Chemistry Reference
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Scheme 10.5 Synthesis of the UPy reversibly unfolding modular cross-linker.
folding cooperativity. Therefore, we designed UPy-based reversibly unfolding
modules for our bulk polymer designs (Kushner et al. 2007). To ensure good rever-
sibility for the refolding, we followed the design of our peptidomimetic DCL system
and designed UPy-based DCL cross-linkers.
The synthesis of the module is provided in Scheme 10.5 (Kushner et al. 2007).
Double alkylation of ethyl acetoacetate followed by guanidine condensation afforded
alkenyl -pyrimidone intermediate 24 (Kushner et al. 2007). Isocyanate 25 was
coupled to pyrimidone 24 to yield 26. Upon dimerization in DCM, RCM effectively
cyclized the two UPy units (Mohr et al. 1997; Weck et al. 1999). A one-pot reduction
and deprotection through hydrogenation using Pearlman's catalyst gave diol module
27. Finally, capping 27 with 2-isocyanatoethyl methacrylate at both ends provided
the UPy sacrificial cross-linker 28, which was thoroughly characterized by
1
H- and
13
C-NMR, Fourier transform IR (FTIR), and mass spectrometry.
In our initial study we chose poly(n-butyl acrylate) as the backbone because this
polymer exhibits elastomeric properties upon cross-linking (glass-transition
temperature ¼ -548C; Brandrup and Immergut 1998) and can be easily synthesized
by free-radical polymerization. A series of transparent rubbery films was prepared by
radical copolymerization of n-butyl acrylate with desired amounts of cross-linker 28
using AIBN as the initiator. In control experiments, a poly(ethylene glycol) (PEG)
dimethacrylate (M
n
¼ 750) was used as the cross-linker instead. When fully stretched,
this molecule has approximately the same length as the fully unfolded 28. The ther-
mosets were characterized by FTIR and differential scanning calorimetry. The films
were then cut with a “dogbone” die and subjected to tensile testing on an MTS tester.
The comparison of the mechanical properties of the UPy samples and the PEG
controls demonstrates that the introduction of our biomimetic module into the
network dramatically enhanced the polymer mechanical properties. As shown in
the stress-strain curves (Fig. 10.8), the network containing the biomimetic cross-
linker has significantly higher modulus, tensile strength, and toughness than the
