The ring-opening polymerization of the cyclic ester amide (cEA, 2) (systematic name, 1-oxa-7-aza-cyclotridecane-8,13-dione)-prepared from adipic anhydride and 1-amino-5-pentanol-in the melt at temperatures above 145 degreesC with Bu2Sn(OMe)(2), Ti(OBU)(4), Al(O-sec-Bu)(3), or Sn(octoate)(2) as initiator yields the poly(ester amide) (PEA, 3) (systematic name, poly(5-(5-oxypentylcarbamoyl)pentanoate) with regular microstructure. This poly(ester amide) is a semicrystalline material with a melting point of 108 degreesC. The elementary chain growth reaction proceeds by a coordination insertion mechanism in analogy to the polymerization of lactones. The monomer-to-initiator ratio and the conversion determine the number-average molecular weight. By using hydroxy telechelic poly(ethylene oxide) with Sn(octoate)2 as initiator poly(ester amide)-block-poly(ethylene oxide) and poly(ester amide)-block-poly(ethylene oxide)-block-poly(ester amide) were obtained. Kinetic studies for different monomer-to-initiator ratios, different temperatures and initiators reveal that the ring-opening polymerization is a first-order reaction with respect to the monomer and shows no termination and transfer reactions.