Controlled Synthesis of Conjugated Polymers in Catalyst-transfer Condensation Polymerization: Monomers and Catalysts

T. YOKOZAWA AND Y. OHTA

Introduction

Semiconducting p-conjugated polymers were first synthesized by the chemical and electrochemical oxidative polymerization of electron-rich aromatic species such as pyrrole, thiophene, and aniline. This polymerization method is still widely used in, for example, the modification of electrodes. Yamamoto later reported organometallic polycondensation reactions involving cross-coupling reactions catalyzed by transition metals, such as the Kumada-Tamao coupling of organo-magnesium halides, Negishi coupling of organo-zinc halides, and Sonogashira coupling of acetylenic compounds, in addition to the polycondensation of dibromoarenes with an equimolar Ni complex (Yamamoto coupling). The Pd-catalyzed polycondensations of organo-borons (Suzuki-Miyaura coupling) and organo-stannanes (Stille coupling) have also been developed by other researchers. As these polymerizations proceeded in a non-chain reaction manner, research was focused on synthesizing high molecular weight p-conjugated polymers by changing the coupling reactions and polymerization conditions rather than on controlling molecular weight, polymer end-groups, and dispersity, which was believed to be achievable only by means of "living" chain polymerization using vinyl monomers and cyclic monomers.

In 2004, our research group and McCullough and coworkers independently found that bromothiophenemagnesium chloride and its zinc chloride counterpart underwent chain polymerization with an Ni catalyst to afford poly(3-hexylthiophene) (P3HT) with a controlled molecular weight, low dispersity, and defined polymer ends. This finding was made during the course of research on the condensative chain polymerization of aromatic polyamides and the synthesis of regioregular P3HT, respectively. We proposed that chain polymerization involves the intramolecular transfer of the catalyst to the polymer C-Br ends after reductive elimination and we named this type of polymerization catalyst-transfer condensation polymerization (CTCP). This finding opened up a new area of research, aimed on extending CTCP to other coupling polymerizations and applying it to develop organic electronic devices, such as photovoltaics and field-effect transistors. This chapter focuses on the monomers and catalysts used for catalyst-transfer Kumada-Tamao coupling polymerization, Suzuki-Miyaura coupling polymerization, and other coupling polymerizations. p-Conjugated polymer architectures such as block copolymers, obtained by virtue of CTCP, are reviewed in Chapter 3.

1.2 Kumada-Tamao Coupling Polymerization of Grignard Monomers

1.2.1 Background and Discovery

We developed condensative chain polymerization by the activation of the polymer end-group based on a difference of substituent effects between the monomer and the polymer. An example of condensative chain polymerization for the synthesis of a well-defined aromatic polyamide is shown in Figure 1.1a. The amide anion of monomer 1 deactivates the ester moiety through its strong electron-donating resonance effect, which serves to suppress self-condensation. Monomer 1 then selectively reacts with the polymer end-group, which has a weaker electron-donating amide linkage at the para position, resulting in chain polymerization. Similar substituent effects are known in organometallic chemistry; for example, an electron-donating group on an aromatic halide retards oxidative addition with a zero-valent transition metal catalyst. We utilized this chemistry for the Pd-catalyzed condensative chain polymerization of bromophenoxide 2 and carbon monoxide. Chain polymerization proceeded until the middle stage of polymerization and transesterification occurred in the final stage, resulting in a non-chain behavior (Figure 1.1b).

We attempted to conduct a simpler Ni-catalyzed condensative chain polymerization of bromothiophenemagnesium chloride 3 on the basis of substituent effects, because bond-exchange reactions such as transesterification do not occur on the polythiophene backbone. Osaka and McCullough reported that the Ni-catalyzed Kumada-Tamao coupling polymerization of 3 yielded regio-regular P3HT with superior electrical properties. We similarly anticipated that the Ni0 catalyst would insert selectively into the terminal C-Br bond of the polymer chain, rather than the C-Br bond of monomer 3, because the strong electron-donating chloromagnesio moiety of 3 would deactivate the C-Br bond for oxidative addition (Figure 1.1c). The polymerization of 3 with Ni(dppp)Cl2 (dppp = 1,3-bis(diphenylphosphino)propane) at room temperature showed chain polymerization behavior; the molecular weight increased in proportion to monomer conversion, and extension of the polymer chain took place upon further addition of the monomer to the reaction mixture. At the same time, McCullough and coworkers reported the Ni-catalyzed chain polymerization of the zinc chloride counterpart and 3.

