Conjugated Polymer Synthesis

Methods and Reactions

Yoshiki Chujo

• Publisher: Wiley-VCH
• Year: 2011
• ISBN:
  9783527632688
• Edition: 1
• Topic: Technology & Engineering
• Subtopic: Materials Science
• Index: Technology & Engineering

Edited and authored by top international experts, this first book on conjugated polymers with a focus on synthesis provides a detailed overview of all modern synthetic methods for these highly interesting compounds. As such, it describes every important compound class, including polysilanes, organoboron compounds, and ferrocene-containing conjugated polymers. An indispensable source for every synthetic polymer chemist.

1

Organometallic Polycondensation for Conjugated Polymers

Takakazu Yamamoto

1.1
Basic Organometallic C–C Coupling

Diorganonickel(II) complexes NiR₂L_m undergo reductive coupling (or reductive elimination) reactions to give R–R (Eq. (1.1)) [1]. The controlling factors of this coupling reaction have long been studied by our research group and by others (L = neutral ligand such as 2,2′-bipyridyl (bpy) and tertiary phosphine):

(1.1)

This basic C–C coupling on Ni introduced the concept of “reductive elimination” to the field of organometallic chemistry [1a–1p] and the first experimental support [1a] for the concept of “back-donation to an olefin [1q,1r]” was given during the study. The coordination of molecules leading to back-donation (e.g., electron-accepting olefin [1a] or aromatic compound [1k,1s]) to the central metal facilitates the reductive elimination of R–R, and the concepts of reductive elimination and back-donation are now widely accepted in chemistry.

The Ni–R bond in NiR₂L_m is considered to be polarized as Ni^δ+–R^δ−, whereas the reductive elimination produces an electrically neutral R–R molecule. Consequently the reductive elimination is assumed to involve electron migration from the R group to Ni, and this electron migration is considered to be enhanced by coordination of an electron-withdrawing olefin (e.g., CH₂=CHCN and CH₂=CHBr) and an aromatic compound (e.g., C₆H₅CN, C₆H₅Br, and C₆F₆) [1a,1k,1s,1t] (Figure 1.1). For NiEt₂(bpy) (bpy = 2,2′-bipyridyl), the enhancement effect is as large as 10¹⁰–10¹³ [1t].

This enhancement effect is similar to that of an electron-withdrawing group on the acid dissociation of substituted benzoic acid (Hammett’s effect), however, the enhancement effect on the reductive elimination is much larger than the Hammett’s

Figure 1.1 Activation of Ni–R bond by coordination of an electron-accepting olefin and aromatic compounds.

effect on the acid dissociation. Because the electron withdrawing ability of the R group increases in the order:

The stability of the Ni–R bond is considered to increase in this order. Actually thermal stability of NiR₂(bpy) increases in the order:

and insertion of CO into an Ni–Et bond is usually easier than into an Ni–Me bond [1u, 1v]. However, the Ni–Ph bond in NiPh₂L_m seems to be less stable and undergoes reductive elimination to give Ph–Ph. Attempts to isolate NiPh₂L_m have not been successful, and they usually give the reductive elimination product Ph–Ph. When the Ph group has a strongly electron-withdrawing substituent(s) as in C₆F₅, the NiPh₂L_m type complex (e.g., Ni(C₆F5)₂(bpy)) can be isolated, and its molecular structure suggests the presence of electronic interaction between the two aromatic ligands through π-electrons in the two aromatic units [1s] (Figure 1.2):

The presence of such an electronic interaction between the two aromatic groups accounts for the ease of reductive elimination of Ph–Ph from NiPh₂L_m. Because of (i) the enhancement effect of electron-accepting aromatic compounds on the reductive elimination and (ii) the ease of the reductive elimination from the NiPh₂L_m type complex, organometallic dehalogenative polycondensation is considered to be especially suited to polymerization of dihaloaromatic monomers, X–Ar–X, which are considered to behave as typical electron-accepting ligands to Ni.

The basic coupling reaction (reductive elimination) is a key step in Ni-promoted organic syntheses (e.g., RMgX + R′X → R–R′; 2RX + Zn → R–R; 2RX + Ni(0)

Figure 1.2 Electronic interaction between two aromatic groups through π-electrons in the aromatic ligands. For aromatic unit = C₆F₅, Ni(C₆F₅)₂(bpy) can be isolated [1s].

complex → R–R; X = halogen) [2]. We have developed further the utilization of this coupling reaction for the polycondensation of dihaloaromatic compounds:

(1.2)

(1.3)

In some cases, Ni(0)L_m formed in situ by chemical (e.g., by Zn) or electrochemical reduction of Ni(II)-compounds are also usable in this polycondensation, thus providing the following catalytic reactions (Eq. (1.4)) [5,6a–e]. It was reported that NaH and hydrazine hydrate could also be used as the reducing agents [6f, 6g]

(1.4)

Polyarylenes can be prepared by the organometallic polycondensation as well as by chemical and electrochemical oxidation of aromatic compounds, and books and reviews have been published concerning the preparation and properties of polyarylenes [1t, 7].

