Synthesis and characterization of telechelic oligoethers with terminal cinnamylidene acetic acid moieties

Purpose: The formation of photoresponsive hydrogels were reported by irradiation of star-shaped poly(ethylene glycol)s with terminal cinnamylidene acetic acid (CAA) groups, which are capable of a photoinduced [2+2] cycloaddition. In this study we explored whether oligo(ethylene glycol)s and oligo(propylene glycol)s of varying molecular architecture (linear or star-shaped) or molecular weights could be functionalized with CAA as terminal groups by esterification or by amide formation.

Methods: Oligo(ethylene glycol) (OEG) and oligo(propylene glycol) (OPG) with varying molecular architecture (linear, star-shaped) and weight average molecular weights between 1000 and 5000 g.mol-1 were functionalized by means of esterification of hydroxyl or amine endgroups with cinnamylidene acetic acid (CAA) or cinnamylidene acetyl chloride (CAC) as telechelic endgroups. The chemical structure, thermal properties, and molecular weights of the oligoethers obtained were determined by NMR spectroscopy, UV spectroscopy, DSC, and MALDI-TOF.

Results: CAA-functionalized linear and star-shaped OEGs or OPGs could be obtained with a degree of functionalization higher than 90%. In MALDI-TOF measurements an increase in Mw of about 150 g∙mol(-1) (for each terminal end) after the functionalization reaction was observed. OEGCAA and OPGCAA showed an increase in glass transition temperature (Tg) from about -70°C to -50°C, compared to the unfunctionalized oligoethers. In addition, the melting temperature (Tm ) of OEGCAA decreased from about 55°C to 30°C, which can be accounted for by the hampered crystallization of the precursors because of the bulky CAA end groups as well as by the loss of the hydroxyl telechelic end groups.

Conclusion: The synthesis of photoresponsive oligoethers containing cinnamylidene acetic acid as telechelic endgroup was reported and high degrees of functionalization could be achieved. Such photosensitive oligomers are promising candidates as reactive precursors, for the preparation of biocompatible high molecular weight Polymers and polymer networks.