Effect of Side Group Chemistry on the Properties of Biodegradable l-Alanine Cosubstituted Polyphosphazenes

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Abstract
Biodegradable polyphosphazenes have been investigated for a variety of applications, such as controlled drug delivery matrixes, tissue-engineering scaffolds, membranes, and bone-type composites. In this study we have evaluated the effect of side group chemistry on the properties of biodegradable phosphazene polymers that contain ethyl alanato side groups together with ethyl glycinato, p-methylphenoxy, or p-phenylphenoxy side groups. The polymers were synthesized by a macromolecular substitution route. The molecular weights of aryloxy/amino acid ester cosubstituted polymers were much higher than the amino acid ester substituted polyphosphazenes described earlier. Polymer properties, such as glass transition temperature, hydrolytic degradation, surface wettability, tensile strength, and modulus of elasticity varied over a wide range following changes to the type of co-substituents on the polymer backbone. The glass transition temperatures varied from −10 to 35 °C and increased with the bulkiness of the side groups. Polymer films in phosphate buffer saline solution showed molecular weight declines ranging from 58% to >80% and mass loss ranging from 4% to 90% over a period of 7 weeks. Water contact angles for polymer films varied from 63° to 107°, with the highest angles for the alanine ethyl ester and p-phenylphenoxy cosubstituted polyphosphazene. The tensile strengths were in the range of 2.4−7.6 MPa and the modulus of elasticity was in the range of 31.4−455.9 MPa. Thus, in this study we have demonstrated the tunability of biodegradable polyphosphazenes to suit a range of biomedical applications.