Benzocyclobutene -containing

The use of benzocyclobutene as the source of the diene in a Diels-Alder polymerization offers a unique solution to the problems described above. Benzocyclobutene containing monomers can be stored indefinitely at room temperature without concern for further advancement of the molecular weight. It is only when benzocyclobutene is heated to temperatures of approximately 200 °C that the reactive diene, o-quinodimethane, is formed at a significant rate and enters into reaction with the dienophile. The only requirement of the dienophile is that it must be stable at these temperatures and not undergo reaction with itself. The most common dienophiles that have been successfully used in the formation of polymers from AB type benzocyclobutene monomers have been acetylenes, olefins and maleimides. 

The diene undergoes Diels-Alder cycloaddition with dienophiles. Polyimides have been synthesized from various imide-containing benzocyclobutenes and dienophiles (83—85). 

This reaction can be applied to substituted benzocyclobutenes. For example, the reaction has been used to form an array of five linear rings containing most of the functionality for the antibiotic tetracycline. 

Further examples of the use of the hDA reaction in dihydropyran synthesis include the formation of the fused pyrans 18 from vinylallenes 17 and aldehydes (Scheme 8) <00TL6781> and a trans-fused dihydropyran containing a phosphonate group 19 . A total synthesis of the 11-oxa steroid system is based on an intramolecular Diels-Alder reaction involving an orthoquinodimethane derived from a benzocyclobutene (Scheme 9) <00TL1767>. 

The steroid estrone (332) contains a benzene ring and thus provides the best structural feature for the synthetic application of the benzocyclobutene-o-xylylene method. Indeed, many successful applications of this methodology have been recorded 115 -118). 

Bisbenzocyclobutenes readily react with molecules which contain sites of reactive unsaturation such as bismaleimides [10,13, 31, 32]. This is in essence, a novel type of Diels-Alder polymerization in which the bis-diene is latently embodied within two benzocyclobutene moieties. The properties of these polymers depends strongly on the mole ratio of the monomers and when it is equimolar, can result in some exceptionally tough high Tg resins [33, 34]. 

The Friedel-Crafts type of technology can also be used for the preparation of monomers that contain one benzocyclobutene and a second functional group which can react with the benzocyclobutene. These types of molecules are commonly called AB monomers. An example of this class of monomer is shown in Fig. 8. 

A study of benzocyclobutene polymerization kinetics and thermodynamics by differential scanning calorimetry (DSC) methods has also been reported in the literature [1]. This study examined a series of benzocyclobutene monomers containing one or two benzocyclobutene groups per molecule, both with and without reactive unsaturation. The study provided a measurement of the thermodynamics of the reaction between two benzocyclobutene groups and compared it with the thermodynamics of the reaction of a benzocyclobutene with a reactive double bond (Diels-Alder reaction). Differential scanning calorimetry was chosen for this work since it allowed for the study of the reaction mixture throughout its entire polymerization and not just prior to or after its gel point. The monomers used in this study are shown in Table 3. The polymerization exotherms were analyzed by the method of Borchardt and Daniels to obtain the reaction order n, the Arrhenius activation energy Ea and the pre-exponential factor log Z. Tables 4 and 5 show the results of these measurements and related calculations. 

An alternative polymerization mechanism and polymer architecture has been proposed by Kirchhoff [1, 2, 3], Tan and Arnold [77], By this mechanism, polybenzocyclobutenes which do not contain reactive sites of unsaturation are proposed to polymerize by the 1,4 addition of the o-quinodimethane intermediates to give a substantially linear poly(o-xylylene) structure. Since the monomers all contain at least two benzocyclobutene units the net result of this reaction will to a first approximation be a ladder type polymer as shown in Fig. 17. The formation of a true ladder polymer however would require that all... 

A second class of AB benzocyclobutene monomers and polymers which have been reported in the literature are those in which a benzocyclobutene and a stilbene double bond are contained within the same molecule (Fig. 36) [4, 5]. 

During the workup of the o-xylene oxidation run, a strong lachrymator made its presence felt. This was probably a-bromo-o-xylene, although it was not detected in the low voltage mass spectrum. We suspected that a strong peak at mass 104, undoubtedly caused chiefly by a fragment ion derived from o-methylbenzyl alcohol by loss of H20 (I), might also contain a contribution from benzocyclobutene from the interaction of a-bromo-o-xylene with the indium tube used to introduce samples into the spectrometer. To test this possibility, benzyl bromide and a-bromo-o-xylene were run separately under the same conditions. 

Many metal-catalyzed reactions are accelerated by light irradiation [88], In the case of the Vollhardt reaction [89], which is performed with conveniently available Co catalysts, the irradiation with visible light is included in the standard conditions. In a triple [2 + 2 + 2] cycloaddition, the nonaalkyne derivative 86 is transformed into the [7]Phenylene 87 (Scheme 5.17) [90]. Compound 87, containing six benzo-cyclobutene moieties, is a partial structure of the archimedene C12o 88. In this way a variety of similar benzocyclobutene structures such as helical phenylenes [91] can be built up. 

Without question, the metal-promoted cycloaddition of three alkynes to produce benzenes is the most extensively studied organometallic cycloaddition in intramolecular versions. Early work indicated the utility of Ni° systems e.g. Ni(CO)2(PPh3)2), Ziegler catalysts and rhodacyclopentadienes in the partially intramolecular cocycloaddition of a,b>-diynes with additional alkynes. Ziegler catalysts were noteworthy in giving rise to products containing the benzocyclobutene moiety from reactions of 1,5-hex-adiyne, while the Rh systems showed considerable utility in the preparation of anthraquinone derivatives from appropriate diyne precursors (Scheme 29). 

The system provided for a wide process window and various crosslink densities according to the variation in the above monomer ratios [96]. Properties of mixtures containing benzocyclobutene end-capped oligoimides and bismaleimides were reported by Denny ei al. [78]. 

Generation of a diene from a polymer-bound benzocyclobutane [320] The polymer-bound benzocyclobutene (358) can be converted into a very reactive diene upon thermal 47r-conrotatory ring opening. The resulting diene (359) can react with a series of dienophiles containing heteroatoms to afford a series of heterocycles (Scheme 75). Lewis acid-catalyzed reductive alkylation concluded the synthesis to give 363. 

Kirchhoff, R. A. and Bruza, K.J. Polymers from Benzocyclobutenes. Vol. 117, pp. 1-66. Kishore, K. and Ganesh, K Polymers Containing Disulfide, Tetrasulfide, Diselenide and... 

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