Masataka Nojima

Precision synthesis of poly(3-hexylthiophene) from catalyst-transfer Suzuki-Miyaura coupling polymerization.

(t)Bu(3) PPd(Ph)Br (1)-catalyzed Suzuki-Miyaura coupling polymerization of 2-(4-hexyl-5-iodo-2-thienyl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (2) was investigated. Monomer 2 was polymerized with 1 at 0 °C in the presence of CsF and 18-crown-6 in THF containing a small amount of water to yield P3HT with a narrow molecular weight distribution and almost perfect head-to-tail regioregularity. The M(n) values increased up to 11,400 g · mol(-1) in proportion to the feed ratio of 2 to 1. The MALDI-TOF mass spectra showed that P3HT with moderate molecular weight uniformly had a phenyl group at one end and a hydrogen atom at the other, indicating involvement of a catalyst-transfer mechanism. Successive 1-catalyzed polymerization of fluorene monomer 3 and then 2 yielded a well-defined block copolymer of polyfluorene and P3HT.

Structural Requirements for Palladium Catalyst Transfer on a Carbon–Carbon Double Bond

Intramolecular transfer of (t)Bu3PPd(0) on a carbon-carbon double bond (C═C) was investigated by using Suzuki-Miyaura coupling reaction of dibromostilbenes with aryl boronic acid or boronic acid esters in the presence of various additives containing C═C as a model. Substituent groups at the ortho position of C═C of stilbenes are critical for selective intramolecular catalyst transfer and may serve to suppress formation of the bimolecular C═C-Pd-C═C complex that leads to intermolecular transfer of (t)Bu3PPd(0).

Alternating Intramolecular and Intermolecular Catalyst-Transfer Suzuki–Miyaura Condensation Polymerization: Synthesis of Boronate-Terminated π-Conjugated Polymers Using Excess Dibromo Monomers

The Suzuki-Miyaura coupling polymerization of dibromoarene 1 and arylenediboronic acid (ester) 2 with a Pd catalyst having a high propensity for intramolecular catalyst transfer is reported. The polymerization of excess 1 with 2 affords high-molecular-weight π-conjugated polymer having boronic acid (ester) moieties at both ends, contrary to Florys principle. This unstoichiometric polycondensation behavior is accounted for by intramolecular transfer of the Pd catalyst on 1. In the polymerization of 1 and 2 having different aryl residues, high-molecular-weight polymer is obtained when the stronger donor aromatic is used as the dibromo monomer and the weaker donor or acceptor aromatic is used as diboronic acid (ester) monomer. The pinacol boronate moieties at both ends of the obtained poly(p-phenylene) (PPP) can be converted to benzoic acid ester, hydroxyl group, and bromine. Furthermore, the reaction of the pinacol boronate-terminated PPP with poly(3-hexylthiophene) (P3HT) having bromine at one end yields a triblock copolymer of P3HT-b-PPP-b-P3HT.

Catalyst-transfer condensation polymerization for precision synthesis of π-conjugated polymers

Catalyst-transfer condensation polymerization, in which the catalyst activates the polymer end-group, followed by reaction with the monomer and transfer of the catalyst to the elongated polymer end-group, has made it feasible to control the molecular weight, polydispersity, and end-groups of π-conjugated polymers. In this paper, our recent progress of Kumada–Tamao Ni catalyst-transfer coupling polymerization and Suzuki–Miyaura Pd catalyst-transfer coupling polymerization is described. In the former polymerization method, the polymerization of Grignard pyridine monomers was investigated for the synthesis of well-defined n-type π-conjugated polymers. Para-type pyridine monomer, 3-alkoxy-2-bromo-5-chloromagnesiopyridine, afforded poly(pyridine-2,5-diyl) with low solubility in the reaction solvent, whereas meta-type pyridine monomer, 2-alkoxy-5-bromo-3-chloromagnesio-pyridine, yielded soluble poly(pyridine-3,5-diyl) with controlled molecular weight and low polydispersity. In Suzuki–Miyaura catalyst-transfer coupling polymerization, t-Bu3PPd(Ph)Br was an effective catalyst, and well-defined poly(p-phenylene) and poly(3-hexylthiophene) (P3HT) were obtained by concomitant use of CsF/18-crown-6 as a base in tetrahydrofuran (THF) and a small amount of water.

Catalyst-dependent intrinsic ring-walking behavior on π-face of conjugated polymers

We show that ring-walking behavior on the π-face of conjugated polymers is catalyst-dependent, and this significantly affects the composition of the products in catalyst-transfer condensation polymerization (CTCP). Systematic mechanistic study by means of density functional calculations combined with experimental verification revealed that the activation energy of the ring-walking process on the π-face and the stability of the oxidative addition state (C–M–X) are key determinants of catalyst mobility. Our findings offer a rational basis for designing innovative catalysts and monomers for CTCP.