Bulk polymerization of e-caprolactone (CL), 1,4-dioxan-2-one (PDX) and mixtures of PDX and CL was carried out by initiation with Al((OBu)-Bu-sec)(3) in a co-rotating twin-screw extruder through a fast single-step process. Both homopolymerizations and copolymerization of PDX and CL proceed very rapidly and reach almost complete (co)monomer(s) conversion as soon as 8 mol% of CL are added in the feed. Even though poly(1,4-dioxan-2-one) (PPDX) is known to thermally degrade mainly through unzipping depolymerization promoted from the hydroxyl endgroups and yielding PDX monomer, it turns out that the thermal stability of PPDX chains is substantially improved by the copolymerization of PDX with limited amounts of CL Interestingly, DSC analysis of the so-obtained P(PDX-co-CL) copolymers has demonstrated that a CL molar fraction as high as 11 mol% does not prevent the crystallization of the resulting copolymer, which retains a melting temperature close to 95 degrees C. This last observation has been explained by the formation of a blocky-like copolymer structure, in which short PPDX and PCL sequences are randomly distributed. (c) 2005 Society of Plastics Engineers.