Rh-Mediated Carbene Polymerization: from Multistep Catalyst Activation to Alcohol-Mediated Chain-Transfer

Rh-mediated polymerization of carbenes gives access to new highly substituted and stereoregular polymers. While this reaction is of interest for the synthesis of syndiotactic polymers that are functionalized at every carbon atom of the polymer backbone, the catalyst activation, chain-initiation, and chain-termination processes were so far poorly understood. In this publication we present new information about these processes on the basis of detailed end-group analyses, dilution-kinetic studies, and a comparison of the activity of well-defined catalysts containing a preformed Rh–C bond. All data point toward complex catalyst activation processes under the applied reaction conditions. The use of well-defined Rh^I(cod)-alkyl, aryl, and allyl complexes does not lead to better initiation efficiencies or higher polymer yields. MALDI-ToF MS of the oligomeric fractions indicates that during the incubation time of the reaction, the precatalysts are first transformed into oligomer forming species with a suppressed tendency toward β-hydrogen elimination, and accordingly a shift to saturated oligomeric chains that are terminated by protonolysis. Further catalyst modifications lead to a shift from atactic oligomerization to stereoregular high molecular weight polymerization activity. Dilution-kinetic studies reveal that under diluted conditions two different active species operate that differ largely in their chain-termination behavior. Analysis of the reaction products by MALDI-ToF MS also allows conclusions about chain-initiation and chain-termination. Chain-initiation can occur by insertion of a preformed carbene into a Rh-ligand or Rh-hydride bond or by (internal or external) nucleophilic attack of water and/or alcohol on a Rh-carbene moiety. Chain-termination takes place mainly by (nucleophilic) protonolysis involving water or alcohols, while β-H elimination plays only a minor role and is only observed for the shorter oligomers. The detection of ethoxy and hydroxyl end-groups demonstrates the importance of trace amounts of water and ethanol toward chain-initiation. Alcohols further function as a chain-transfer agent, and increasing the alcohol concentration accelerates the chain-transfer process (which remains however relatively slow compared to chain-propagation). On the basis of the chemical properties of the alcohols, we propose a chain-transfer mechanism involving nucleophilic attack of the alcohol (nucleophilic, σ-bond metathesis type, protonolysis). This further allows us to draw some (careful) new conclusions about the oxidation state of the actual polymerization species.