As the preparation of semiconducting all-conjugated polymers always involves the formation of C-C bonds, the tools allowing the synthesis of these materials are confined to a few methods.
At the early stages of semiconducting polymer research, electropolymerization was a valuable tool due to its simplicity and the access to insoluble and conducting polymer films.[110-114] This approach has now, however, be replaced almost completely by superior transition metal catalyzed C-C cross couplings[115] that allow immense structural variability, control over the precise molecular structure and defined polymer geometries. These cross-couplings include Stille,[116] Suzuki-Miyaura,[117-118] Yamamoto,[119] Negishi,[120] Kumada-Tamao[121-122] and Heck[123]
cross couplings as well as more recent methods such as direct arylation[124-127] and direct catalytic cross-coupling of organolithium compounds.[128] In recognition of the achievement of palladium catalyzed C-C cross coupling reactions the Nobel prize in chemistry was awarded to Suzuki, Negishi and Heck in 2010.[129]
1.1.4.1 Transition metal catalyzed cross-couplings
In this thesis, the syntheses of both small molecules and polymers employs either Stille[130] or Suzuki-Miyaura[118, 131] cross-couplings, which are, therefore, discussed in greater detail. Stille pioneered the Pd(0) catalyzed coupling reaction between an organostannane and an organic electrophile (e.g. a halide) to form a new C-C bond.[116] Albeit the potentially toxic organotin compounds that are used as starting materials, the Stille cross-coupling is still very popular for the synthesis of functional conjugated polymers. This is primarily due to its tolerance against a wide variety of functional groups and the storage stability of the organotin compounds. One reason for this stability is its moisture and oxygen insensitivity. Furthermore, its almost quantitative yields are a crucial prerequisite for polycondensations in order to obtain high molecular weight polymers. The catalytic cylce for the Stille coupling is shown in Figure 1-12.[132-137] One cycle comprises three distinct steps, i.e. oxidative addition, transmetallation and finally reductive elimination. The reaction is catalyzed by an active 14-electron Pd(0)
complex[138] which can be generated in situ either by addition of a Pd(0) catalyst or a Pd(II) catalyst, which is subsequently reduced to Pd(0) by organostannane homocoupling.[139]
Commonly used catalysts include Pd(PPh3)4, Pd2(dba)3, Pd(OAc)2 which can be used with or without an additional ligand (L). After oxidative addtion of an aryl halide (red) a Pd(II) intermediate is formed. An organostannane subsequently transmetallates to the Pd(II) intermediate in the rate determining step. Finally, the coupled product (red-blue) is formed by reductive elimination with the regeneration of the Pd(0) species. The reaction rate can be enhanced by choosing the correct substitution pattern for the two different aryl units, i.e. the electron-rich aryl unit should be stannylated whereas the more electron-deficient aryl unit should be halogenated.[130]
= alkyl, allyl, alkenyl, alkynyl, aryl
X = Cl, Br, I, OTf, OPO(OR)2 L = Ligand, e.g. PR3
base = M+(-OR) with M = metal, R = alkyl or Na2CO3, K3PO4, K2CO3,...
Key for Stille Cross-Coupling: Key for Suzuki-Miyaura Cross-Coupling:
R = alkyl, OH, O-alkyl
Figure 1-12. Mechanistic schemes (adapted from Kürti et al.[137]) of the Stille and Suzuki-Miyaura cross-coupling reactions. Solid boxes represent the desired products, dashed boxes represent the employed starting materials. Whereas both reactions can also be used with a variety of substrates, such as alkenyl, acyl, allyl or even alkyl residues, commonly aryl compounds (bold) are used for the synthesis of all-conjugated polymers.
