Polymerization of Alkynes in Aqueous Systems

As for other coordination polymerizations, the utilization of water as a reaction medium raises issues such as catalyst stability towards water, and miscibihty of the monomers, polymers and the catalyst with water. Concerning the catalyst stability towards water, to date polymerizations of alkynes in aqueous systems have only been reported with rhodium and iridium catalysts. Unfortunately, the mechanism of polymerization with these complexes in non-aqueous, organic reaction media has not been completely elucidated (vide supra). 

In the cyclotrimerization of propynoic acid, trimellitic (l,2,4-fb,l l3((X)2H),) and trimesic acid (l,3,5-CgH3(CO2H)3) are obtained as the product of head-to-head or head-to-tail linkage, respectively. For the sake of comparison, reactions were conducted both in neat water and in neat THF. In all cases, the yield of the trimer was higher in the aqueous system, evidencing the high tolerance of the rhodium catalyst towards water. 

Subsequently, Tang et al. studied the polymerization of phenylacetylene and of para-substituted phenylacetylenes with lipophilic and water-soluble catalysts [148]. 

Besides this work with rather lipophilic catalyst precursors, two water-soluble analogues have been investigated [Rh(tos)(cod)(H2O)] (30 tos = tosylate) and the similar compound [Rh(tos)(nbd)(H20)] (31) [148]. The authors have verified that water is still the best reaction medium for polymerization of phenylacetylene compared to toluene, THF and neat monomer, thus indicating a possible involvement of water in the formation of the active catalyst (830 TO in water). With catalyst precursor 30, low polydispersities and a cis content of 90% are found, whereas catalyst precursor 31 affords a larger polydispersity but a higher cis content (100%). Interestingly, these catalysts proved to be tolerant to air since similar polymerization results are obtained with tap water without exclusion of air. 

Joo et al. utilized the highly water-soluble rhodium complex [RhCl(CO)(TPPTS)2] for the polymerization of terminal alkynes (phenylacetylene and (4-methylphe-nyl)acetylene for the structure of TPPTS cf. Section 7.2.2.3) [150]. This catalyst selectively produces cis-transoid polymers at room temperature in homogeneous solution in water/methanol mixtures, as well as in biphasic mixtures of water and chloro- 

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