Macromolecular design and architecture of aliphatic polyesters

Abstract: Public health care has reached a level where tissue or organ reconstruction by means of biodegradable short-term implants via e.g. tissue engineering will be practicable in the near future. The vital issue now is to be able to reproducibly fabricate and design new materials with the appropriate properties and three-dimensional shape, and to facilitate their sterilization. In this thesis, macromolecular design and polymer architecture techniques are used to synthesize well-defined polymers with narrow molecular weight distributions (MWD), and to control the reactions upon sterilization, and the degradation rate and profile.A model system for the ring-opening polymerization of L-lactide (LLA) initiated by a spirocyclic tin initiator was developed to synthesize star-shaped polymers. It was shown that an increase in temperature and a decrease in the dielectrical constant of the solvents increased the reaction rate. The versatility of the spirocyclic tin initiator system was subsequently established, by homopolymerization of ?-caprolactone (CL) and 1,5-dioxepan-2-one (DXO) and this system was compared with a conventional system using stannous octoate (Sn(Oct)2) and pentaerythritol ethoxylate. Two different strategies were assessed for the two initiator systems for the synthesis of copolymers that are random or blocky in their nature. Random copolymers with distinct sequence lengths were synthesized using both initiator systems, together with block poly(DXO-co-LLA) and poly(CL-co-LLA) with narrow MWD.Three different types of copolymers of LLA, CL and DXO were synthesized using Sn(Oct)2 and ethylene glycol. A solvent casting and particulate leaching technique was developed and applied to construct porous scaffolds of the copolymers. The porous scaffolds were subsequently sterilized using electron beam or ?-irradiation and it was shown that the reactions induced by radiation can be used to tailor the end-properties of the materials.Homo- and copolymers of CL and DXO with different macromolecular designs (triblock and multiblock) and different polymer architectures (linear and cross-linked) were synthesized and degraded in a phosphate buffer solution for up to 364 days. By altering the network composition, the release pattern of acidic degradation products was controlled, where an increase in DXO content led to an increase in the release of both monomeric hydroxy-acids. Varying the distribution of the more hydrolysis-susceptible DXO-sequences in the linear copolymers also enabled the amount of monomeric hydroxy acids released to be controlled, where the triblock copolymer showed the highest release.