Amphiphilic Star Polymers with a Hyperbranched Core

Amphiphilic Star Polymers with a Hyperbranched Core 

In the search to develop new materials for immobilization of homogeneous transition metal catalyst to facilitate catalyst-product separation and catalyst recychng, the study of dendrimers and hyperbranched polymers for application in catalysis has become a subject of intense research in the last five years [68], because they have excellent solubility and a high number of easily accessible active sites. Moreover, the pseudo-spherical structure with nanometer dimensions opens the possibility of separation and recycling by nanofiltration methods. Although dendrimers allow for controlled incorporation of transition metal catalysts in the core [69] as well as at the surface [70], a serious drawback of this approach is the tedious preparation of functionalized dendrimers by multi-step synthesis. 

Starting from a hyperbranched polyester based on 4,4 -bis(hydroxyphenyl)valeric acid, terminal -OH groups were derivatized to yield the hyperbranched macroinitiator. The Hgand precursor was introduced as the first block in the grafting from reaction, followed by 2-methyl-2-oxazoHne polymerization to give the second block and allow for water-soluble polymers. The triphenylphosphine-functionalized am-phiphihc star block copolymer was obtained after transformation of the iodoaryl 

In a first test reaction, the polymer support was used for the hydroformylation of 1-octene in water in the presence of Rh(CO)2acac. Within a reaction time of 10 h at 120°C and a H2/CO (1 1) pressure of 5 MPa, conversion of up to 94% was obtained. The n iso ratio was 3 1, sometimes reaching 9 1. The quantities of 3-and 4-nonanal resulting from olefin isomerization, were below 5% [74]. The versa-tihty of the polymer support was further demonstrated with the Heck reaction of styrene and iodobenzene in the presence of Pd(OAc)2 as the metal catalyst The reaction was again performed in pure water at 50 °C and product yield of up to 80% was obtained after 20 h reaction time [75]. 

Recently, Mecking et al. reported the synthesis of inverse micelles based on a hy-perbranched polyglycerol polymer. Terminal -OH groups were modified with palmi-toyl chloride and gave a polymeric catalyst soluble in organic solvents with hydrophilic core to immobilize water-soluble guest molecules such as PdCl2 or Pd(OAc)2. 

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Scheme 6.10 Synthesis of the amphiphilic star polymers with a hyperbranched core and amphiphilic diblock graft arms [59].

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