Cyclopolymerization of Disiloxane-Tethered Divinyl Monomers To Synthesize Chirality-Responsive Helical Polymers
Synthetic polymers mimicking the stimuli-responsive coiled-coil structures of naturally occurring macromolecules such as motor proteins remain challenging. Here we report that chirality switchable quasi-double helical polymers can be synthesized by the free radical cyclopolymerization of the tetramethyldisiloxane-tethered divinyl monomers. Two cyclopolymers (pbSt and pbMA) are obtained in this work, which in solid state are helical polymers investigated by both ECD and VCD. According to VCD analysis, the C–H deformation modes including C–H bending (mode I) and C–H rocking (mode II) in the main-chain helix show strong signals in VCD spectra and the plots of the division of VCD by IR, which have different responses to solvation histories or thermal treatment compared with other vibrational signals arising from side rings (especially signals from tetramethyldisiloxane groups). After thermal treatment, all the side-ring VCD signals of both pbSt and pbMA are inverted, while the VCD signals from mode II of the main chain are unchanged. In cast films from chloroform solutions, pbSt shows only and inverted VCD signals arising from mode I and mode II of the main-chain helix, but not any VCD signals from side rings. Ab initio quantum calculation reveals that the change in the orientation of side-ring distortion against the main chain may account for the inversion of side-ring VCD signals. The responses of the experimental VCD spectra of the cyclopolymers can thus be rationalized based on the quantum calculation results. The main-chain helices of the cyclopolymers remained unchanged, while the side rings changed their distortion orientation during thermal treatment. In cast films from chloroform solutions, the polymer main-chain helix was inverted, hence inverted main-chain VCD; while the side rings might be randomly distorted, causing loss of VCD from side rings. Overall, the main-chain helices and side-ring pseudohelices can be switched independently. This work presents the first example that a single macromolecule can display such (quasi-)double-helical conformation. Besides the theoretical value, this kind of chirality switchable quasi-double-helical polymer may find application in asymmetric catalyst, smart molecular devices, etc.
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