Direct Probing of Regioregularity for Polycarbodiimide Systems via 15 N NMR Analysis James F. Reuther, † Joseph D. DeSousa, ‡ and Bruce M. Novak ‡, * † Department of Chemistry, North Carolina State University, Raleigh, North Carolina 27695, United States ‡ Department of Chemistry and Alan G. MacDiarmid NanoTech Institute, University of Texas at Dallas, Richardson, Texas 75080, United States * S Supporting Information ABSTRACT: After more than a decade of ambiguity, polycarbodiimides have been discovered to be fully regiore- gular when containing two sterically inequivalent pendant groups. To directly probe the regioregularity, a series of nitrogen-15 isotopically enriched polycarbodiimides with various combinations of pendant groups was synthesized using a variety of catalysts. Subsequent 15 N NMR analysis was performed on each of the labeled polymers to accurately determine the preferred regioisomer(s) and any particular bias present for monomer insertion. More sterically hindered substituents, i.e., aromatics, were found to be relegated to the imine nitrogen while the less hindered aliphatic groups were, in all cases, located on the amine nitrogen. No electronic biases were observed and the use of different titanium(IV) catalysts yielded the same regioisomer. Carbodiimides bearing sterically equivalent groups were polymerized to form regioirregular polymers with a 1:1 mixture of both regioisomers. ■ INTRODUCTION The monomer connectivity of polymer systems, i.e., the regioregularity, can play a large role in specific polymer properties. For this reason, the absolute determination of polymer microstructures is of considerable importance. For example, the electronic and photovoltaic properties of various polythiophenes are greatly affected by the extent of regioregularity along the polymer backbone. 1-6 Altering the degree of regioregularity in certain polyesters displays a significant change in the thermotropic liquid crystalline order of the polymer. 7 The formation of highly regioregular, fluorene- based polyolefins by acyclic diene metathesis (ADMET) allowed for the precise tuning of the polymer microstructure in order to amplify electroactive properties. 8 Extensive research has also been carried out on the formation of catalyst systems capable of polymerizing various olefins with high regio- and stereo-regularity in order to improve thermal properties of the polymer. 9-12 Polycarbodiimides, a class of rigid rod, helical macro- molecules with a wide variety of potentially interesting properties, have been a continuing area of interest ever since 1994 when Goodwin and Novak polymerized carbodiimide monomers in a living fashion using organotitanium(IV) catalysts. 13 Other catalyst systems employing various transition metals such as copper, 14 nickel, 15 and zirconium 16 have also been shown to polymerize carbodiimides. Polycarbodiimides adopt a 6/1 helical conformation 15 and, structurally, are a hybrid of polyisocyanates and polyisocyanides which both contain only one functional nitrogen atom per repeat unit. The presence of two nitrogen atoms per repeat unit, as is the case for polycarbodiimides, allows one to tailor polymers with a variety of substituents and provides ample opportunities to selectively tune specific polymer properties. In addition, the incorporation of asymmetric carbodiimides units can yield two possible regioisomers with each pendant group relegated to the imine or amine nitrogen, a short hand designation adopted for use in this paper, upon polymerization. (Scheme 1) The polymerization is hypothesized to be initiated by the insertion of one of the imine-π-bonds into the Ti-X bond. Upon initiation, the titanium center donates an initiator ligand (X in Scheme 1, typically either an alkoxide or amine ligand) to the electrophilic carbon of the carbodiimide, creating a titanium- (IV) amidinate complex. Sequential monomer insertions into the complex propagate the polymer in a living chain growth fashion. 13 The formation of the preferred regioisomer stems directly from the proposed mechanism of propagation. The monomer insertion can be governed by a particular bias created by steric values of pendant groups, substituent coordination to the catalyst center, and/or electronic effects of substituents, to name a few. If no bias exists, a regioirregular polymer will ensue with a 50/50 mixture of both regioisomers randomly placed throughout the backbone. Early identification of regioregularity in polycarbodiimides focused on the imine CN stretch in FTIR spectroscopy. Received: July 12, 2012 Revised: September 6, 2012 Published: September 19, 2012 Article pubs.acs.org/Macromolecules © 2012 American Chemical Society 7719 dx.doi.org/10.1021/ma301448b | Macromolecules 2012, 45, 7719-7728