Investigation of SNAr for the synthesis of poly(aryl ether)s
Nucleophilic aromatic substitution by an addition-elimination process (SNAr) has been widely studied to produce high temperature poly(aryl ether)s and poly(aryl sulfide)s. The number and types of electron-withdrawing activating groups has been expanded by the drive for high temperature polymers to include appropriately substituted benzoxazoles, benzimidazoles, azomethines, and many others. All of the groups known to activate SNAr are strong electron withdrawing groups with electronegative atoms that can accept the electron density upon attack to form a Meisenheimer complex intermediate.
We have been looking at alternative means of promoting an SNAr polymerization by investigating fluoro-substituted monomers that lack obvious strongly electron withdrawing activating groups. In the course of our preliminary research, we have found that a phthalide ring, while not a traditional electron-withdrawing group, can serve as an activating group by accepting electron density to produce a stable ring-opened Meisenheimer complex upon nucleophilic attack.
(D. M. Knauss and J. T. Bender, "Phthalide as an Activating Group for the Synthesis of Poly(aryl ether phthalide)s by Nucleophilic Aromatic Substitution", J. Polym. Sci., Part A Polym. Chem., 2002, 40, 3046-3054.)
Scheme 1. Phthalide as activating group for SNAr polymerization.
By lowering the energy of the Meisenheimer complex intermediate, the reaction is able to proceed and expulsion of fluoride ion results in the formation of an ether linkage. Numerous bisphenols were used in this reaction to make poly(aryl ether phthalide)s of different compositions. Under the reaction conditions investigated, polar aprotic solvents (DMPU, DMAc) and temperatures up to 200 C, only low molecular weight polymers (6000-7000 g/mole) could be achieved. While that corresponds to conversions of aryl fluoride up to 98%, high molecular weight requires the reaction to be closer to quantitative.
Continuing this area of research, we have begun to look at aryl sulfides as activating groups. Sulfides, with unshared electrons, are well-established electron donating groups for many reactions. However, by exploiting potential dp-pp interactions (or other low lying unoccupied molecular orbitals interacting with the p system), sulfide can be considered as a group capable of accepting electron density. In the nucleophilic aromatic substitution polymerization to form poly(phenylene sulfide) at high temperatures, it has been demonstrated that a sulfide can activate SNAr, although no further investigation of this capability has been investigated. We are looking to expand this idea to look at aryl sulfide activated compounds to further understand this reaction. So far we have demonstrated 2,7-difluorothianthrene as an activated monomer for polymerization by SNAr. The thianthrene monomer has been shown to produce high molecular weight, thermally stable, high glass transition polymers.
(J. B. Edson; D. M. Knauss, "Thianthrene as an Activating Group for the Synthesis of Poly(aryl ether thianthrene)s by Nucleophilic Aromatic Substitution", J. Polym. Sci., Part A: Polym. Chem., In Press (2004).)