Hydrolyzable Aromatic Copolyesters of p-Dioxanone G. Giammanco, A. Martı ´nez de Ilarduya, A. Alla, and S. Mun ˜ oz-Guerra* Departament d’Enginyeria Quı ´mica, Universitat Polite ` cnica de Catalunya, ETSEIB, Diagonal 647, 08028 Barcelona, Spain Received June 23, 2010 Entropically driven ring-opening copolymerization of mixtures of a fraction of cyclic oligo(hexamethylene terephthalate)s composed of cycle sizes from 2 to 5 and p-dioxanone was used to prepare random copolyesters covering a range of aromatic (HT) to aliphatic (DO) units ratios from 9 to 1.3. The composition and microstructure of the copolyesters were accurately determined by 1 H and 13 C NMR, respectively. The copolyesters showed thermal degradation and glass transition temperatures in good agreement with their comonomeric composition and microstructure, and they crystallized for contents in DO less than 30%, adopting the same crystal structure as poly(hexamethylene terephthalate). The copolyesters appeared to be sensitive to hydrolytic degradation, which was observed to take place superficially with the generation of non-water-soluble degraded fragments and with the release of water-soluble dioxanoic acid to the aqueous medium. Introduction Semicrystalline aromatic polyesters are polymers displaying excellent mechanical and thermal properties, which combined with other specific properties such as for example high transpar- ency or thermal stability, confer them an outstanding techno- logical interest. 1,2 Poly(ethylene terephthalate) (PET) and poly(butylene therephthalate) (PBT) are the best known mem- bers of this family, both of them being widely used in a large variety of industrial applications, either as homopolyesters or as copolyesters containing minor amounts of a diversity of second comonomers. 2 However, the utilization of these materials in niches requiring either biodegradability or easy practical chemical recycling of the used polymer has been prevented by their extremely high reluctance to chemical and biological degradation. On the other hand, poly(hexamethylene tereph- thalate) (PHT) is a member of the terephthalate family that has not found practical use to date. This compound has considerably lower T m and T g than PET and PBT due to the higher chain flexibility provided by the hexamethylene segment but continues being highly resistant to hydrolysis and biodegradation. The relative low T m of PHT facilitates its handling and processing and places it in an advantageous position to be explored for its potential in frontier biomedical applications. Very recently, copolymers of PHT containing lactic acid have been shown to be hydrolyzable under physiological conditions 3 and random copolyesters of PHT with ε-caprolactone have been demon- strated to be sensitive to biodegradation by P. fluorescens. 4 In this paper we wish to report on copolyesters of PHT containing p-dioxanone (DO). The polyester of p-dioxanone (PDO) is well-known in the biomedical field as a biocompatible polymer usable for the manufacture of bioreadsorbible surgical sutures and prostheses. 5 It is well-known that PDO is readily hydrolyzed in aqueous medium under physiological conditions, and this is the degrading mechanism that seems to operate mainly in the in vivo degradation of the polyester. 6-8 Further- more, Nishida et al. isolated several PDO-degrading bacteria that utilize PDO as the only carbon source. 9 PDO is a semicrystalline polymer melting around 105 °C and showing glass transition in the -15 to -8 °C range. Its low stiff modulus makes it flexible enough to be used as a monofilament. Some aliphatic copolymers of DO with other aliphatic lactones have been synthesized, and their reabsorbing properties were evalu- ated as a function of composition. 10,11 Copolymers of PHT and PDO are expected to combine beneficially the properties of the two parent homopolyesters, and exploring this approach is indeed the aim of this work. The target is to generate aromatic-aliphatic copolyesters displaying good thermal and mechanical properties but being sensitive to hydrolysis under physiological conditions. Such a pattern of behavior will be the first step toward a development of these materials for biomedical applications requiring reabsorbability. Although aromatic units have been inserted in PDO to improve the resistance to γ-radiation, 11 copolyesters of PDO containing terephthalate units have not been investigated so far. Entropically driven ring-opening polymerization (ROP) 12-16 will be used here for the synthesis of the copolyesters made from HT and DO. This method, which has been traditionally applied for the synthesis of certain aliphatic polyesters, 17-20 including the synthesis of PDO, is achieving recently increasing importance as an alternative option for the synthesis of aromatic polyesters. We will make use of this synthetic procedure instead of traditional two-step melt polycondensation because lower temperatures are required for the reaction, which prevents DO monomer degradation and volatilization. The preparation of PET, PBT, PPT, and PHT by ROP of their corresponding cyclic oligo(alkylene phthalate)s is well described in the literature. 21-24 In our preceding works on this subject, fractions of cyclic oligo(hexamethylene terephthalate)s differing in the average ring size were prepared and copolymerized with ε-caprolactone to obtain copolyesters with composition and microstructure es- sentially unaffected by cycle size. 4,25 In this work we are using a cyclic HT fraction composed of two- to five-membered rings; the ROP of these cycles can be carried out at temperatures notably lower than those required for carrying out conventional polycondensation minimizing, therefore, the depolymerization of PDO, which is known to begin above its melting temperature. * To whom correspondence should be addressed. E-mail: sebastian.munoz@upc.edu. Biomacromolecules 2010, 11, 2512–2520 2512 10.1021/bm1007025  2010 American Chemical Society Published on Web 08/04/2010