Dendrimers and Hyperbranched Polymers
Highly branched polymeric materials with large number of end groups can offer unique physical properties. Dendrimers differ from linear polymers in viscosity and thermal behavior. A variety of applications have been forecast for these highly branched globular macromolecules. One early approach to dedrimer synthesis relied upon the Michael reaction using repeated sequential additions of an amine to a,ẞ-unsaturated ester [248]:
In the first step, ammonia is added to an excess of methyl acrylate. The product is reacted with an excess of ethylenediamine to form a star molecule with three arms. The product of the second step is then reacted with an excess of methyl acrylate. This is followed by a reaction with an excess of ethylenediamine to form a molecule with six arms. The sequences are repeated to yield a star polymer. Such star polymers are sometimes referred to as starburst polymers.
A recent publication described preparation of dendrimers from functional aliphatic polyesters that are based on 2,2-bis(hydroxymethyl)propionic acid [249]. N,N'-dicyclohexylcarbodiimide was used as the coupling agent in a double-stage convergent approach that reduced the number of synthetic and liquid chromatographic steps required in the preparations and purifications. The hydroxyl functional dendrimers were then subjected to a variety of surface modifications by reactions with different acid chlorides [249].
Another publication describes formation of polyphenylene dendrimers that formed nanoparticles with 102 benzene rings [250]. In this preparation, an A-B type monomer, 2,3,4,5-tetrakis(4- triisopropylsilylethynylphenyl)-cyclopenta-2,3-dienone, is condensed in a [4 + 2] cycloaddition with 3,3',5,5'-tetraethynyl-biphenyl:
The above condensation reactions were carried out at 180-200°C in aromatic solvents [250]. Preparation of dendrimers usually involves numerous synthetic steps. Some attempted to simplify the process by merely forming hyperbranched polymer. The products have less perfect structures than dendrimers, but offer the advantage of being synthesized in only one step. A recent publication describes preparation of hyperbranched poly(e-caprolactone) s [251]. Living ring opening polymerizations were carried out with the aid aluminum benzyl oxide as the initiator. The resultant polymers were then functionalized with benzylidine-protected 2,2'bis(hydroxymethyl)propionic acid. After a subsequent deprotection, a-carboxylic-o-dihydroxyl functional AB2 macromonomers formed. These AB2 polyesters were then condensed into hyperbranched polymers through a room- temperature esterification synthesis using dicyclohexylcarbodiimide and 4-(dimethylamino) pyridinium 4-toluenesulfonate [251].
