Three types of copolymers were synthesized and characterized. First, triblock ABA copolymers [where A is a homopolymer of epsilon-caprolactone and B is poly(ethylene glycol)] were prepared by the ring-opening polymerization of poly(ethylene glycol) with epsilon-caprolactone in the presence of stannous octoate (Sn(Oct)(2)). The spectral, thermal, and mechanical properties of one sample of these copolymers were studied, and it was discovered that these types of copolymers were more hydrophilic, possessed lower melting points, and had superior mechanical properties (greater toughness) than poly(epsilon-caprolactone). Second, triblock ABA copolymers [where A is a homopolymer of L-lactide and B is poly(ethylene glycol)] were prepared by the ring-opening polymerization of poly(ethylene glycol) with L-lactide in the presence of Sn(Oct)(2). The mechanical properties of these copolymers were studied, and it was found that they were tougher and softer than poly(L-lactide). Third, novel ABA triblock copolymers [where A is a copolymer of epsilon-caprolactone and L-lactide and B is poly(ethylene glycol)] were prepared, and H-1-NMR and C-13-NMR spectra of these copolymers indicated a microblock structure for the two end blocks. The stress-strain behavior revealed low yields and high toughness for these copolymers.