Synthesis of Star- and Figure-Eight-Shaped Polyethers by t-Bu-P₄-Catalyzed Ring-Opening Polymerization of Butylene Oxide

The synthesis of well-defined four-armed star-shaped poly(butylene oxide) and figure-eight-shaped poly(butylene oxide)s ( s -PBO and 8-PBO, respectively) with predicted molecular weights and narrow molecular weight distributions ( M w / M n s) was achieved by the t -Bu-P 4 -catalyzed ring-opening polymerization (ROP) of butylene oxide (BO). The t -Bu-P 4 -catalyzed ROP of BO using 1,2,4,5-benzenetetramethanol as the initiator produced s -PBOs having number-average molecular weights ( M n,NMR s) ranging from ca. 4000 to 12 000 g mol –1 and narrow M w / M n s of <1.03. Cleavage of the linkage between the initiator residue and PBO arms in s -PBO provided evidence for the homogeneous growth of each arm during the polymerization. The synthesis of 8-PBO was carried out through three reaction steps including (1) the synthesis of a PBO possessing two azido groups at the chain center ((N 3 ) 2 -(PBO) 2 ) by the ROP of BO using 2,2-bis((6-azidohexyloxy)methy)propane-1,3-diol as the initiator, (2) the introduction of an ethynyl group at the two ω-chain ends by etherification using propargyl bromide to give the ω,ω′-diethynyl poly(butylene oxide) with two azido groups ((N 3 ) 2 -(PBO-C≡CH) 2 ), and (3) the intramolecular click cyclization of (N 3 ) 2 -(PBO-C≡CH) 2 using the copper(I) bromide/ N,N,N′,N″,N″ -pentamethyldiethylenetriamine catalyst in DMF under high dilution conditions. Size exclusion chromatography, FT-IR, and 1 H NMR measurements confirmed that the click reaction proceeded in an intramolecular fashion to give 8-PBOs having M n,NMR s ranging from ca. 3000 to 12 000 g mol –1 and narrow M w / M n s of <1.06. The viscosity property of s -PBO and 8-PBO was evaluated together with linear and cyclic PBOs ( l -PBO and c -PBO, respectively). The intrinsic viscosity ([η]) of l -PBO, c -PBO, s -PBO, and 8-PBO decreased in the order of l -PBO > s -PBO > c -PBO > 8-PBO.