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Overview
The Suzuki Polycondensation reaction (SP) is a polymer forming derivative of the Suzuki-Miyaura Reaction where the monomer(s) are chosen so that a continuous sequence of Suzuki-Miyaura couplings occurs to form a polymer. The reaction can involve either a monomer with both halide and boronic acid substituents (AB-type reaction), or two monomers, each with two substituents of halide/halide and boronic-acid/boronic-acid (AA/BB-type reaction).



The SP in particularly allows a synthetic route to poly-arenes (linked aromatic substituents) which are otherwise difficult to synthesize. The SP reaction has seen particular interest in recent years for its ability to synthesize polymers with organic light-emitting diode (OLED) properties.

Origin and Development
C-C bond formation reactions are very significant step in the field of chemistry, which usually is a key to build up complex molecules from some simple precursors. Suzuki and his colleagues at the Max Planck Institute for Polymer Research (MPI-P) discovered the first feasible method to synthesize a poly(p-phenylene)(PPP) and other polyarylenes in 1989, which is now referred to as the Suzuki polycondensation. Until SP was developed, there were few methods of coupling two sp2 hybridized carbon atoms forming a new C-C bond, and many of those methods came with their own difficulties. Other methods of synthesizing PPP were the Ullmann reaction, Wurtz-Fittig reaction, Since SP was discovered to be effective for polymer synthesis, a great deal of work has been done to optimize activity and selectivity of the catalyst, as well as the properties of the synthesized polymer itself.

There are several areas of active research in recent years. For instance, the solvent heterogeneity affects the reaction behaviour and resulting polymer, so both phase transfer catalysts and homogeneous reactions are being actively explored. For catalysts, reducing the amount of catalyst and increasing the efficiency of catalyst remain a constant goal. In terms of the product polymers, there are two main concerns: product purity and chain length. For the product purity, ICP-MS is most commonly employed to get quantitative data on trace element such as Pd, and techniques like dissolution, precipitation and filtration are used for purification. For chain length, all reaction conditions play a role, so optimization of each reaction is necessary. Finally, chemists have employed more complex and variously substituted monomers to increase the variety of polymers that are generated.

Catalytic Cycle
The mechanism of the SP reaction is presumed to follow that of the Suzuki-Miyaura Reaction, as the same reaction components and similar conditions are used. The catalytic cycle proceeds by the standard mechanism of palladium cross-coupling reaction, involving oxidative addition of the aryl-halide, transmetalation with the aryl boronic acid, and reductive elimination of the coupled biaryl product. The mechanism of the Suzuki-Miyaura reaction is reasonably well understood for both the oxidative addition and reductive elimination steps, but some controversy still exists about the active species involved in transmetalation.



The most popular mechanism for transmetalation involves the formation of an PdL2(Ar)(OH) intermediate (L is usually PPh3 and Ar is the aryl group resulting from oxidative addition) which then reacts with the neutral aryl-boronic acid to transmetalate the second aryl group to the palladium center giving PdL2(Ar)(Ar’). The role of the base in this mechanism is to form a hydroxyl anion which coordinates with [PdL2(Ar)]+ to give the reactive transmetalation species.



Another mechanism was recently proposed where [PdL2(Ar)]+ (cation) reacts directly with the borate [ArB(OR)3]- (anion form of the aryl-boronic acid, where R is an alkyl group or H). The [PdL2(Ar)]+ cation forms readily in solution by the dissociation of the halide anion, and the role of the base in this mechanism is to form the borate anion.



SP is believed to progress via Step-growth polymerization, where the extended polymer is removed from the catalyst before the oxidatively adding again to couple the next unit. However, there is some evidence for Chain-growth polymerization in the SP reaction, and it is possible that either growth mechanism could be active depending on the conditions and monomers used.

"AA/BB" Type Reaction
This “AA/BB” derivative of the SP reaction uses two different monomers in the same reaction. One of the monomer units is disubstituted with two halides, and the other is disubstituted with two boronic acid functional groups ("AA" and "BB" referring to two of the same substituents on each aryl ring). The resulting reaction will couple these two units together to give an alternating aryl polymer. This reaction is useful for introducing different properties into the polymer by varying the monomer units used.

