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

Abstract

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 19 F 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 CHCl 3 solutions.

Facile Synthesis of Rod–Coil Block Copolymers with Chiral, Helical Polycarbodiimide Segments via Postpolymerization CuAAC “Click” Coupling of Functional End Groups

“…The opposite terminus of the polymer was previously shown to become protonated upon precipitation in MeOH during work-up as evident of MALDI-TOF MS. 40 The proton associated with this end group is also visible at 3.96 ppm in the 1 H NMR of PPMC-1. To confirm this, the polymer was redissolved in THF-d 8 , and ∼2 drops of D 2 O were added to the NMR sample, causing the peak to disappear in the spectra.…”

“…The appearance of the single peak at 21.0 ppm in the 31 P NMR of this initiator confirms the formation of the desired trans-substituted nickel(II) complex and is comparable to previously reported chemical shifts of similar complexes ( Figure S1). 40,49 The 1 H NMR, 13 C NMR, and HRMS-ESI characterization data for Ni-1 can also be found in the Supporting Information. This complex was applied in the polymerization of the chiral PMC monomer forming the rigid-rod, mesogenic polycarbodiimide PPMC-1 with the protected alkyne functional end group (Scheme 2).…”

“…Adding small amounts (∼0.5 vol %) of specific additives, in this case 1,8-diazabicycloundec-7-ene (DBU), to the GPC solvent mobile phase has been shown to decrease the affinity of polycarbodiimides, but some trailing is still observed causing the slightly larger than expected Đ values reported herein. 40 1 H NMR integration of the o-methyl substituent on the end group relative to the methine hydrogen on the pendant group provided accurate estimations of M n for subsequent "click" postpolymerization modifications when coupled with GPC. Unfortunately, neither the terminal alkyne proton nor the resulting triazole proton following the click reactions is observable in the 1 H NMR spectra most likely due to signal overlap with chemical shifts corresponding to pendant methyl group and pendant aromatic protons, respectively.…”

“…37−39 Recently, we reported the new living polymerization of carbodiimides initiated and mediated by air and moisture stable bis(triphenylphosphino)aryl nickel(II) bromide complexes. 40 In a similar manner to the alkoxide ligand on Ti(IV) initiators, the η 1 -aryl ligand acts as the necessary initiator ligand transferring from the Ni(II) metal center to the electrophilic center carbon of the carbodiimide monomer. Upon work-up, the aryl ligand remains covalently attached to the terminus of the polymer chain as confirmed by 1 H NMR and MALDI-TOF MS.…”

Synthesis of amphiphilic diblock copolymers of isotactic polystyrene-block-isotactic poly(p-hydroxystyrene) using a titanium complex with an [OSSO]-type bis(phenolate) ligand and sequential monomer addition

“…[13][14][15][16][17][18][19][20][21] Macroinitiators were prepared from end-functionalized polymers available, which initiate or catalyze living polymerizations of desired monomers to obtain welldened diblock copolymer. [22][23][24][25][26] Moreover, click coupling reaction of two end-functionalized macromolecular chains has been employed to synthesize diblock copolymers in recent years.…”