Catalytic polycondensations of bis(trimethylsilyl) p‐xylylene ethers with elimination of hexamethyldisiloxane and their related reactions

Abstract

AbstractFor establishment of the catalytic polycondensation with elimination of hexamethyldisiloxane, etherification of benzylic trimethylsilyl ethers 2 as a model reaction and polymerization of bis(trimethylsilyl) p‐xylylene ethersSystematic name: α,α′‐bis(trimethylsiloxy)‐p‐xylenes. 1 were carried out with Lewis acids. Benzyl trimethylsilyl ether (2a) was reacted with 5 mol‐% of trimethylsilyl trifluoromethanesulfonate (Me3SiOTf) in dichloromethane at 25°C for 24 h to afford dibenzyl ether (3) in 40% yield. The yield of 3 decreased with time after 24 h because the ether linkage of 3 was cleaved also with Me3SiOTf to generate benzyl cation which underwent Friedel‐Crafts reaction. During the study of catalysts and solvents in this reaction, the reaction of 2a with Me3SiOTf in diethyl ether at 25°C was found to depress considerably the Friedel‐Crafts reaction. Further, the etherification of a secondary benzylic silyl ether, α‐phenethyl trimethylsilyl ether (2b), with Me3SiOTf proceeded rapidly in diethyl ether at 25°C to yield the corresponding dibenzylic ether in good yield. On the basis of the results of the model reaction, the polymerizations of difunctional benzylic silyl ethers, bis(trimethylsilyl) p‐xylylene ether (1a) and α,α′‐dimethyl‐p‐xylylene bis(trimethylsilyl) ether (1b), were carried out with Me3SiOTf in diethyl ether. In the polymerization of 1a, a polyether with low molecular weight (Mn = 2000) was obtained as well as a cross‐linked polymer formed by the Friedel‐Crafts reaction. In the polymerization of 1b, the molecular weight of the polymer increased with time to 9000, then decreased gradually accompanied by the production of cross‐linked polymer after 5 days. The polyether obtained from 1b could not be prepared by the Williamson ether synthesis, because the intramolecular elimination of hydrogen bromide from α,α′‐dimethyl‐p‐xylylene dibromide predominantly took place.