BMI polymer

Bismaleimide (BMI) polymers are produced by reaction of a diamine and a bismaleimide (Eq. 2-211) [de Abajo, 1988, 1999 Mison and Sillion, 1999]. Polymerization is carried out with the bismaleimide in excess to produce maleimide end-capped telechelic oligomers (XLVI). Heating at temperatures of 180°C and higher results in crosslinking via radical chain... 

BMI polymers have glass transition temperatures in excess of 260°C and continuous-use temperatures of 200-230°C. BMI polymers lend themselves to processing by the same techniques used for epoxy polymers. They are finding applications in high-performance structural composites and adhesives (e.g., for aircraft, aerospace, and defense applications) used at tem-peratrues beyond the 150-180°C range for the epoxies. Bisnadimide (BNI) polymers are similar materials based on bisnadimides instead of bismaleimides. 

Micha.elAdditions. The reaction of a bismaleimide with a functional nucleophile (diamine, bisthiol, etc) via the Michael addition reaction converts a BMI building block into a polymer. The non stoichiometric reaction of an aromatic diamine with a bismaleimide was used by Rhc )ne Poulenc to synthesize polyaminobismaleimides as shown in Figure 6 (31). 

Toughening of BMIs with thermoplastics is a promising approach however, more information is required about the toughening mechanism involved in order to select the most promising polymers in terms of backbone chemistry, molecular weight, and reactive groups. 

BMI was also used as a crosslinking agent for poly(iminoethylene). The Michael addition takes place with the nucleophilic nitrogen of the imino group and the double bonds of the electrophilic BMI. The Michael addition of BMI is now adopted as a crosslinking reaction for polymers with amino end groups [2]. 

A typical maleimide resin is synthesized by the Michael addition of MDA and BMI (Fig. 4). If the stoichiometrically equal amounts of MDA and BMI are added into the reaction solvent under controlled temperature, linear, high molecular weight polyaminoimide (PAI) results. To obtain crosslinkable oligomer (pre-polymer) with maleimide end groups, a calculated 1.1-1.8 times an excess... 

Another original approach is exemplified by Ciba-Geigy s XU292. BMI is mixed with o,o -diallylbisphenol A (DAP, 2,2-bis(3-allyl-4-hydroxyphenyl)pro-pane Fig. 9) followed by heating to 110-125°C to obtain a liquid pre-polymer. The pre-polymer could be cured in the same way as common maleimide and epoxy resins are done [ 16,17]. 

The Diels-Alder reaction mechanism is discussed in greater detail in reference 3. As seen above the reaction between a benzocyclobutene terminated resin and a BMI results in the formation of linear polymer chains. 

An interesting approach to maleimide-terminated phenoxy resin has recently has described (42). para-Maleimidobenzoic acid was reacted with diglyci-dylbisphenol-A epoxy resin in the presence of catalyst to provide the bismale-imide of Fig. 13. Instead of diglycidyl bisphenol-A, linear epoxy resin pre-polymers can be used in this reaction to form a maleimide terminated phenoxy resin. Another suitable functionalized monomaleimide is m- or p- N-(hydroxyphenyl) maleimide which is synthesized from maleic anhydride and m-aminophenol in DMF as a solvent at 70 °C. The purified hydroxyphenyl maleimide was reacted with epoxy resin to form novel BMIs as outlined in Fig. 14. The new BMI and phenoxy oligomers polymerize at temperatures of 200-220 °C, but the cure temperatures can be significantly lowered when catalysts such as imidazoles or triphenylphosphine are added. The cured homopolymers show Tg of 140 and 230 °C for the n = 2 and the n = 1 polymer, respectively(43). 

BMI intermediate synthesis [COMPOSITE MATERIALS - POLYMER-MATRIX - THERMOSETS] (Vol 7) -coumann reduction [COUMARIN] (Vol 7)... 

Many side reactions can occur because when BMIs are heated above 200°C, double bonds can also polymerize by a chain polymerization mechanism with the formation of a crosslinked polymer. A typical formulation contains a molar excess of BMI with respect to a diamine. Linear maleimide end-capped aspartimides are formed, which can then be cross-linked by further heating with or without an initiator ( a catalyst ). 

In the field of high thermomechanical performance polymers, linear and thermosetting systems offer complementary properties. Among the thermosetting materials, BMIs and BNIs have been extensively studied and are now commercially available. In this chapter, firstly the main preparation and characterization methods are reviewed, and then the chemistry of the polymerization processes is discussed for both families. For the BMIs, due to the electrophilic character of their double bond, different polymerization pathways have been published, which is not the case for BNIs. Special attention has been paid to thermal polymerization which has already been used in industrial achievements however, on the other hand, the structure of these materials has been considered for the purpose of establishing relationships between processability, stability and thermomechanical properties. 

Blends of BNIs and BMIs with Linear Polymers ... 

The non-conjugated BNI double bond is less reactive. However, the polymerization mechanism has not up to now been definitely established although a radical process is postulated for the propagation. Recent approaches towards the structure determinations of crosslinked networks and, by the way, towards the best knowledge of polymerization mechanisms, were undertaken in order to obtain BMI and BNI polymers presenting improved properties. In this section particular attention will be paid to this point. 

Table 6. Thermomechanical properties of some linear polymers used for the preparation of semi-IPNs with BMIs and BNIs...

The BMIs and BNIs which were used for the semi-IPN preparations are summarized in Table 7. By heating, the melt viscosity of such oligomers decreases before the crosslinking and allows the process with classical tools. Two questions arise Is it possible to get an intimate mixture of crosslinked BMI or BNI with a linear heterocyclic polymer and what will be the behavior of such semi-IPNs ... 

The first question to be discussed is the polymer miscibility which governs the blend morphology. The solubility parameters of BMIs is 12-135 (cal cm 3)0,5 vs. 11-12 (cal cm"3)0,5 for the high-performance thermoplastics [112]. We can expect an important non-miscibility however, the morphology will also depend on some other factors (conversion at the gel point, viscosity...) and as a result different types of morphologies were identified. 

Fig. 35. Linear heterocyclic polymers blended with BMIs...

The blending of a BMI with a powder of a thermoplastic polymer without solvent offers a great advantage for the processing. Some results have been published based on a non-disclosed formula BMI blended with the CIBA GEIGY Matrimide 5218. The addition of 20% of powder did not require a modification of the cure cycle and an improvement in the mechanical properties (inhibition of crack propagation) was observed [115]. 

A partial miscibility between a linear polyimide and a crosslinked BMI was evidenced when the polyimides and the BMIs were prepared with the same diamine (Fig. 36) [116]. As a consequence the adhesive properties of the blend were better at high temperature than the ones of the linear polymer alone. 

Polystyrene modified dicyanate polymer was obtained by simultaneous polymerization of styrene monomer and BPA/DC. The handling of low viscous dicyanate solution in styrene is more convenient that the processing of crystalline BPA/DC or high-viscous dicyanate prepolymers [52]. A similar patent specification describes the polymerization of a mixture of styrene, BPA/DC and p-isopropenyl cyanate BMI (cf. Sect. 5) was also used [53]. 

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