Transition Metal Dimer Complexes in Reactions with Polymers

Monomerization of Transition Metal Dimer Complexes in Reactions with Polymers 

Different routes are defined by the reaction conditions, nature of the functional groups, MX , and the level of macroligand crosslinking. For example, at a low level of crosslinking (ca. 2% of divinylbenzene), the polymer is flexible enough and the dimer structures B and C form or remain unchanged on complex formatiotL Dimer complexes were nearly unobserved for a 20% of divinylbenzene crosslinked polymer, and coordinatively unsatinated structures (A) can possibly form as well. 

The nature of the fimctional groups of the macroligands is also of great importance. For instance, monomeric and polymeric amines dissociate the [Rh(CO)2RS]2 dimer complex in contrast to thiols that yield addition products. Similarly, the nature of the solvent affects chain flexibility and long-range action. For example, the Co complex with a crosslinked PS polymer has a Co L composition of 1 1 (L-monomer unit) in weakly swelling n-hexadecane, whereas a thermodynamically stable 1 2 complex forms in xylene. This observation may be a result of the fact that a PS support is glassy at 70°C in n-hexadecane and gum-like in xylene. 

Therefore, for highly flexible polymers, metal complex dimerization occurs either in a single chain (linear polymer) or in the chain site between the crosslinked knots. It was reported that a ratio of mononuclear and binuclear complexes of Cu with polymeric acids depends on the degree of neutralization of the latter low charge density provides binuclear complexes and monomer formation as the charge density increases. 

In general, many properties of MMC, including catalysis, depends on a type of complex that is formed (monomer or dimer). Unfortunately, contradictory data do not allow generalized conclusions. 

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