Special Reactions in Homogeneous Aqueous Systems

For obvious reasons, cationic and anionic polymerizations, as well as any other technique that propagates through water-sensitive intermediates, are not applicable to emulsion conditions. Currently, only radical emulsion processes are used commercially. 

In order to avoid these side reactions, the most obvious modification would be to remove the phosphine ligands altogether [16]. This can be accomplished, although not without penalty, because the reduced palladium centers tend to form colloidal particles that eventually show attenuated activity. The details of this phosphine-free approach have been reported [17]. Convenient sources of Pd° include CpPdft/ -CaHs) and Pd2(dba)3 CsHs (where dba = dibenzylidene acetone). Some of these reactions can be exceedingly fast (99% yield with 0.02 % Pd2(dba)3 CeH, in 45 min). 

The reactive intermediates used in chain-growth polymerizations include radicals, carbanions, carbocations, and organometallic complexes. Of the three common metal catalyzed polymerizations - coordination-insertion, ring-opening metathesis and diene polymerization - the last appears to possess the greatest tolerance toward protic solvents. The polymerization of butadiene in polar solvents was first reported in 1961 using Rh salts [18]. It was discovered that these polymerizations could be performed in aqueous solution with an added emulsifier (sodium dodecyl sulfate, for example). 

This Rh-catalyzed reaction is selective for the formation of highly crystalline rrans-l,4-polybutadiene. The activity of the catalyst shows a marked dependence 

It was discovered that the addition of 1,3-cyclohexadiene to the Rh -catalyzed reactions increased the rate of butadiene polymerization by a factor of over 20 [20]. Considering the reducing properties of 1,3-cyclohexadiene, this effect could be due to the reduction of Rh to Rh and stabilization of this low oxidation state by the diene ligands. With neat 1,3-cyclohexadiene, Rh is reduced to the metallic state. These emulsion polymerizations are sensitive to the presence of Lewis basic functional groups. A stoichiometric amount of amine (based on Rh) is sufficient to inhibit polymerization completely. It was also discovered that styrene could be polymerized using the Rh catalyst. However, the atactic nature of the polymer, along with the kinetic behavior of the reaction, indicated that a free-radical process, rather than a coordination-insertion mechanism, was operative. 

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