However, molecular weight was not controlled by the addition of active aryl halides bearing an electron-withdrawing group, implying that the anticipated condensative chain polymerization based on a change in the substituent effect was not involved in this polymerization. After a detailed study of the mechanism, four important points were clarified: (1) the polymer end-groups are uniform among molecules - one end-group is Br and the other is H; (2) the propagating end-group is a polymer-Ni-Br complex; (3) one Ni molecule forms one polymer chain; and (4) the chain initiator is a dimer of 3 formed in situ. On the basis of these results, we proposed that the Ni0 catalyst is inserted into the intramolecular C-Br bond after transmetallation of the polymer-Ni-Br complex with 3 and reductive elimination during propagation (Figure 1.1d). The intramolecular transfer of Ni catalysts has been reported in organic chemistry; van der Boom and coworkers demonstrated that the Ni atom on an ?2-C=C complex of bromostilbazole underwent intramolecular transfer to the aryl-Br bond followed by oxidative addition, even in the presence of reactive aryl iodide (Figure 1.2). Nakamura and coworkers, in a study of kinetic isotope effects and theoretical calculations of Ni-catalyzed coupling reactions, showed that the p-complex of Ni0 and a haloarene does not dissociate and proceeds quickly to the oxidative addition step in an intramolecular manner.

1.2.2 Mechanistic Studies

The mechanism of the CTCP of 3 with an Ni catalyst has been studied in detail and the whole mechanism is shown in Figure 1.3. In the initiation step, two equivalents of 3 are reacted with Ni(dppp)Cl2, followed by reductive elimination to form a tail-to-tail thiophene diad, accompanied by the generation of an Ni0 complex, which inserts itself into the intramolecular C–Br bond of the dimer. Propagation involves the transmetallation of the polymer–Ni–Br complex with 3, reductive elimination, intramolecular transfer to the terminal C–Br bond, and then oxidative addition (unidirectional growth). However, the C–Br bond of the tail-to-tail thiophene diad at the other terminal can also undergo intramolecular oxidative addition after "long walking" of the Ni0 complex on the backbone. Similar propagation takes place to yield P3HT containing the tail-to-tail thiophene diad inside the backbone (bidirectional growth). In the termination step, the polymer–Ni–Br propagating end is hydrolyzed with hydrochloric acid to form the H-terminal. Therefore P3HTs containing the tail-to-tail thiophene diad at the terminal end and inside the backbone both bear H at one end and Br at the other. Koeckelberghs identified the signals of all possible end-groups in the 1H NMR spectrum of P3HT obtained with Ni(dppp)Cl2 and found that the ratio of Br-TT/(Br-HT + Br-TT) – where Br-TT represents a Br-terminal attached to the tail-to-tail diad and Br-HT represents a Br-terminal attached to the head-to-tail diad – decreased with an increasing degree of polymerization, indicating bidirectional growth.

Before this finding, Kiriy and coworkers had demonstrated bidirectional growth in the polymerization of 3 with Br–C6H4-Ni(dppe)Br (dppe = 1,2-bis(diphenylphosphino)ethane) as an initiator (Figure 1.4). The polymerization afforded not only bromophenyl-ended P3HT via unidirectional growth, but also P3HT containing the phenylene unit inside the backbone via directional growth. They also showed the presence of a single tail-to-tail defect distribution over the whole chain by means of an evaluation of the crystallinity of P3HT obtained with Ni(dppp)Cl2.

