Ligand variation, kinetic investigations, and computational studies have been used to elucidate the mechanism of rhodium-catalyzed diazoalkane polymerization. Variations in the "N,O" donor part of the catalyst precursors (diene)Rh-1(NO) result in different activities but virtually identical molecular weights, indicating that this part of the precursor is lost on forming the active species. In contrast, variation of the diene has a major effect on the nature of the polymer produced, indicating that the diene remains bound during polymerization. Kinetic studies indicate that only a small fraction of the Rh (1-5%) is involved in polymerization catalysis; the linear relation between polymer yield and M, suggests that the chains terminate slowly and chain transfer is not observed (near living character). Oligomers and fumarate/maleate byproducts are most likely formed from other "active" species. Calculations support a chain propagation mechanism involving diazoalkane coordination at the carbon atom, N-2 elimination to form a carbene complex, and carbene migratory insertion into the growing alkyl chain. N-2 elimination is calculated to be the rate-limiting step. On the basis of a comparison of NMR data with those of known oligomer fragments, the stereochemistry of the new polymer is tentatively assigned as syndiotactic. The observed syndiospecificity is attributed to chain-end control on the rate of N-2 elimination from diastereorneric diazoalkane complexes and/or on the migratory insertion step itself.