Polycaprolactone (PCL) is a biodegradable synthetic polymer that is currently used in a number of biomedical applications. A number of concerns have been raised over the toxicity of initiators commonly employed for the synthesis of PCL. Therefore, more biocompatible initiators have been studied. The biocompatibility of PCL, itself, is adequate; however, improved bioactivity is desirable for several applications. Copolymerisation, and incorporation of bioactive fillers can both be used as ways of enhancing the bioactivity of PCL. Therefore, the global objective of this project was to enhance the bioactivity of PCL by copolymerisation of PCL with poly(ethylene glycol) (PEG) using a biocompatible calcium-based initiator. This calcium-initiator was expected to leave potentially bioactive calcium-initiator residues in the synthesised copolymers. A study of the ring-opening polymerisation of epsilon-caprolactone (CL) in the presence of a poly(ethylene glycol) (PEG) / calcium hydride (CaH2) co-initiation system was performed. Polymerisation kinetics were monitored by following the degree of conversion of CL by Fourier transform-Raman (FT-Raman) spectroscopy and 1H nuclear magnetic resonance spectroscopy (NMR). Resultant PCL-b-PEG-b-PCL (PCL/PEG/PCL) triblock copolymers were analysed by NMR and gel permeation chromatography (GPC). The observed rates of polymerisation for the synthesis of PCL/PEG/PCL triblock copolymers using the PEG / CaH2 co-initiator were much lower than expected. 1H NMR and Raman microspectroscopy analysis showed that the concentration of the active calcium-PEG alkoxide was much lower than the initial feed concentration of PEG. Even so, the molecular weight of PCL/PEG/PCL triblock copolymers could be predicted from the CL : PEG feed ratio. This was found to be due to a fast reversible transfer process. Inductively coupled plasma-atomic emission spectroscopy (ICP-AES) analysis of solutions containing acid digested, pure PCL/PEG/PCL copolymers showed calcium concentrations at equal to or greater than 77 % of the calcium feed concentration. These calcium-initiator residues were isolated and their structures confirmed by Fourier transform infrared-attenuated total reflectance spectroscopy (FTIR-ATR). They were found to be a mixture of calcium hydroxide (Ca(OH)2) and calcium carbonate (CaCO3). The effect of calcium-initiator residues on the in vitro mineralisation of PCL/PEG/PCL triblock copolymers, as well as the same effect on a model calcium-salt-doped PCL homopolymer system, was studied by immersion in simulated body fluid (SBF). In the model studied, PCL homopolymer was doped with low concentrations (0.2 - 2 w / w % Ca) of Ca(OH)2, or CaCO3. Results from the model study showed calcium phosphate (CaP) mineral deposition on Ca(OH)2-doped PCL, and not on CaCO3-doped PCL. This was attributed to the higher solubility of Ca(OH)2, compared to CaCO3. Minimal CaP deposition was observed on PCL/PEG/PCL triblock copolymers. This was attributed to the low Ca(OH)2 concentration in these samples. For all mineralised samples in the SBF studies, the formation of carbonated HAP was observed. Overall, the synthesis of PCL/PEG/PCL copolymers using the PEG / CaH2 co-initiator was found to be a suitable method for preparing reproducible materials. The calcium-based initiator was also found to have potential for increasing the bioactivity of PCL-based materials.
| Identifer | oai:union.ndltd.org:ADTP/265497 |
| Date | January 2006 |
| Creators | Colwell, John Michael |
| Publisher | Queensland University of Technology |
| Source Sets | Australiasian Digital Theses Program |
| Detected Language | English |
| Rights | Copyright John Michael Colwell |
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