Lanni and McNeil conducted kinetic studies and showed that the rate-determining step is transmetallation in the polymerization of 3 with Ni(dppp)Cl2 and reductive elimination in the polymerization with Ni(dppe)Cl2. This means that an intermediate p-complex in the CTCP of 3 with the Ni catalyst could not be directly observed as a resting state in the rate-determining oxidative addition. However, McCullough and coworkers reacted dibromothiophene with 0.5 equivalent of tolylmagnesium chloride in the presence of Ni(dppp)Cl2 to exclusively obtain disubstituted thiophene. This result strongly supported the view that the second substitution would proceed via an intermediate p-complex between Ni0 and thiophene (Figure 1.5). Bryan and McNeil reacted the bromophenyl Ni complex 4 with an aryl Grignard reagent in the presence of active aryl bromide 5 with an electron-withdrawing CN group (Figure 1.6). If intramolecular transfer takes place via the p-complex, then the aryl-substituted Ni complex 6 would be formed. However, if the p-complex does not form, then the resulting free Ni0 should be selectively trapped by 5 to produce 7. The experiment resulted in a 95:5 ratio of 6: 7, consistent with the involvement of an intermediate Ni0 p-complex.

Monomers

Since the CTCP of 3 was first published, it has been reported that many monomers undergo Kumada-Tamao CTCP (Figure 1.7). Donor monomers were the first to be developed, including thiophenes, selenophenes, pyrroles, phenylenes, fluorenes, cyclopentadithiophenes, dithienosilole, and non-conjugated bithienylmethylene. Not only acceptor monomers such as pyridines, benzotriazoles, and thiazoles, but also diaryl monomers, have been reported. The CTCP of most of these monomers proceeds with phosphine-ligated Ni complexes such as Ni(dppp)Cl2 or Ni(dppe)Cl2, whereas the CTCP of the benzotriazole monomer proceeds with the Ni diimine complex, which bears electron-donating groups; Ni(dppp)Cl2 and Ni(dppe)Cl2 were ineffective (see Section 1.2.4).

Several monomers have been reported not to undergo CTCP (Figure 1.8). Polymerization of the pyridine monomer was accompanied by disproportionation, probably due to the coordination of the pyridine N adjacent to the C-Ni-Br end to the Ni in another pyridine-Ni-Br end, yielding a polymer with broad molecular weight distribution and Br/Br ends. The thienothiophene monomer was not polymerized with an NiII catalyst because the dimer, formed by the reaction of the NiII catalyst with the monomer in the initiation step, strongly coordinated to the Ni0 generated in this initiation step, blocking oxidative addition of the C-Br bond in the dimer. Phenylenevinylene monomers did not efficiently undergo Kumada-Tamao coupling, affording only low molecular weight polymers. Dibromophenanthrene was not quantitatively converted to the Grignard monomer with i-PrMgCl • LiCl.

These Grignard monomers are generated in situ via a halogen-magnesium exchange reaction with i-PrMgCl or t-BuMgCl. The iodo-magnesium exchange reaction was fast enough to be practical; it was completed in 1 h at 0 °C. However, the bromo-magnesium exchange reaction takes longer. For example, it took 20 h to generate the Grignard monomer when 2,5-dibromo-3-hexylthiophene was treated with t-BuMgCl at room temperature. It is crucial to use an exactly equimolar amount of the Grignard reagent to generate the Grignard monomers. Excess Grignard reagent reacts with the polymer-Ni-Br end-group to afford a polymer with a broad molecular weight distribution and unexpected end-groups. Mori and coworker sgenerated Grignard monomers by proton abstraction with TMPMgCl • LiCl (TMP = 2,2,6,6-tetramethylpiperidine) or by using a combination of a Grignard reagent and a catalytic amount of secondary amine.

1.2.4 Catalysts

The catalysts used for Kumada-Tamao CTCP are summarized in Figure 1.9. The first CTCP of 3 was achieved with Ni(dppp)Cl2. This catalyst is broadly effective for many monomers: selenophenes, m-phenylenes, cyclopentadithiophenes, dithienosilole, pyridines, thiazoles, and diaryl monomers. The polymerization of the fluorene Grignard monomer with Ni(dppp)Cl2 was poorly controlled, whereas the Ni(acac)2 (acac = acetylacetonate)/dppp system mediated well-controlled CTCP. Ni(dppe)Cl2 was effective for methoxyethoxyethoxymethylthiophene, p-phenylene, N-alkylpyrrole, and bithienylmethylene. McNeil and coworkers studied CTCP with Ni(depe)Cl2, anticipating that the more electron-donating phosphine ligand would increase the binding affinity of the polymers to the Ni catalyst and thus minimize the side-reactions. The polymerization of the phenylene monomer with Ni(depe)Cl2 was significantly slower than that with Ni(dppp)Cl2 at room temperature. The polymerization at 60 °C yielded a polymer with a low dispersity, as well as a small amount of oligomers, which resulted from the disproportionation of the polymer-Ni-Br end-group. The stronger binding affinity of Ni(depe)Cl2 was demonstrated by means of small molecule competition experiments (Figure 1.6), as well as polymerization in the presence of a competing molecule, 2-bromobenzonitrile; the dispersity of the polymer obtained with Ni(depe)Cl2 was narrower than that of the polymer obtained with Ni(dppp)Cl2 under the same conditions.

