Synthesis and characterization of hyperbranched aromatic poly(ether imide)s with varying degrees of branching

A series of high molecular weight hyperbranched poly(ether imide)s (HBPEI), derived from the AB₂ monomer N-[3,5-di(tert-butyldimethylsilyloxy)phenyl]-4-fluorophthalimide (1), were synthesized in a single-step process involving a rapid fluoride-catalyzed arylation reaction. NMR analysis of the degree of branching shows that changes in the polymerization reaction time, from 2.5 to 20.0 min, result in hyperbranched polymers with degrees of branching ranging from 66% to 42%, respectively. Reactions of selected model compounds show that changes in the degree of branching are a result of molecular rearrangements occurring via a transetherification mechanism. Such rearrangements lead to an increase in M_w while M_n remains relatively constant. Correlations between rheological data and the degree of branching of the hyperbranched poly(ether imide)s were observed. For solutions of the hyperbranched poly(ether imide)s, rheological measurements showed that shear thinning and normal stress effects increased as the degree of branching decreased and that birefringence as a function of shear rate increased with decreasing degree of branching. While strong correlations between the degree of branching and rheological properties exist, light scattering GPC measurements show that M_w increases significantly with reaction time. This variation in M_w suggests that the differences in rheological properties may be largely due to changes in molecular weight and makes it difficult to determine the role that changes in the degree of branching, which are manifested in altered molecular architecture, have on macroscopic physical properties.