Maleimides, Michael addition

The Michael addition of nucleophiles to the carbon—carbon double bond of maleimide has been exploited ia the synthesis of a variety of linear polymers through reaction of bismaleimide with bisthiols (39). This method has been used to synthesize ethynyl-terminated imidothioether from the reaction of 4,4 -dimercaptodiphenyl ether [17527-79-6] and A/-(3-ethynylphenyl)maleimide (40). The chemical stmcture of this Michael addition imide thermoset is as follows ... 

The Michael addition reaction of amines and thiols with bismaleimides or functionalized monomaleimides is a versatile tool ia the synthesis of chain-extended maleimide-terroinated prepolymers. These prepolymers generally are soluble ia organic solvents from which they can be processed to prepreg and molded to high quaUty, void-free laminates. 

Maleimides have three principal reaction pathways. These are radical addition to vinyl compounds the Michael addition with compounds having active hydrogens and the Diels-Alder reaction with dienes (Fig. 3). Any of the three can be tools for forming thermosetting adhesives. 

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... 

Fig. 5. Curing mechanism of a maleimide resin via Michael addition.

As already mentioned, aromatie polymers are thermally stable but aliphatic portions of them are not as thermally stable. Typical maleimide resins have aliphatic units. This is inevitable because the Michael addition was used to prepare the maleimide-based oligomers. On the other hand, if an adhesive consists of a linear thermoplastic polymer, it is not usable at temperatures above its softening temperature. Introdueing chemical crosslinking is one way to prevent thermal weakening of a material. 

Chiral pyrrolo[d]thiepine 166 can be obtained efficiently in 63% yield starting from N-alkylated maleimide 164. Successive Michael addition of phenethyl thiol and regioselective reduction are followed by spontaneous loss of hydrogen by N-acyliminium intermediates 165 and 7r-aromatic intermolecular a-amidoalkylation... 

Tan also found that guanidine 21, acting as a base to activate the o [3], X [3] tautomers of diaryl phosphine oxides, catalyzes the asymmetric phospha-Michael reachon of aryl nitroalkenes (Scheme 5.42) [76]. He later employed 21 to realize highly enantioselective Michael additions of dithiomalonate and 3-keto thioesters with a range of acceptors, including maleimides, cyclic enones, furanone, and acyclic 1,4-dicarbonylbutenes [77]. 

The maleimide group can undergo a variety of chemical reactions. The reactivity of the double bond is a consequence of the electron withdrawing nature of the two adjacent carbonyl groups which create a very electron-deficient double bond, and therefore is susceptible to homo- and copolymerizations. Such polymerizations may be induced by free radicals or anions. Nucleophiles such as primary and secondary amines, phenates, thiophenates, carboxylates, etc. may react via the classical Michael addition mechanism. The maleimide group furthermore is a very reactive dienophile and can therefore be employed in a variety of Diels Alder reactions. Bisdienes such as divinylbenzene, bis(vinylbenzyl) compounds, bis(propenylphenoxy) compounds and bis(benzocyclobutenes) are very attractive Diels Alder comonomers and therefore some are used as constituents for BMI resin formulations. An important chemical reaction of the maleimide group is the ENE reaction with allylphenyl compounds. The most attractive comonomer of this family is DABA particularly when tough bismaleimide resins are desired. 

The Michael addition reaction of dimercaptodiphenylether with N-(3-ethynyl phenyl) maleimide allowed the synthesis of ethynyl-terminated imido-thioether as shown in Fig. 50 (139). This acetylene terminated imidothioether was blended with acetylene terminated polyarylene ether oligomers of different molecular weights and tested as composite resins (140). Blends of functionalized thermoplastics such as the acetylene terminated polyarylene ethers with brittle high-Tg imide resins are finding increased attention for tough high-Tg composites. 

A variety of routes for incorporation of the succinimide group into a polymer backbone are provided by the use of bis(maleimide) monomers (8 Scheme 5). Examples include Michael addition of diamines or dithiols (or H2S) to produce poly(imides) (9) (B-80MI11101), and photopolymerization (via the triplet state) to prepare cyclopolymers (10) (73PAC213) having molecular weights of about 30 000. 

We rationally launched this line by exploiting the Michael addition of thiols to enones with example 82 [157], The n-electron system of maleimide is... 

FR 2612195 (French) 1988 Thermally stable polymers from bismaleimides, siloxane diamines and optionally maleimide-termi-nated siloxanes Rhone-Poulenc Chimie, Fr. R Barthelemy, Y Camberlin Heat-resistant maleimide-termi-nated siloxane Thermally stable polymers were prepared by co-curing MDA based bismaleimides and male-imide-terminated siloxanes. Michael addition with amino-terminated siloxanes and MDA was also utilized. 

Scheme 2 Michael addition reaction of acrylates, vinylsulfones, and maleimides with thiol...

Michael addition is a facile reaction between nucleophiles and activated olefins and alkynes in which the nucleophile adds across a carbon-carbon multiple bond [25], For the preparation of hydrogels, the hydroxyl, thiol or amine functionalities have been reacted with vinyl sulfones [26-28], acrylates [29-31], acrylamides [32], and maleimides [33, 34] (Scheme 2). 

As electron-rich olefins are more reactive, vinyl-sulfones are the most reactive species and are capable of reacting with thiols, amines, and even with small nucleophilic alcohol groups. Less reactive are acrylamides and acrylates, which are reactive towards amines and thiols. Maleimides are the least reactive of the mentioned species and allow selective addition of thiols in the presence of amines in the pH range 6.5-7.5. However, hydrolysis of the imide, especially at elevated pH values [35], may be a concern for certain applications. The mentioned Michael addition reactions do not require organic solvents and can be carried out at physiological temperature and pH [36], In acidic conditions, the reaction is either very slow or does not proceed because protonation removes the nucleophilic form in the case of amines, and the thiolate anion is usually the active species in Michael additions involving thiols [25],... 

Okumoto, S., Yamabe, S. A theoretical study of curing reactions of maleimide resins through Michael additions of amines. J. Org. Chem. 2000, 65, 1544-1548. 

NIR spectrophotometry in the region from 8000 to 4000 cm-1 was used to measure the kinetics of copolymerization of an aromatic bismaleimide (72) derived from an aromatic diamine (e.g. 5a), taking place at 160 to 180 °C. The following NIR spectral ranges were useful for this study primary amine first overtones (vn h) at 7000 to 6400 cm-1, double bond first overtone (vc=c-h) at 6100 cm-1, aromatic first overtones (vc-h) at 6000 to 5750 cm-1, aliphatic first overtones (vc-h) at 5750 to 5350 cm-1 and primary aromatic amine combination bands first overtones (vn h + <5nh2) at 5150 to 4800 cm-1. The process consisted mainly of a second-order Michael addition, as depicted in equation 14, and not the plausible imide opening to yield a maleic dianilide (119), as shown in equation 15. A Michael addition between maleimide moieties and secondary amine moieties present in the products (118) also takes place, however at a rate of about one fourth of that of the primary amine moieties. To improve the SNR of the measurements, usually the results of... 

For a carbohydrate microarray, a glass slide is modified by the thiol group as solid support. Carbohydrates in the form of glycosylamines are converted into maleimide via a hydrocarbon tether chain and covalently bound to the glass surface by hetero-Michael addition reaction (Park and Shin, 2(X)2) between the thiol group and the maleimide moiety of the glycosyl derivative (Fignre 14.4C). 

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