Synthesis of Star- and Figure-Eight-Shaped Polyethers
by <i>t</i>‑Bu‑P<sub>4</sub>‑Catalyzed
Ring-Opening Polymerization of Butylene Oxide
- Publication date
- Publisher
Abstract
The synthesis of well-defined four-armed
star-shaped poly(butylene oxide) and figure-eight-shaped poly(butylene
oxide)s (<i>s</i>-PBO and 8-PBO, respectively) with predicted
molecular weights and narrow molecular weight distributions (<i>M</i><sub>w</sub>/<i>M</i><sub>n</sub>s) was achieved
by the <i>t</i>-Bu-P<sub>4</sub>-catalyzed ring-opening
polymerization (ROP) of butylene oxide (BO). The <i>t</i>-Bu-P<sub>4</sub>-catalyzed ROP of BO using 1,2,4,5-benzenetetramethanol
as the initiator produced <i>s</i>-PBOs having number-average
molecular weights (<i>M</i><sub>n,NMR</sub>s) ranging from
ca. 4000 to 12 000 g mol<sup>–1</sup> and narrow <i>M</i><sub>w</sub>/<i>M</i><sub>n</sub>s of <1.03.
Cleavage of the linkage between the initiator residue and PBO arms
in <i>s</i>-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<sub>3</sub>)<sub>2</sub>-(PBO)<sub>2</sub>) 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<sub>3</sub>)<sub>2</sub>-(PBO-CCH)<sub>2</sub>), and (3) the intramolecular click cyclization of (N<sub>3</sub>)<sub>2</sub>-(PBO-CCH)<sub>2</sub> using the copper(I)
bromide/<i><i>N,N</i>,N′,N″,N″</i>-pentamethyldiethylenetriamine catalyst in DMF under high dilution
conditions. Size exclusion chromatography, FT-IR, and <sup>1</sup>H NMR measurements confirmed that the click reaction proceeded in
an intramolecular fashion to give 8-PBOs having <i>M</i><sub>n,NMR</sub>s ranging from ca. 3000 to 12 000 g mol<sup>–1</sup> and narrow <i>M</i><sub>w</sub>/<i>M</i><sub>n</sub>s of <1.06. The viscosity property of <i>s</i>-PBO and 8-PBO was evaluated together with linear and cyclic
PBOs (<i>l</i>-PBO and <i>c</i>-PBO, respectively).
The intrinsic viscosity ([η]) of <i>l</i>-PBO, <i>c</i>-PBO, <i>s</i>-PBO, and 8-PBO decreased in the
order of <i>l</i>-PBO > <i>s</i>-PBO > <i>c</i>-PBO > 8-PBO
