Living β-selective cyclopolymerization using Ru dithiolate catalysts

Abstract

Cyclopolymerization (CP) of 1,6-heptadiyne derivatives is a powerful method for synthesizing conjugated polyenes containing five- or six-membered rings via α- or β-addition, respectively. Fifteen years of studies on CP have revealed that user-friendly Ru-based catalysts promoted only α-addition; however, we recently achieved β-selective regiocontrol to produce polyenes containing six-membered-rings, using a dithiolate-chelated Ru-based catalyst. Unfortunately, slow initiation and relatively low catalyst stability inevitably led to uncontrolled polymerization. Nevertheless, this investigation gave us some clues to how successful living polymerization could be achieved. Herein, we report living β-selective CP by rational engineering of the steric factor on monomer or catalyst structures. As a result, the molecular weight of the conjugated polymers from various monomers could be controlled with narrow dispersities, according to the catalyst loading. A mechanistic investigation by in situ kinetic studies using ¹H NMR spectroscopy revealed that with appropriate pyridine additives, imposing a steric demand on either the monomer or the catalyst significantly improved the stability of the propagating carbene as well as the relative rates of initiation over propagation, thereby achieving living polymerization. Furthermore, we successfully prepared diblock and even triblock copolymers with a broad monomer scope.

Scheme 1. (a) Two possible pathways for CP of 1,6-heptadiynes, (b) proposed scheme showing the effects of the pyridine additive and the limitations of this method, and (c) living β-selective CP using fast-initiating Ru2.

Fig. 1. (a) Plots of the obtained M_nvs. M/I (solid line shows a fit of the data) and the corresponding Đ values for P2, and (b) SEC traces of P2 from entries 3–7 in Table 1.

Scheme 2. (a) Diblock copolymerization of with M2 as the first monomer, and M3 (above) and M4 (below) as the second monomers. SEC traces of homopolymer P2 and diblock copolymers: (b) P2-b-P3, and (c) P2-b-P4.

Fig. 2. (a) Modifying ligands for living polymerization, and (b) model for improved β-selectivity and controllability of CP using Ru2.

Fig. 3. The plot of ln([Ru]/[Ru]₀) vs. time to measure the initiation rates of Ru1-2 at –20 °C by monitoring the disappearance of the benzylidene signal using ¹H NMR.

Fig. 4. Scheme for the ¹H NMR kinetic experiments (top), monitoring the changes in carbene proton signals during CP of M5 using Ru2 (middle) and their corresponding plots in real time (bottom).

Fig. 5. (a) Summary of the results from the kinetic experiments showing the relationship with the binding affinity of the additives and (b) THF-SEC traces of the corresponding polymers.

Fig. 6. Plots of M_nvs. M/I and corresponding Đ values of (a) P6, (b) P3, (c) P7, and (d) SEC traces of P3 from entries 7–11.

Scheme 3. Diblock and triblock copolymerization for exclusively β-selective conjugated polyenes (a and c), and THF-SEC traces of the corresponding polymers (b and d).