Synthesis of Vinylic Macromolecular Rotaxane Cross-Linkers Endowing Network Polymers with Toughness

Macromolecular rotaxane cross-linkers having two radically polymerizable vinyl groups (RCs) were first synthesized and used to prepare network polymers. A crown ether/sec-ammonium-type pseudorotaxane initiator having an OH terminal-containing axle and a crown ether wheel with a vinyl group was subjected to the living ring-opening polymerization of δ-valerolactone followed by end-capping with a bulky isocyanate to yield a polyester axle-tethering macromolecular [2]rotaxane cross-linker (RC). Rotaxane cross-linked polymers (RCPs) were prepared by the radical polymerization of n-butyl acrylate in the presence of RCs (0.25, 0.50 mol %). The properties of the RCPs and covalently cross-linked polymers (CCPs) were characterized mainly by mechanical properties. Both fracture stress and strain values of RCPs were much higher than those of CCPs, probably owing to the increased network homogeneity by the rotaxane cross-link. The hybrid-type RCPs obtained from a mixture of RC and covalently connected cross-linker (CC) showed poorer mechanical properties similar to that of CCPs, indicating the importance of RCs in increasing the toughness of the network polymers.