A widely used alternative to Stille cross-couplings was established by Suzuki and Miyaura, namely the Suzuki-Miyaura cross-coupling.[117-118] In this reaction, a organoboron compound and an organic halide are coupled to form a new C-C sigma bond.[137, 140-141] The advantage over the Stille cross-coupling is the safer and less toxic nature of the organoboron compounds, many of which are also commercially available. Furthermore, the reaction is insensitive to water and is in fact mostly performed in biphasic aqueous mixtures. Similar to the Stille cross-coupling the catalytic cycle (Figure 1-12) consists of an oxidative addition, a transmetallation and a reductive elimination which work identically. In the course of the additional metathesis in the Suzuki-Miyaura coupling, the anion attached to the palladium is exchanged for the anion of
the base. As the organoboron compounds are less reactive compared to their organostannane analogs, the addition of a base becomes necessary. The base leads to the quaternization of the boron atom, thus increasing the nucleophilicity of the attached aryl group leading to an accelerated transfer to the palladium in the transmetallation step.[142]
1.1.4.2 AABB and AB type polycondensations
The stepwise polymerizations which are carried out using these transition metal catalyzed cross coupling reactions are also referred to as polycondensations because of the generation of low molecular weight byproducts from the former functional groups. The two distinct pathways leading to a polycondensation are shown in Figure 1-13. Both pathways involve the use of bifunctionalized monomers carrying two different functional groups, labelled A and B. In a Stille polycondensation for example these groups would be the stannane and a halide, respectively.
A A B B A B A B
n
A B
n
AB AB
M1 M1 M1
M1 M2
M2 M2
M2
EC EC
EC EC
AABB route AB route
Figure 1-13. Two different polycondensation routes: AABB and AB approach to obtain a strictly alternating copolymer comprising the two comonomers M1 and M2. At the end of the polymerization the polymer is usually endcapped on both ends ussing an endcapping reagent EC.
In the “AABB route”, two monomers M1 and M2 are used that both are bifunctionalized with either A or B groups. Catalytic cross-coupling of these two monomers leads to a step-growth polymerization. This AABB approach has various advantages, primarily the ease of monomer synthesis as all units show symmetric functionalization which greatly enhances yields and simplifies purification. Second, this modular approach allows the efficient synthesis of a polymer library, keeping one monomer constant whilst strategically varying the second one.
Further, even ternary copolymers are easily accessible. A considerable drawback of this route is that the stoichiometry of both monomers has to be exactly kept at 1:1 in order to allow high degrees of polymerization and large molecular weights. The degree of polymerization (DP) can be described by Carothers equation (eq. 4) and is strongly dependent of the molar ratio r of AA-type (NAA) and BB-type (NBB) monomers with r = NAA/NBB.[143]
= +
+ −
DP r
r rp
11 2 (4)
Here, p is the conversion ranging from 0 to 1. For this reason, polymerizations with r deviating only by a small fraction from 1, only low molecular weight oligomers can be obtained, even at
or partial decomposition of one of the functional groups, making maximal monomer purity a crucial prerequisite for sucessful polymerization.[131] The degree of polymerization is also limited if the conversion p < 1, e.g. caused by low catalyst turnover numbers or generally sluggish reactivity. Another problem for AABB polycondensation arises from the homocoupling (i.e.
cross-coupling of an A functionality with another A functionality rather than A and B)[144] that can be observed as a side reaction both in Stille and in Suzuki-Miyaura cross-couplings.[137]
Solvent-dissolved oxygen can be one possible origin for homocoupling.[137]
The alternative “AB route” (Figure 1-13) employs only one monomer that carries both functional groups A and B. The monomer itself can either be a single aryl unit or comprise several moieties, e.g. M1-M2, inbetween the functional groups A and B. Whereas this route is usually much more laborious from a synthetic point of view due to asymmetric functionalization, the stoichiometry parameter r is eliminated from the carother’s equation (eq. 5).
= −
DP p
1
1 (5)
As stoichiometry can only deviate from 1:1 by decomposing functional groups or imperfect functionalization in the first place, these systems are much more tolerant against weighing errors, making them ideal for small batches. Furthermore the AB route allows chain-growth polymerizations as will be discussed later on.
Following either route, after polymerization a strictly alternating (M1-M2)n copolymer chain is obtained which carries a functional group at either polymer end. These functional end groups can deteriorate the device performance in organic electronic devices[145] and thus have to be scavenged by the addition of endcapping reagents (EC) at the end of the polymerization reaction. Two different endcapping reagents carrying exactly one of the two distinct functional groups have to be added subsequently and in excess to obtain the final endcapped polymer EC-(M1-M2)n-EC. EC can be any aryl unit that is desired to be at the end of the polymer chain. The endcapping reagents can also carry additional orthogonal functionalities as a starting point for polymer analogous reactions (e.g. azides or alkynes for click reactions). In case of the AB route, the resulting copolymer carries two different functional groups A and B at the either end. These polymers can thus either be monofunctionalized or bifunctionalized orthogonally by choosing appropriate endcapping reagents.
