Macromolecular metamorphosis of thermoplastic poly(thio)urethanes containing boronic acid esters

Abstract

Biodegradable thermoplastic polyurethanes (TPUs) are promising materials for vascular grafts, due to their excellent mechanical properties and the possibility to incorporate degradable moieties. Especially degradability is important in the field of tissue engineering (TE) or regenerative medicine, because the synthetic scaffold should be replaced by human tissue after a certain time. We synthesized thermoplastic polythiourethanes (TPTUs) based on degradable chain extenders and tested their mechanical, thermal and degradation characteristics. As soft-block we used polytetrahydrofuran (pTHF) and as hard-block the aliphatic hexamethylenediisocyanate (HMDI) in combination with a variety of chain extenders. The focus was set on the boronic acid-based dithiol 2,2′-(1,4-phenylene)-bis[4-mercaptan-1,3,2-dioxaborolane] (BDB). As dithiol-based reference 1,2-bis(2-mercaptoethoxy)ethane (BMEE) was used and bis(2-hydroxyethyl)-terephthalate (BHET) as well-known ester-based reference chain extender. By using BDB as degradable chain extender we could accelerate the degradation rate in terms of mass erosion at 37 and 90°C compared to BHET-based TPUs. However, a strong increase in molecular weight was observed under degradation conditions. Further investigation by ATR-FTIR revealed a release of benzene-1,4-diboronic acid (BDBA) in the polymer. This leads to the formation of a hydroxy-terminated polyol, which then undergoes an intramolecular functional group metamorphosis with the thiourethane group to form a crosslinked polymer. BMEE-based TPTUs in contrast show excellent mechanical properties, even better than BHET-TPUs but a slightly lower mass erosion at 37 and 90°C.