With the aim to develop novel biodegradable materials with good flexibility and fast degradation rate, random copolymers of -caprolactone (CL) and p-dioxanone (PDO) with a full range of compositions were synthesized in bulk using stannous octoate as the ring-opening catalyst. The chemical composition and number average sequence lengths of CL and PDO units determined by H-1-NMR were used to correlate with various properties of the copolymers. Although both CL and PDO are crystalline components, only one crystalline phase could be present for each copolymer. The low limit of average block length for the copolymers that could crystallize is 3.22 for L-CL and 3.43 for L-PDO, respectively. The crystallinity and crystalline morphology of the copolymers are dependent on the crystalline component as well as its number average sequence length. Irrespective of composition, all the copolymers have good solubility in chloroform with glass transition temperature much below room temperature, implying good flexibility of the materials. The incorporation of PDO component could significantly increase the water wettability of the copolymer surfaces and thereby accelerate the degradation rate of the materials. In conclusion, flexible biodegradable polymers with adjustable degradation and crystalline properties were acquired by random copolymerization of CL and PDO, which are expected to use in tissue engineering and drug delivery fields. (c) 2013 Wiley Periodicals, Inc.