1.1.4.3 Catalyst transfer polycondensation
Beyond the classical step-growth polycondensation, elegant catalyst design also allows chain-growth polycondensations of AB type monomers.[146-148] This novel route was first discovered by McCullough et al. for the Kumada-Tamao coupling of Grignard reagents with aryl halides.
The resulting living[149] polycondensation is abbreviated as KTCTP (Kumada-Tamao Catalyst
Transfer Polycondensation).[150-152] Furthermore, this approach has been adapted for Suzuki-Miyaura cross couplings by Yokozawa et al., leading to SMCTP (Suzuki-Suzuki-Miyaura Catalyst Transfer Polycondensation).[153-155] The mechanism is schematically shown in Figure 1-14a and is very similar for both types of reactions with the key step in the catalytic cycle being the oxidative addition. After the metal (M) complex has undergone a reductive elimination, diffusion of the catalyst from the polymer chain (leading to step-growth) competes against intramolecular migration of the catalyst along the π-conjugated polymer backbone over several metal-aryl π-coordination complexes (shaded orange). This migration is also referred to as ring walking and can occur along the polymer backbone over the order of even more than 10 nm.
[156-157] When the affinity of the catalyst to the polymer chain is high enough and the subsequent oxidative addition at the chain end is fast enough, this intramolecular catalyst transfer leads to a chain-growth polymerization. These chain-growth polymerizations can either be initiated in-situ by reduction of a metal(II) complex by two monomers or alternatively by use of an external initiator (RMXL) which allows the synthesis of defined α-functionalized polymer chains (here with R) as shown in Figure 1-14a.[155, 158-160]
When the blockcopolymer synthesis by sequential monomer addition to the living chain end was studied, it was found that the order of monomer addition has a large influence on the final blockcopolymer (see Figure 1-14b).[161-162] When a polymer block comprising a weak π-donor monomer (red) is grown first, addition of second monomer with stronger π-donor ability (blue) results in the growth of a second block with excellent control and very low polydispersities.
When, however, the block from the strong π-donor monomer is grown first, subsequent addition of the weaker π-donor does not lead to blockcopolymer formation.[162-163] This was attributed to the higher affinity of the metal catalyst to the strong π-donor moieties than to the weak ones.[164]
Ring walking therefore occurs in the wrong direction and the catalyst will not reach the polymer chain end in order to undergo oxidative addition but rather dissociate from the polymer chain.[163] At this point supposedly a step-growth mechanism takes over which results in bad control and high polydispersities.[163]
This issue considerably limits the concept of chain-growth catalyst transfer polycondensation for the synthesis of low bandgap polymers which consist of aryl units with inherently different π-donor strengths (see section 1.1.3). In case of diketopyrrolopyrroles for example, a monomer unit consists of a weakly donating core (red) and strongly donating thiophene moieties (see Figure 1-14c). Upon initiation with an external Pd(0) species, the catalyst will not be able to migrate to the chain end due to the central core with its electron deficient π-system. This prevention of migration by catalyst trapping has been experimentally proven for a thiophene-benzothiadiazole-thiophene (TBT) unit in the KTCTP system.[165] This issue has still not completely been overcome, but catalyst systems have emerged that allow the chain-growth polycondensation of units with slightly different π-donating strengths, such as copolymerization
A B
Figure 1-14. Catalyst transfer polycondensation concept: (a) Reaction mechanism for Suzuki-Miyaura Catalyst Transfer Polycondensation (SMCTP) and Kumada-Tamao Catalyst Transfer Polycondensation (KTCTP); (b) Blockcopolymer formation by sequential monomer addition for two monomers with different π-donor strengths – the success of the blockcopolymerization is depending on the order of monomer addition; (c) Problem of inherently different π-donor strengths in a low-bandgap building block.