal olefins.

[1] R. Hourani, A. Kakkar, Macromol. Rapid Commun., DOI10.1002/marc.200900712; B. I. Voit, A. Lederer, Chem. Rev., 2009, 109, 5924. [2] T. M. Trnka, R. H. Grubbs, Acc. Chem. Res., 2001, 34, 18. [3] A. B. Lowe, Polym. Chem., 2010, 1, 17.