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

For 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 ethers a) 1 were carried out with Lewis acids. Benzyl trimethylsilyl ether (2a) was reacted with 5 mol-% of trimethylsilyl trifluoromethanesulfonate (Me 3 SiOTf) 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 Me 3 SiOTf to generate benzyl cation which underwent Friedel-Crafts reaction. During the study of catalysts and solvents in this reaction, the reaction of 2a with Me 3 SiOTf 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 Me 3 SiOTf 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 Me 3 SiOTf in diethyl ether. In the polymerization of 1a, a polyether with low molecular weight (M n = 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.