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.
Compared to rigid microspheres that consist, for example, of polystyrene or silica, soft and deformable elastomer microspheres can be used to generate colorless transparent films upon evaporating the solvent from microsphere-containing dispersions. To obtain tough films, a post-polymerization reaction to crosslink the microspheres is usually necessary, which requires extra additives during the drying process. This restriction renders this film-formation technology complex and rather unsuitable for applications in which impurities are undesirable. In the present study, it is demonstrated that tough elastomer microspheres that are crosslinked with rotaxanes can form tough bulk films upon evaporation of water from microsphere dispersions, so that post-polymerization reactions are not required. The results of this study should thus lead to new applications including coatings for biomaterials that need complete removal of all impurities from the materials prior to use.
Three component mobility controlling vinylic rotaxane crosslinkers with two radically polymerizable vinyl groups (RC_Rs) were synthesized to prove that the mobility of the components of the RC_Rsp laysacrucial role in determining the properties of rotaxane-crosslinked polymers (RCPs). RC_Rs( R= H, Me,o rE t) were obtained from living ringopening polymerization. RCP_Et was prepared using RC_Et, which exhibits the lowest component mobility.T he low component mobility is reflected in inferior mechanical strength and stretching ability in tensile stress tests compared to components with good (R = Me) and high (R = H) mobility. However, RCP_Et exhibited significantly higher stress and strain values than the corresponding covalentlyc rosslinked polymers (CCP_Rs). These results indicate that as uitable component mobility substantially enhances the mechanical strength of RCPs.T his behavior could serve as ag uiding principle for the molecular design of advanced RCs.
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