Full
control over the ceiling temperature (Tc) enables a selective transition between the monomeric and
polymeric state. This is exemplified by the conversion of the monomer
2-allyloxymethyl-2-ethyl-trimethylene carbonate (AOMEC) to poly(AOMEC)
and back to AOMEC within 10 h by controlling the reaction from conditions
that favor ring-opening polymerization (Tc > T0) (where T0 is the reaction temperature) to conditions that favor ring-closing
depolymerization (Tc < T0). The ring-closing depolymerization (RCDP) mirrors the
polymerization behavior with a clear relation between the monomer
concentration and the molecular weight of the polymer, indicating
that RCDP occurs at the chain end. The Tc of the polymerization system is highly dependent on the nature of
the solvent, for example, in toluene, the Tc of AOMEC is 234 °C and in acetonitrile Tc = 142 °C at the same initial monomer concentration of
2 M. The control over the monomer to polymer equilibrium sets new
standards for the selective degradation of polymers, the controlled
release of active components, monomer synthesis and material recycling.
In particular, the knowledge of the monomer to polymer equilibrium
of polymers in solution under selected environmental conditions is
of paramount importance for in vivo applications, where the polymer
chain is subjected to both high dilution and a high polarity medium
in the presence of catalysts, that is, very different conditions from
which the polymer was formed.
The understanding of cell-material interactions is important for creating personalized implants for tissue engineering. This has resulted in an interest in developing polymers with functional groups with the possibility of controlling the macromolecular surface. We have in a one-pot reaction synthesized a series of amorphous and degradable polyester-based copolymers with active functional groups by copolymerization of 2-methylene-1,3-dioxepane and glycidyl methacrylate. The properties of the final polymers were varied by varying the feed ratios of the monomers, and it was seen that it was possible to control the amount of active functional groups. The resulting epoxy-functionalized polyester was further modified by covalent immobilization of heparin. The heparinization was done in order, in a future aspect, to enhance the osteogenic differentiation of mesenchymal stem cells. Heparin binds directly with the growth factor bone morphogenetic protein-2 and helps to retain its activity. The molecular structure of the copolymers was characterized by nuclear magnetic resonance, size exclusion chromatography, and Fourier transform infrared spectroscopy. Differential scanning calorimetry and tensile testing showed that the monomer feed ratio had a great influence on the properties of the final polymer and that it thus was possible to control the mechanical properties to suit an intended application. The presence of heparin was verified by toluidine blue staining, and all of the films tested showed positive signals for heparin.
Tuning the properties of materials toward a special application is crucial in the area of tissue engineering. The design of materials with predetermined degradation rates and controlled release of degradation products is therefore vital. Providing a material with various functional groups is one of the best ways to address this issue because alterations and modifications of the polymer backbone can be performed easily. Two different 2-methylene-1,3-dioxepane/glycidyl methacrylate-based (MDO/GMA) copolymers were synthesized with different feed ratios and immersed into a phosphate buffer solution at pH 7.4 and in deionized water at 37 °C for up to 133 days. After different time intervals, the molecular weight changes, mass loss, pH, and degradation products were determined. By increasing the amount of GMA functional groups in the material, the degradation rate and the amount of acidic degradation products released from the material were decreased. As a result, the composition of the copolymers greatly affected the degradation rate. A rapid release of acidic degradation products during the degradation process could be an important issue for biomedical applications because it might affect the biocompatibility of the material. The cytotoxicity of the materials was evaluated using a MTT assay. These tests indicated that none of the materials demonstrated any obvious cytotoxicity, and the materials could therefore be considered biocompatible.
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