SmI2-catalyzed polyaddition of 1,12-dodecanedial followed by treatment with benzaldehyde gives a polyester (1) containing three structural units, [OCH2-(CH2)(10)-CH2O], [OCH2-(CH2)(10)-CO], and [CO-(CH2)(10)-CO], in the main chain and an OCH2Ph end-capping group. GPC analysis of 1 shows molecular weights of M-n = 5.5 x 10(3) and M-w = 14 x 10(3). The H-1-NMR spectrum reveals the polymer structure with the COOCH2Ph end group as well as the M-n value (2.6 X 10(3)) calculated based on an amount of the end-capping group and lower than that estimated from GPC. Mixtures of terephthalaldehyde and 1,12-dodecanedial in several molar ratios undergo similar polyaddition catalyzed by SmI2 to give the corresponding copolyesters after treatment with benzaldehyde. Increase in the ratio of 1,12-dodecanedial causes increase in yield and molecular weight of the copolymer. Terephthalaldehyde shows a tendency to give alcohol-side -O-R-O- unit in the polyester, whereas 1,12-dodecanedial is mostly incorporated as the acid-side -CO-R’-CO- unit. Terminal aldehyde group derived from dodecanedial is capped effectively by benzaldehyde to give the COOCH2Ph end group.