Propagation and termination steps in Rh-mediated carbene polymerisation using diazomethane

Area of Science:

• Organometallic Chemistry
• Polymer Science
• Catalysis

Background:

• Rh(III) catalyst precursors enhance carbene (co)polymerization of diazomethane.
• Chain termination via β-hydride elimination is suppressed, but unsaturated chain ends form.
• Chain walking, leading to branched polymers, is not observed.

Purpose of the Study:

• Investigate the role of Rh(III) catalysts in diazomethane carbene polymerization.
• Elucidate the mechanisms of chain propagation and termination.
• Explain the observed linearity of the resulting polymers.

Main Methods:

• Experimental carbene (co)polymerization of diazomethane using Rh(III) precursors.
• Computational studies modeling active species and reaction pathways.
• Analysis of polymer yield, molecular weight, and chain end structures.

Main Results:

• Rh(III) catalysts increase polymer yield and molecular weight.
• Chain propagation via carbene insertion is energetically favored over β-hydride elimination and branched polymer formation.
• Lower molecular weight oligomers and dimers result from Rh(I) species favoring termination.

Conclusions:

• Rh(III) catalysts promote linear poly-methylene formation by favoring carbene insertion.
• Energetic barriers explain the absence of branching in experimental polymerizations.
• Non-selective catalyst activation leads to competing Rh(I) and Rh(III) species, impacting polymer characteristics.