The thermal properties and biodegradability of the block copolyesterethers with copoly[succinic anhydride (SA)/ethylene oxide (EO)], synthesized by ring-opening copolymerization as a hard segment and the triblock copolyethers of poly(oxyethylene)-poly(oxypropylene)-poly(oxyethylene)(PN) as a soft segment, were studied. The block copolyesterethers synthesized from higher than 8000 number-average molecular weight (M-n) of copoly(SA/EO)s showed a microphase separation structure as determined by the thermal properties [melting point (T-m) and glass transition (T-g)], at any polymer composition [EO/propylene oxide (PO)] or the determination of M-n of PN. A decrease in the M-n of copoly(SA/EO) or an increase in PO content in PN resulted in depression of heats of fusion (Delta H) of these block copolyesterethers. The enzymatic degradation of the block copolyesterethers by the lipase from Rhizopus arrhizus showed a substantial increase with a decrease in their Delta H, whereas it was depressed with an increase in the M-n of polyoxyethylene or polyoxypropylene segment in the block copolyesterethers. The block copolyesterethers were degraded by microorganisms in activated sludge. The biodegradability of the block copolyesterethers showed a pronounced drop, with an increase in the polyoxyethylene chain length or polyoxypropylene content in PN. The polycondensation was also conducted without a catalyst at 190 degrees C, similarly, to the reaction catalyzed with Ti[OCH(CH3)(2)](4) at 170 degrees C. The effect of the residual titanium on the biodegradability of the block copolyesterethers was negligible.