Organopalladium(II) complexes also undergo C–C coupling on Pd [8]. We applied C–C coupling to the following polycondensation [9a–e] which is based on Pd-promoted synthetic reactions of arylacetylenes [8b, 8c, 10a, b]. Acetylenic ligands of Cu complexes can migrate to Pd [8b, 8c], and this migration reaction seems to occur in the C–C coupling reaction and the polycondensation to give PAE (poly (aryleneethynylene)) type polymers.

(1.5)

Successful polycondensation usually requires highly effective basic coupling reactions. However, the polycondensations expressed by Eqs. (1.2)–(1.5) give polymers with high molecular weights even when the basic C–C coupling reaction is not so effective. One of the reasons for the successful polycondensation seems to be an energetic advantage of the polycondensation leading to poly(arylene)s which seem to be stabilized by forming the extended π-conjugation system along the polymer chain. In relation to this, it was reported that polymerization of propylene to give crystalline stereoregular poly(propylene) proceeded at a much faster velocity than that giving amorphous stereo-irregular poly(propylene), presumably due to the stabilization energy attained by forming the crystal in the stereoregular polymerization [10c].

As described above, the basic C–C coupling reaction in the polycondensation is considered to proceed well, especially when the dihalo compounds, such as X–Ar–X, come (or coordinate) to the propagating species ((polymer)_a–NiL_m–(polymer)_b) to produce (polymer)_a–(polymer)_b in the polycondensation. A concept that the propagation reaction proceeds selectively when the monomer comes to the propagating species also explains the smooth polycondensation and the high molecular weight polymer obtained by the polycondensation.

Because organometallic polycondensations can give the π-conjugated aromatic polymers effectively, various analogous polycondensations have been developed. For example, organostannanes and organoborons undergo similar Pd-catalyzed C–C coupling reactions [1t, 7, 11a–d]:

(1.6)

Pd-promoted coupling reactions between ArX and olefin are also known [11e, 11f]. They have also been applied to the polymerization [12]. The polymerization expressed by Eq. (1.2) is applicable to dihaloalkanes (e.g., X–(CH₂)_n–X) by using Cu catalyst [3g]. Use of C–OY (Y = tosyl, etc.) compounds, instead of C–X compounds (OY = leaving group or pseudo-halogen), is also possible for the polycondensation [3g, 5e], which is considered to proceed through oxidative addition of C–OY to a transition metal as studied previously [13a–c].

(1.7)

(1.8)

When the polymerization is carried out using Ni(0)L_m(Eq. (1.3)), the polymerization is considered to proceed through the following fundamental reactions [2f, 4c];

(1.9)

(1.10)

(1.11)

The oxidative addition of C–X [13d–f] and C–OY [13a–c] to Ni(0)L_m (Eq. (1.9)) is well known, and the disproportionation reaction [1s, 13g] is also known. When the Ni–C bond has high stability, the Complexes I [2e] and II [1k, l], as well as a complex of type L_m(X)Ni–Ar...

Table of contents

• Cover
• Title page
• Copyright page
• Preface
• List of Contributors
• 1: Organometallic Polycondensation for Conjugated Polymers
• 2 Catalyst-Transfer Condensation Polymerization for Precision Synthesis of π-Conjugated Polymers
• 3 Regioregular and Regiosymmetric Polythiophenes
• 4 Functional Hyperbranched Polymers Constructed from Acetylenic An-Type Building Blocks
• 5 Through-Space Conjugated Polymers
• 6 Fully Conjugated Nano-Sized Macrocycles: Syntheses and Versatile Properties
• 7 Organoboron Conjugated Polymers
• 8 Recent Developments in π-Conjugated Macromolecules with Phosphorus Atoms in the Main Chain
• 9 Organo-Arsenic, Phosphorus, and Antimony Conjugated Polymers
• 10 Synthetic Strategies to Conjugated Main-Chain Metallopolymers
• 11 Helical Polyacetylene Prepared in a Liquid Crystal Field
• Index