3158
Macromolecules 1998, 31, 3158-3161
Linear and Hyperbranched m-Polyaniline: Synthesis of Polymers for the Study of Magnetism in Organic Systems
Scheme 1
Nikolaos Spetseris, Rachel E. Ward, and Tara Y. Meyer* Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260 Received March 4, 1998
There is a developing interest in preparing organic polymers that have the conductive or magnetic properties more often associated with metals. While the study of conducting polymers has now progressed to the point of producing commercially viable products,1 the investigation into magnetism is still in its early stages and has yet to produce a material that exhibits the requisite long-range ferromagnetic coupling of unpaired electrons.2 However, it is clear that progress in this field, even more than in conducting systems, depends directly on the regularity of the polymers since the transmission of spin information must proceed serially through bonds down the polymer chain.3 m-Polyaniline and derivatives thereof have been acknowledged as some of the most important target polymers since they represent an optimum combination of a stable radical site and short ferromagnetic coupling unit.4-7 Despite its potential importance in the study of magnetic polymers and despite previous attempts at its synthesis, the mpolyaniline that has been reported appears to exhibit extensive cross-linking and possible regioirregularity. Herein, we report the preparation of well-characterized, soluble m-polyaniline prepared by a pathway that minimizes the potential for cross-linking and precludes regioirregularities. We also describe the preparation of the first example of a new class of polymers that we believe will be interesting magnetic substrates, 1,3,5hyperbranched polyanilines. Our strategy is to employ the elegant palladiumcatalyzed coupling strategy that has recently been developed into a useful reaction by the Hartwig and Buchwald groups.8,9 Previous efforts to prepare the meta-substituted polymer have focused on the Ullmann coupling to form the aryl-N linkages.10 Although this reaction is useful for the synthesis of small molecules that can be easily purified,6,7,11-13 the application of this strategy to the preparation of polymers has produced intractable materials whose insolubility has prevented complete characterization.10 m-Polyaniline was prepared both by palladiumcatalyzed condensation of 3-bromoaniline to give polymer 1a and by reaction of the monomers 1,3-dibromobenzene and 1,3-diaminobenzene to give polymer 1b (Scheme 1). Both polymerizations were carried out in THF at 90 °C in the presence of BINAP ligand and 1.1 equiv of NaO-t-Bu per amine. After neutralization, the polymers were isolated by precipitation using hexanes. The crude polymer was then extracted with hot CH2Cl2 to remove the majority of the free BINAP ligand. The tan polymers were isolated in >95% yield.14 CH2Cl2 extraction also removes low molecular weight oligomers and therefore decreases the PDI relative to the crude sample. GPC molecular weights of these polymers in both NMP and THF are reported in Table 1.
Scheme 2
Polymers 1a and 1b have significant solubility in THF and NMP,15 moderate solubility in toluene and benzene, and poor solubility in CH2Cl2, CHCl3, and alkanes. The hyperbranched polymer, 2, was prepared in a 35% yield by an analogous palladium-catalyzed condensation of 3,5-dibromoaniline. The yield decrease is attributed to the increased solubility of the polymers in the CH2Cl2 used for extraction. A model trimer, 3,16,17 was also synthesized for comparison with the linear polymers. The reaction of 1 equiv of m-dibromobenzene with 2 equiv of aniline for 24 h gave a 98.8% GC yield18 and a 75% isolated yield after column chromatography (Scheme 2). The reaction mixture also contained small amounts of unreacted aniline (0.7%), diphenylamine (0.4%), N-methyldiphenylamine (1.2%, possibly arising from alkoxide methyl transfer), biphenyl (
