Stimuli- ​responsive switchable networks: Click and un-​click reaction of furanyl-​functionalized (co)​poly(2-​oxazoline)​s with bis(maleimide)

Co)poly(2-oxazoline)s have been thoroughly investigated due to their multifaceted properties that can be varied and manipulated by the incorporation of dedicatedly substituted repetition units [1]. The crosslinking of functionalized copoly(2-oxazoline)s can be performed either in polymeranalogous fashion or in situ during the polymerization [2].
In order to find new synthetic routes for ‘switchable’ self-healing networks, the monomer 2-furanyl-2-oxazoline has been synthesized according to the Witte-Seeliger reaction protocol from the reaction of 2-furonitrile and ethanol amine. This monomer can undergo microwave-assisted cationic ring-opening polymerizations, and the poly(2-furanyl-2-oxazoline)s as well as corresponding copoly(2-oxazoline)s can be crosslinked by UV-induced Diels-Alder click-reaction of the furan moiety with (at least) bisfunctional maleimides (Figure 1) [3].
The crosslinking reaction may be subjected to click-unclick cycles involving the so-called retro-Diels-Alder reaction, namely the cleavage of the crosslinked bonds in the course of increased temperatures. The density of the networks can be quantified by swelling degrees: dense networks have comparably low swelling degrees. Hence, this type of switchable copoly(2-oxazoline)-based networks can be applied as sensors if dyes or other tracers are included in the network, which are released upon a temperature stimulus.