Poly , thermomorphic

Soluble polymers can be used in a variety of ways in liquid/liquid systems. An approach originally developed in our group concerned polar polymers in liquid/liquid systems whose miscibility changes with temperature. The first of these so-called thermomorphic liquid/liquid systems studied used polar polymers like poly(AT-isopropylacrylamide) (PNIPAM) or poly(ethylene oxide) (PEO) [151]. Experiments using the dye-labeled PNIPAM (110) or PEO (111) where the dye served as a surrogate for a catalytic species, showed that these polymers had excellent (>500 1) phase-selective solubility in the polar phase of an equivolume thermomorphic mixture of heptane and 90% aqueous ethanol. 

Dendrimers too can be used in thermomorphic systems. In a report describing dendrimer-bound Pd(0) catalysis [163], Kaneda compared the liq-uid/liquid thermomorphic separation scheme with more established membrane and solvent precipitation procedures. Starting with commercially available third-, fourth-, and fifth-generation poly(propylene imine) dendrimers, the primary amine groups at the periphery were converted into chelating phosphines. The resulting phosphines were in turn allowed to react with Cl2Pd(PhCN)2 to form Pd(II) complexes that were reduced by hydrazine in the presence of triphenylphosphine to form the Pd(0) catalyst 126 (Eq. 63). This catalyst was successfully used in the allylic amination shown in Eq. 64. In this example, solvent precipitation, membrane filtration, and thermomorphic liq-uid/liquid separation were all used to recycle 126. The latter procedure proved to be simplest with the best recovery of active catalyst. 

A second nucleophilic catalyst supported by PtBS is the polymer-bound di-methylaminopyridine analog that was also used in latent biphasic catalysis with the poly(JV-alkylacrylamide) support 129 [131]. This example of a nucleophilic catalyst (133) was used to catalyze formation of a t-Boc derivative of 2,6-di-methylphenol (Eq. 70). In this case, the extent of recovery of the catalyst and the yields of product were directly comparable to those seen with thermomorphic systems. The isolated yield for the first five cycles of this reaction were 34.3, 60.9,82.2,94.6, and 99%. In this case we reused catalyst 133 through 20 cycles. Yields after the first few cycles were essentially quantitative (ca. 93% average for each of 20 cycles). Separation of the polymer from the aqueous ethanol phase was quantitative as judged by either visual observation or UV-visible spectroscopic analysis. 

The first application of dendrimers in a thermomorphic system was described by Kaneda and co-workers. Poly(propylene imine) dendrimer-bound Pd(0) complexes were synthesized by reduction of dendritic Pd(II) systems with hydrazine and used for the allylic substitution of trans-cinnamyl acetate with dibutylamine [Eq. (5)] [14]. 

The idea of thermomorphic catalysts, which is largely similar to the concept of catalysts with modified poly(ethylene oxide)s, was implemented by the example of modified poly(A -isopropylacrylamides) (PNIPAMs). At low temperatures, PNIPAMs can dissolve in water and at high temperatures, in two-phase systems, PNIPAMs pass into the organic phase, and also retain partly in water (Fig. 11-4). The upper critical solution temperature depends on substituents at the amide group [83, 91-93]. Several macromolecular metal complexes with PNIPAM ligands have been designed. 

Palladium nanoparticles, stabilized in micelles formed by polystyrene-co-poly(ethylene oxide) copolymer (PS-PEO) and acetylpyridinium chloride (CPC) as a surfactant, have been used to catalyze heterocyclization of N-methylsulfonyl-o-iodoaniline with phenylacetylene leading to formation of a substituted indole. The activity of the colloidal palladium catalytic system is comparable to that of the low-molecular-we ht palladium complexes, whereas the stabUity of the colloidal palladium system is much h her. The reuse of the catalyst PS-PEO-CPC was demonstrated in experiments with fresh starts as well as by thermomorphous separation of the catalyst from products (20060M154). 

Lin, S.-T, Fuchise, K., Chen, Y. et al. (2009) Synthesis, thermomorphic characteristics, and fluorescent properties of poly[2,7-(9,9-dihexylfluorene)]-block-poly(N-isopropylacrylamide)-block-poly(N-hydroxyethylacrylamide) rod-coil-coil triblock copolymers. Soft Matter, 5,3761-3770. 

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