Synthesis of a Polymerizable Benzocyclobutene that Undergoes Ring-Opening Isomerization at Reduced Temperature

Abstract

1-Ethoxyvinylbenzocyclobutene is a substituted benzocyclobutene that undergoes radical polymerization to produce polymers that can be crosslinked at 100-150°C. The 4-and 5-vinyl isomers are synthesized in a 1:4 ratio via a halogenated benzyne intermediate produced from anthranilic acid, followed by cycloaddition with ethyl vinyl ether and replacement of the halogen atom with a vinyl group.Key words: benzocyclobutene, cycloaddition, ring-opening isomerization, Kumada coupling, benzyne Bicyclo[4.2.0]octa-1,3,5-triene, or benzocyclobutene (BCB), 1,2 has been widely used in polymer science based on the ring-opening ability of the strained cyclobutene ring. 3It is stable at room temperature, but undergoes an electrocyclic rearrangement at elevated temperature (>200°C) to the highly reactive o-quinodimethane. The o-quinodimethane undergoes Diels-Alder reactions, and in the absence of an external dienophile, generates 1,2,5,6-dibenzocyclooctane (~25%) and higher oligomers (~75%) (Scheme 1). 4,5Since the thermally unstable spirodimer does not generate any 1,2,5,6-dibenzocyclooctane upon heating, it apparently forms by a different mechanism than the oligomers. 5 This thermal activation and the lack of condensation byproducts makes benzocyclobutene chemistry especially attractive for crosslinking 6 and for polymer-forming reactions 3 by a step (condensation-like) mechanism. Benzocyclobutene-containing vinyl monomers have also been polymerized by chain mechanisms. 7-10Although the high temperature of the ring-opening isomerization reaction is useful for applications such as the crosslinking of engineering polymers, which are synthesized at elevated temperatures, it also limits the use of benzocyclobutene in other applications, such as those involving temperature-sensitive materials. For example, it would be difficult to exploit the benzocyclobutene crosslinking reaction to create nanostructured flexible films by field-assisted roll-to-roll processing if the glass transition temperature of the substrate polymer film and/or the available thermal field are significantly less than 200°C. We are particularly interested in developing nanostructured flexible films using block copolymers that can be aligned and crosslinked at more reasonable temperatures than those used to crosslink benzocyclobutene. Scheme 1 Electrocyclic ring-opening of benzocyclobutene and its subsequent [4+2] cycloaddition with another o-quinodimethane 4The ring-opening isomerization temperature of benzocyclobutenes can be lowered by introducing both electrondonating (raises the ground state energy) and electronwithdrawing (lowers the transition state energy) substituents onto the cyclobutene ring. 5For example, 1-methoxybenzocyclobutene isomerizes at ≥110°C, 11 and the cyclobutene rings of poly(1-benzocyclobutenyl vinyl ether) isomerize at ≥60°C. 12(The temperature range over which the exothermic isomerization transitions occur is usually broad.) Block copolymers with controlled chain lengths are prepared by chain polymerizations, such as living ...

Effect of Ring Functionalization on the Reaction Temperature of Benzocyclobutene Thermoset Polymers

“…Most significant are the results of compounds 6b and 6c, which have exothermic transitions at significantly reduced temperatures (176 and 148 °C, respectively). It should be noted that compound 6c performs similarly to the 1-ethoxy styrene synthesized by Pugh et al (135 °C as reported), 9 and thus lends credence to the hypothesis that the 1-alkoxy group is a key contributor to cross-linking temperature because the delta-trifluoromethyl groups appear to have no effect. The work by Harth et al did not utilize DSC data; therefore, a direct comparison of cross-linking temperature cannot be made.…”

“…It has been reported that functionalization of the alkyl ring on BCB can significantly lower the temperature at which the electrocyclic ring opening reaction responsible for cross-linking takes place . Pugh et al described the synthesis of 1-ethoxy-4-vinylbenzocyclobutene, which cures at a significantly reduced temperature. The key step in their synthesis of substituted BCB involves use of an ortho -diazonium carboxylate salt as a benzyne precursor.…”

Thermo-cross-linkable fullerene for long-term stability of photovoltaic devices

“…Due to insolubility of the nal cross-linked products, further structural characterization could not be carried out. However, results of thermally annealing different combinations of the precursor (PCBCB), PCBM and the BCB compounds and previous studies on these materials in literature 31,32,34,36…”

Recent Advances in Benzocyclobutene Chemistry

“…These benzocyclobutenes 39 and 40 underwent ring opening isomerisation and cross linking at 100-150 °C, which is about 100 °C lower than the unsubstituted vinylcyclobutene containing polymers (Scheme 6). [26] Synthesis of benzene ring containing more than one annulated cyclobutene ring are not available in the literature except Suzuki's work. [27] Compounds 43 and 44 are of great interest as they can act as precursors to the synthesis of anthracene and higher acene-derived compounds, a valuable building blocks in medicinal chemistry and material science.…”