Polyimides

It is interesting that formation of a linear aromatic polyimide was observed as early as 1908 when a polyimide was formed by heating 4-aminophthalic anhydride:

Formation of polymers, however, was at the time considered undesirable, so the material was not pursued [87]. It was learned since that an imide link is more thermally stable than an amide one and that polyimides can be very useful materials. Many polyimides were developed since. Aromatic structures in the polymeric backbone raise the melting temperatures and yield stiffer and tougher materials. Most sought-after polyimides, therefore, are products from aromatic tetra acids (or dianhydrides) and aromatic diamines. Many commercial preparations of aromatic polyimides include a preliminary step of forming polyamic acids first [88]:

This is followed by imidation, often after the polymer has been applied to a substrate as a coating or was cast as a film:

The polyamic acids are usually prepared in solution. Suitable solvents are N,N-dimethyl formamide, dimethyl sulfoxide, and n-methyl-2-pyrrolidone. The reactions require anhydrous conditions at relatively low temperatures, like 50C (or lower). Some, however, need high temperatures, as high as 175C[88]. The two reagents are combined in solution. The order of addition and reagent purity can influence the molecular weight of the products that may range from 13,000 to 55,000 [88]. Some imidation accompanies the first step. It is desirable that during polyamic acid formation, the degree of imidation does not exceed 50%. The step of conversion of polyamic acids to polyimides can take place at about 300C in thin films. With cyclizing agents, however, it can take place at much lower temperatures [88]. Lozano et al. reported on an improved method for forming high molecular weight polyimides [88]. They used in situ sililation of the diamines to carry out the reaction. Trimethylchlorosilane is added to the diamine solution prior to the addition of the electrophilic monomer. Commercially, the most commonly used aromatic dianhydrides are pyromellitic dianhydride and benzophenone tetracarboxylic dianhydride. The common amines in industrial practices are meta-phenylene diamine, methylene dianiline, and oxy-dianiline. A typical polyimide preparation can be shown as follows:

The above shown polymer melts above 600C and is heat stable up to 500C in an inert atmosphere. It is sold under the trade name of Kapton. A different polyether-imide sells under the trade name of Ultem. Another route to polyimides is through reactions of diisocyanates with dianhydrides [89, 90]:

The reaction is kept between 120 and 200C[91]. In place of isocyanates, it is also possible to use aldimines or ketimines, as well as silylated diamines.

When silylated diamines are used, trimethylsilyl esters of polyamic acids are formed first and then desilylated with methanol. Due to increased solubility of silylated aromatic amines, the initial condensations and formations of polyamic acid trimethylsilyl esters can be done in various solvents, yielding high molecular weight polymers. The highest molecular weights are obtained in dimethyla- cetamide at 50°C. Other solvents like tetrahydrofuran and chloroform can be used as well, though they appear to yield slightly lower molecular weight products [91].

The films of the silylated precursors of polyamic acid convert directly by heat treatment to yellow, transparent, and tough films of aromatic polyimides with the elimination of trimethylsilanol [91]. Other preparations of polyimides include the use of di-half esters of tetracarboxylic acids [92]:

Polyimides can also be prepared by reactions of diimides with dihalides [93]:

Reactions of bis maleimides with compounds containing active hydrogens can also lead to formations of polyimides [94]:

Also, unsaturated diimides can be reacted with sulfur halides to form polyimides [95]:

Diepoxides can add to pyromelitimide in the presence of a base to form polyimides with pendant hydroxyl groups [96]:

Tertiary amines and quaternary ammonium halides catalyze this reaction. Acetylation of the pendant hydroxyl groups of the product yields polymers that are soluble in solvents like dioxane and dimethylformamide. Pyromelitimide can also add to double bonds to form polyimides [97]:

Polyimides also form by photo additions of aliphatic or aromatic bis maleimides to benzene [98]. The reactions involve 2 + 2 cycloadditions that yield homoannular diene intermediates. Diels–Alder additions follow and result in formations of the polymers:

Diels–Alder reactions yield other polyimides, as, for instance, the following [99]:

Also, N, N-bis(ethoxycarbonyl) pyromelitimide condenses with diamines to yield polyimides [100]:

The first step in the above preparation takes place in solution. After casting a film, the second step takes place at 240C under vacuum. Interesting polyimides also form from reactions of 2,20,6,60 biphenyl tetracarboxylic acid anhydride [101] with aromatic diamines, like 4,40-diaminodiphenyl ether:

As in the previous cases, the polyamic acid forms first in solution at approximately 40C. It is converted to the polyimide by heating in acetic anhydride at reflux for 18 h. Heterocyclic dianhydrides, like pyrazine tetracarboxylic dianhydride, also react with diamines to form polyimides [102]. Such polymers are harder to form, however. Two types of thermoset polyimides are currently prepared commercially. They are based on low molecular weight bis imides such as bis maleimides or bis-5-norbornene-2,3-dicarboximides. Due to unsaturations, the materials cross-link by free-radical mechanism into tight networks. Michael type additions of primary and secondary amines to the bis maleimides are often used to chain-extend them before cross-linking. This reduces the cross-linking density and the brittleness [115]. The materials are designated by the term PMR, for polymerizable monomeric reactants. Ueda et al. [102] synthesized a polyimide from 3,6-bis-(4-aminophenylenesulfanyl) pyridazine.

The polymer has a high refractive index, high transparency, and is aimed at optical devices.

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مواضيع ذات صلة

Poly(p-xylylene) s

Polyacetals and Polyketals

Phenoxy Polymers

Aromatic Polyamide-Imides and Aromatic Polyester-Imides

Aromatic Polyamides

Special Reactions for Formation of Polyamides

Fatty Polyamides

Nylons

Polyesters from Lactones

Polycarbonates

Network Polyesters for Surface Coatings

Linear Unsaturated Polyesters