A variety of aliphatic- and aromatic-type unsaturated polyesters (UPs) were prepared by transition-metal-catalyzed single-component polycondensation of bis(diazoacetate)s under a mild condition. With the second-generation Grubbs catalyst, the polycondensation proceeded exclusively through an intermolecular highly cis-selective C=C forming coupling of diazo-bearing carbons with N-2 release, giving well-defined UPs. The cis-C=Cs of the resulting polymers could be isomerized quantitatively into trans-C=Cs with a catalytic amount of diethylamine. Additionally, other metal complexes, the first-generation Grubbs catalyst, rhodium(II) acetate, and copper(II) acetylacetonate, also produced UPs from the bis(diazoacetate)s, with lower stereoselectivities, although an unexpected carbene oligomerization of the monomers partially occurred along with the C=C bond-forming coupling.