Controlled Living Polymerization of Carbodiimides Using Versatile, Air-Stable Nickel(II) Initiators: Facile Incorporation of Helical, Rod-like Materials

The new polymerization of carbodiimides using two, simple [bis­(triphenylphosphino)­aryl]­nickel­(II) bromide complexes has been discovered to occur in a controlled, living fashion. These initiators are substantially more air and moisture stable compared to their titanium­(IV) counterparts making them significantly easier to synthesize, purify, and utilize. The polymerization is initiated via aryl ligand transfer to the electrophilic center carbon of the carbodiimide. Sequential insertions of the carbodiimide π-bond into the nickel–nitrogen amidinate coordination bond propagates the polymer chain in a living chain growth manner as evident by the linear relationship in the plots of percent conversion vs M n, ln­([M]o/[M]) vs time, and monomer: initiator ratio vs M n. The transferred aryl ligand was confirmed to be appended to the terminus of the polymer chain by MALDI–TOF and 19F NMR. This added control element offers new opportunities to end functionalize rigid-rod, helical polycarbodiimides. This new technique also provides the ability to generate the active Ni­(II) initiation sites on potentially any aryl bromide species for the facile incorporation of rod-like, helical polycarbodiimides into such systems as block copolymers, graft copolymer, polymer functionalized surfaces, etc. To demonstrate this, poly­(4-bromostyrene) was employed as a polymer-supported aryl bromide source to generate the active [bis­(triphenylphosphino)­aryl]­nickel­(II) bromide macroinitiator. The “grafting from” reaction was then carried out upon addition of the chiral (S)-PEMC monomer forming the excess single-handed helical polycarbodiimide appended graft copolymer. The morphology of this novel polymer system was studied using TMAFM, revealing nanofibular aggregation behavior when spin coated from dilute CHCl3 solutions.