Synthesis technology of diamine monomers for polyimide raw materials and structure design of low dielectric polyimide

With the development of 5G extremely high-frequency communication, stringent performance challenges for interlayer insulation materials for flexible printed circuit boards (FPCBs) are on the rise. Traditional polyimide films can not fully comply with high-frequency communication materials because of their high loss and dielectric constant. The article will cover the synthetic technology of diamine monomers based in the polyimide type and the design of low-dielectric polyimide structures.

The syntax of diaminodiphenyl ether synthesis involves condensation reaction, amination reaction, and purification technology. Condensation reaction synthesis can take place by mainly 3 ways: 

① dinitrobenzene coupling method

② p-nitrochlorobenzene condensation method

③ p-nitrophenol salt condensation method. 

Taking dinitrobenzene coupling method as an example, this method generates dinitrodiphenyl ether intermediates (DNDPE) through the coupling reaction of p-dinitrobenzene. 

This synthesis process is to react p-dinitrobenzene with potassium carbonate in a solvent of dimethyl sulfoxide (DMSO) to obtain DNDPE. However, since DNDPE contains nitro groups, the increase in DNDPE concentration in the later stages of the reaction will further react with p-dinitrobenzene, resulting in the formation of by-products, which reduces the selectivity, so the yield is only about 60%. In addition, p-dinitrobenzene is expensive, highly toxic and explosive, making it undesirable for industrial mass production.

The p-nitrochlorobenzene condensation method uses 1-chloro-4-nitrobenzene (CNB) as a raw material, and is catalyzed by sodium nitrite, sodium formate and copper catalyst to finally produce DNDPE. The reaction mechanism of this method is divided into two main stages: the first stage is the oxidation and hydrolysis of CNB. Under the action of sodium nitrite and sodium formate, CNB is first oxidized and hydrolyzed to generate sodium p-nitrophenolate (4-NPS); the second stage is the Ullmann condensation reaction. Under the catalysis of copper catalyst, NPS and another molecule of CNB undergo Ullmann condensation reaction to form DNDPE. This process involves the breaking and formation of chemical bonds. It is usually necessary to increase the reaction temperature to above 200°C to increase the reaction activity. However, higher reaction temperatures increase the risk of intermediate or product cracking, thereby generating by-products, resulting in a decrease in selectivity (80~90%), but the overall yield of this method can still reach more than 80%, which is better than the dinitrobenzene coupling method. Patent JPS56164146 published by Mitsui of Japan uses copper metal, sodium nitrite and sodium formate, and its yield reaches 80~95%.

The p-nitrophenol salt condensation method directly condenses p-nitrophenol salt with p-nitrochlorobenzene to obtain DNDPE, as shown in Figure 3. This method has the advantages of high conversion rate, and the reaction temperature is relatively low, which makes it easier to inhibit the occurrence of side reactions. International giants Mitsui, Shin Nippon Chemical and DuPont have all published patents for synthesizing DNDPE using this method. DuPont's US patent US3442956 uses dimethylacetamide (DMAc) as a solvent, which can promote the condensation reaction of CNB and NPS at a relatively low reaction temperature of 140~160˚C, and the yield can reach 90~98%. Japanese Mitsui patent JPS61200947 uses polyethylene glycol (PEG) as solvent, and DNDPE can be obtained at 170~180˚C for 15 hours with a yield of 88%. The advantages of this method over the p-nitrochlorobenzene condensation method are higher operable solid content and lower reaction temperature to inhibit the formation of by-products, but the disadvantage is that the raw material p-nitrophenol salt is more expensive.

In order to improve the dielectric properties of MPI materials, the design and optimization of the molecular structure of the material is involved. The main strategy is to reduce the polarization ability of the molecular dipole, because the polarization of the dipole has a significant effect on the dielectric constant. The literature often uses the following methods for structural design, such as: introducing fluorine atoms, alicyclic structures, branched structures and large side groups. These changes can effectively improve the dielectric properties of the material while retaining the excellent stability of PI in high temperature environments. In addition, the introduction of branched structures or alicyclic structures can simultaneously improve the solubility and processing properties of the material by changing the stacking mode of the molecules.

Introducing alicyclic structures into PI can destroy the conjugated structure in the molecular chain, weaken the interaction between the molecular chains, and increase the distance between the molecular chains, thereby reducing its dielectric constant. Zhang's team synthesized a series of PI materials with high fluorine content (over 14.6%) through high-temperature solution condensation of alicyclic dianhydrides and trifluoromethyl-containing aromatic diamines (Figure 6). This type of fluorinated PI film exhibits excellent performance in many aspects. Due to the low polarizability of the alicyclic unit itself, coupled with the electron-withdrawing effect and volume effect of the trifluoromethyl group, its dielectric constant can be reduced to 2.61~2.76; while the mechanical properties of the material remain good, showing that the introduction of the alicyclic structure does not have a negative impact on the strength of the material; the glass transition temperature (Tg) ranges from 285~390˚C, showing high thermal stability; in addition, the PI film also has excellent optical transparency, and its cut-off wavelength is as low as 298 nm, showing good light transmittance in the ultraviolet region. In the visible light band (500 nm), the transmittance of these materials exceeds 85%, and the film appearance is almost colorless, which makes it potential in optical applications, especially optoelectronic devices that require high transparency. The above is a partial excerpt of the information, please see the attachment below for the full content.