Dianhydride monomers, structure

Basically, the first approach to correlate the polyimide chain organization to the monomer structure was to take into consideration the electron affinity of the anhydride and the ionization potential of the diamine,10 as shown in Fig. 5.3. The strongest interactions between the polymeric chain are expected when the polyimide is prepared with the dianhydride having the highest electron affinity and die diamine with the lowest ionization potential. The strongest interchain interaction leads to high Tg and low solubility. 

One of the most attractive and successful attempts in attaining processable aromatic polyimides is the introduction of fluorine atoms in the polymer structure, either as substituents of carbon atoms on the polymer backbone (as mentioned before for perfluoroalkane containing polyimides), or as perfluoromethyl or perfluoroalkyl side substituents. The most popular approach has been the introduction of the hexafluoroisopropylidene group in the main chain as a bulky separator group in the dianhydride monomer or in the diamine one [70]. 

In addition to the polyimide structures discussed, dianhydride monomers have also been found to be useful in the preparation of polypyrroles. Reaction of dianhydrides with tetramines in polyphosphoric acid leads to these thermally stable polymers as indicated below. 

The investigation revealed that the incorporation of fluorinated side groups into polyimide membranes decreased their surface free energies (TgS), increased solubility parameters and FFVs, and therefore substantially enhanced gas permeability for CO2, O2, N2, and CH4 gases with reduced selectivity for CO2-CH4,02-N2, and CO2-N2 gas pairs depending on the structure of the dianhydride monomers. 

There have been many attempts to use pyrolysis in a controlled manner to produce molecularly well-defined materials an example is shown in Figure 4.8. Pyrolysis of the dianhydride monomer shown in Figure 4.8 under conditions of high temperature and high vacuum leads to the deposition of thin, lustrous, and chemically inert films which display an undoped conductivity of 250 (Qcm)" The expected structure was the ribbon polymer, poly(peri-napthalene), shown in Figure 4.8, but since hydrogen as well as carbon monoxide and carbon dioxide were eliminated this cannot be the real structure (12). 

Figure 1. (a) Chemical structure of dianhydride monomers, (b) Chemical structure of diamine monomers. 

FIG U RE 3.1 Chemical structures of various dianhydride monomers for preparation of SPIs. 

FIGURE 3.2 Chemical structures of sulfonated dianhydride monomers for preparation ofSPIs. 

Conditions for low temperature solution polymerizations of pyromellitic dianhydride (PMDA) have been developed for a wide variety of aromatic 1,4-phenylene [54, 55] and 4,4 -biphenylene [56-58] diamine monomers in a number of aprotic solvents to give high molecular weight prepolymers referred to as polyamic acids. Since the imidized structures are insoluble, they must be processed in the form of their polyamic acids which are subsequently imidized thermally or by chemical dehydrating agents. Although this procedure is acceptable for thin film or fibers, the fabrication of thick parts is complicated by the water of imidization. 

Kaneda et al. synthesized [61] a series of high molecular weight extended chain copolyimides (XV) by the reaction of PMDA and 3,3, 4,4 -biphenyltetra-carboxylic dianhydride (PPDA) with 3,3 -dimethyl-4,4 -diaminobiphenyl. Solvents used for the one-step synthesis to the fully cyclized imide structure were phenol, p-chlorophenol, m-cresol, p-cresol and 2,4-dicholorophenol. The polycondensations were performed at 180°C for 2h with a monomer concentration of 6% by weight and p-hydroxybenzoic acid used as a catalytic accelerator. A maximum of 50 mol % of PMDA could be used before the copolymer precipitated from solution. Reconstituted copolymers as isotropic dopes (8-10% by weight) in p-chlorophenol were dry-jet wet spun between 80 and 100 °C [62]. 

Further, since dianhydride and diamine structures of wide structural variety are commercially available, an extensive range of polyimide homo- and copolymers can be easily synthesized. This allows structural optimization for applications or fundamental studies. A representative, though far-from-complete, list of monomers can be found in Tables 13.1 and 13.2. 

Table 13.1. Chemical Structures of Dianhydrides from Polyimides Where One or Both Monomer Components Are Fluorinated...

Perusal of the property and chemical structure tables will demonstrate the astonishing range of diamines that are paired with the 6F dianhydride. The 6F group in the dianhydride imparts good solubility and nearly precludes significant molecular ordering. For these and other reasons, it is an ideal monomer to be paired with diamines burdened by specialized functional tasks. 

Monomer Reactivity. The poly(amic acid) groups are formed by nucleophilic substitution by an amino group at a carbonyl carbon of an anhydride group. Therefore, the electrophihcity of the dianhydride is expected to be one of the most important parameters used to determine the reaction rate. There is a close relationship between the reaction rates and the electron affinities, H, of dianhydrides (12). These Ea were independendy determined by polarography. Structures and electron affinities of various dianhydrides are shown in Table 1. 

Some diamines and dianhydrides with a flexible linkage in their structure have been listed in Table 2. The combination of those dianhydrides and diamines, and also the combination of some of them with conventional rigid monomers like benzenediamines, benzidine, pyromellitic dianhydride or biphenyl-dianhydride, offer a major possibility of different structures with a wide spectrum of properties, particularly concerning solubility and meltability [48-56]. 

Table 3 shows the Tg values and solubility of some selected polyimides among those prepared from monomers of Tables 1 and 2. The combination of non-pla-nar dianhydrides and non-planar, raefa-oriented aromatic diamines containing flexible linkages provides the structural elements needed for solubility and melt processability. Some aromatic polyimides marketed as thermoplastic materials are based on these statements [9,57-60]. 

Alston and Grants [19] described the structure-property relationship of polyimides derived from phenyl substituted monomers including the following dianhydride. 

The cyclopentane-type dianhydride CpDA was synthesized by thermal dehydration of c/s,c/s,cw,ds-l,2,3,4-cyclopentanetetracarboxylic acid at 170-190 °C. The norbomane-type dianhydride BHDA was prepared from endo-nadic acid via three steps.[5] The H-NMR spectrum revealed that BHDA consisted of the two isomers, bicyclo[2.2.1]heptane-2-emfo,3-era/o,5-exo,6-ejco-tetracarboxylic dianhydride and the all-exo derivative. The ratio was estimated to be 1 1 from the spectrum. The structures and abbreviations of alicyclic monomers are illustrated in Figure 1. 

Of particular interest is the use of 4,4 -diamino-2,2 -diphenylsulfonic acid [ 124-126] produced on a semi-industrial scale as a sulfonated monomer for preparation of polyimides. The reactions of a mixture of this monomer and 4,4 -diaminodiphenyhnethane and 4,4 -diaminodiphenyloxide with di-phenyloxide-3,3, 4,4 -tetracarboxyUc acid dianhydride resulted in sulfonated polyimides [ 124] with the following structure ... 

TAPOB was synthesized by reduction of l,3,5-tris(4-nitrophenoxy)benzene with palladium carbon and hydrazine in methanol [14], 6FDA was kindly supphed from Daikin Industries, Ltd. Pyromellitic dianhydride (PMDA) and 4,4 -oxidiphthahc anhydride (ODRA) were obtained from Daicel Chemical Industries, Ltd. and Manac Incorporated, respectively. TMOS, MTMS, 3-aminopropyltrimethoxysilane (APTrMOS), and N,N-dimethylacetamide (DMAc) were purchased from Aldrich. Chemical structures of monomers and alkoxysilanes are shown in Figure 8.2. 

---