Precise Synthesis of Linear Polysiloxanes End-Functionalized with Alkynylsilyl Groups by Organocatalytic Ring-Opening Polymerization of Cyclotrisiloxanes

The precise synthesis of linear polysiloxanes end-functionalized with disubstituted (Si–CCR) and monosubstituted alkynyl (Si–CCH) groups was achieved by the controlled/living ring-opening polymerization of cyclotrisiloxanes using water or silanols as initiators, guanidines as catalysts, and alkynyl­(amino)­silanes as end-capping agents. Two alkynyl­(amino)­silanes, (diethylamino)­dimethyl­(phenylethynyl)­silane (PhCCSiMe₂NEt₂) and (diethylamino)­ethynyldimethylsilane (HCCSiMe₂NEt₂), were synthesized by alkynylation of chloro­(diethylamino)­dimethylsilane. Both symmetrically and asymmetrically terminated polysiloxanes, such as poly­(dimethylsiloxane), poly­[dimethylsiloxane-co-allyl­(methyl)­siloxane] [P­(DMS-co-AMS)], and poly­[dimethylsiloxane-co-(3-cyanopropyl)­methylsiloxane], were obtained from the polymerization of hexamethylcyclotrisiloxane, allylpentamethylcyclotrisiloxane, or (3-cyanopropyl)­pentamethylcyclotrisiloxane (CNPD2). Furthermore, the resulting alkynyl-terminated polysiloxanes were confirmed to undergo non-catalytic and catalytic Huisgen reactions with organoazide compounds. The non-catalytic Huisgen reactions between the alkynyl-terminated polysiloxanes and 4-bromobenzyl azide (N₃BnBr) proceeded at 82 °C to produce adducts with high regioselectivity. In addition, α,ω-bis­(ethynyldimethylsilyl)-terminated polysiloxanes were found to be tolerant toward a catalyst consisting of copper­(I) bromide and triethylamine in the catalytic Huisgen reaction with N₃BnBr and 6-azido-1-hexanol. Moreover, α,ω-bis­(ethynyldimethylsilyl)-terminated P­(DMS-co-AMS) underwent the catalytic Huisgen reaction without the loss of allyl groups on its side chain, which shows the orthogonality of the ethynyl groups and the allyl groups for independent modification reactions.