Non-toxic catalysts for ring-opening polymerizations of biodegradable polymers at room temperature for biohybrid materials

Abstract

Biodegradable biopolymers are versatile materials with a wide range of applications including suture, scaffolds, growth chambers and matrix materials for tissue-engineering and regenerative medicine. Their synthesis is usually based on tin-salts at high temperatures. Strategies to replace tin are interesting due to the toxicity of tin for aquatic organisms and potentially problematic effects in higher organisms. In the framework of this study we investigate nontoxic iron catalysts (iron chloride, bromide and perchlorate) and their potential to facilitate ring-opening polymerizations (ROP) at room temperature. To find the best combination of a catalyst and an initiator for the ROP of ε-caprolactone, three iron-catalysts were analysed with four different initiators (water, isopropyl alcohol, benzyl alcohol and 2-allyl phenol). Functional entities such as the allyl group are interesting for bioconjugation and may allow functional access to functional biohybrid materials. The polymerizations were carried out solvent free in bulk and yielded polymers in the range 10–20 kDa for large monomer to catalyst/initiator ratios analysed by GPC, NMR and MALDI-TOF. We find that commercial iron salts can replace tin salts in many cases and allow for the ROP of ε-caprolactone at room temperature while tin(II) 2-ethylhexanoate Sn(Oct)₂ does not facilitate ROP at room temperature. The use of iron based ROP catalysts opens interesting access to biopolymers utilizing a nontoxic metal ion at low temperatures in a solvent free reaction scheme saving energy and the environment.