N-Heterocyclic carbene (NHC)-ligated Pd and Ni catalysts were studied because NHC is s-stronger a-donor than phosphine. McNeil and coworkers conducted the polymerization of thiophene, phenylene, and fluorene monomers with PEPPSI-IPr. Both the thiophene and phenylene monomers underwent CTCP, whereas the polymerization of the fluorene monomer was not well controlled. Mori and coworkers used NiCl2(PPh3)IPr for the polymerization of the chlorothiophene Grignard monomer, which was formed by means of proton abstraction of the corresponding chlorothiophene with TMPMgCl • LiCl. Ni(dppp)Cl2 and Ni(dppe)Cl2 were not effective in this polymerization. They also found that the polymerization of the bromothiophene monomer, similarly generated with TMPMgCl • LiCl, with CpNiCl(SIPr) gave P3HT with Mn 220 000, although the dispersity was rather broad (Mw/Mn = 1.85). Geng and coworkers reported that Ni(IPr)(acac)2 enabled the CTCP of the thiophene monomer to yield P3HT with Mn up to 350 000; the Mn value increased in proportion to the feed ratio of the monomer to the catalyst, while Mw/Mn remained < 1.50.

NiII diimine complexes have been developed as catalysts for olefin polymerization by Brookhart and coworkers.' The steric and electronic effects at the nickel center can be varied by changing the substituents of the diimine ligands. Stefan and coworkers used Ni(t-BuAn)Br2 for the CTCP of the thiophene monomer. A kinetic study indicated the presence of termination reactions, although the molecular weight increased with increasing conversion. Seferos and coworkers investigated the CTCP of the benzotriazole monomer with several Ni catalysts. Ni(dppe)Cl2 resulted in a polymer with lower Mn than expected and broad dispersity. NiII diimine complexes with bulky (Ni(t-BuAn)Br2), donor, and acceptor substituents were examined, and it was found that Ni(OMeAn)Br2 mediated well-controlled CTCP. This catalyst also enabled the block copolymerization of the benzotriazole monomer and thiophene monomer by chain extension from either electron-deficient poly(benzotriazole) or electron-rich P3HT.

1.2.5 Initiators

In CTCP with the described catalysts, the chain initiators are dimers formed in situ (see Section 1.2.2). The bromine of the dimers induces bidirectional growth, which becomes problematic in the one-pot synthesis of block copolymers of A and B, resulting in not only the AB diblock, but also BAB triblock copolymers. If external initiators ArNiLX (L = dppp or dppe, X = Cl or Br) can be formed, only the AB diblock copolymer is synthesized and the functional group in Ar in the initiator is introduced at one end of the p-conjugated polymers. Three procedures have been reported for the generation of ArNiLX initiators (Figure 1.10). (1) Bronstein and Luscombereacted ArCl with Ni(PPh3)4 to generate ArNi(PPh3)2Cl, the ligand of which was replaced with dppp. (2) Kiriy and coworkers used Et2Ni(2,2'-bipyridine) instead of Ni(PPh) for the generation of the primary NiII complex and replaced the ligand. (3) Kiriy and coworkers found that the reaction of o-tolymagnesium bromide or (3-hexylthiophene-2-yl)magnesium chloride with Ni(dppe)Cl2 or Ni(dppp)Cl2 yielded ArNiLCl. The steric hindrance of the ortho-substituent of Grignard reagents is responsible for the monotransmetallation.