"AB" Type Reaction
The “AB” derivative of the SP reaction uses a single monomer aryl unit that is substituted with both a halide and a boronic acid (the "AB" refers to two different substituents on a single monomer unit). This reaction results in a polymer comprised of a single repeating aryl unit.

Reaction Conditions
The general conditions for the reaction are as with the Suzuki-Miyaura reaction, where both a palladium catalyst and base are required, in addition to a small amount of water. The purposes of the base and water are dependent on the mechanism (still under debate), but their presence is nonetheless required for the reaction to proceed. Most academic synthetic applications of the Suzuki-Miyaura reaction are conducted in a single solvent, but SP reactions are generally conducted in biphasic conditions (where two immiscible solvents are used). Recent work on the Suzuki-Miyaura reaction has indicated that the reaction itself requires a source of ROH (where R can be alkyl or a proton), suggesting that the reaction is likely only taking place in one of the two phases in a biphasic reaction. This results in a optimization issue where the solubility of the polymer must be balanced against the solvent's ability to facilitate the reaction itself.

The reaction is usually terminated by "end-capping" with an appropriate coupling partner for the grown polymer chain. Usually this is accomplished by the addition of phenylboronic acid and a phenylhalogen. This cleaves the polymer chain from the catalyst with a final Suzuki-Miyaura coupling, and the "capped" polymer can be isolated.

Other Important Conditions
The purity of monomers and precise stoichiometry have a significant influence on the properties of synthesized polymer, particularly in molar mass and yield percent. The purity of the synthesized polymers is usually tested by High-Performance Liquid Chromatography, Gas Chromatography and Nuclear Magnetic Resonance spectroscopy. If the monomers are impure, calculation of the mole fractions becomes extremely challenging, so extensive purification of the monomers is done before use.

In particular, the purity of the boronic acids used in SP reactions is of great importance for the effective generation of long polymer chains. Boronic acids have a tendency to form dehydrated species which makes quantitative measurement difficult. A common method used to mitigate this dehydration is to synthesize the pinacol ester of the boronic acid, which does not easily dehydrate. This is particularly important for AA/BB type reactions, where the molar ratio of AA:BB is held as close to 1:1 as possible for maximum polymer length.

Catalyst Variations
For the Suzuki-Miyaura reaction itself, the palladium catalysts used tend to incorporate phosphine ligands to promote the appropriate reactivity. However, it has been found that these phosphine ligands will scramble with the polymer and incorporate themselves into the polymer chain. This means that catalysts that are normally good for the Suzuki-Miyaura reaction may not be effective for the SP reaction. The mechanism for this scrambling is still not fully understood, so most catalyst variations used in the SP reaction avoid the use of phosphine ligands.

When talking about catalysts, various catalysts have been utilized. As described below, Most catalysts are palladium catalysts, such as Pd[P(p-tolyl)3]3, prepared Pd[P(p-tolyl)3]3 in situ, Pd(PPh3)4, Pd(Ph3)2Cl2, or Pd(II). Catalyst variation mainly focuses on the following aspects. Firstly, the catalyst amount employed in the synthesis might result in some changes on properties of synthesized polymers. The usage amount of catalyst in SPCs is generally 1-3mol-%. In contrast to different dosage of the same catalysts in the synthetic route, the less catalyst amount just lead to a light decrease in molecular mass of products and make little difference on their functional groups. However, little residual Pd would have some adverse impacts on the process of industrial scale. Decreasing the loading of catalysts has been achieved in Suzuki coupling reactions. Secondly, it is easy for phosphine-based Pd-catalysts to be oxidized, which leads to deactivation when rigorous reaction conditions are not employed. Also, Pd catalysts would trigger dehalogenation of organic halides (ArX) when alcohol involves in reactions simultaneously. Thus, the careful control on reaction conditions is necessary. And when boron-based monomers are applied in the synthesis of PPPs, the mild Pd-catalyst is required.

For the synthesis of polyfluorenes, phase-transfer catalysts such as a tris-o-tolylphosphine palladium catalyst are used to help the more boronate anion transfer into the organic layer where the coupling reaction happens. In addition, when apply SPC into synthesis of regioregular polythiophenes, Ir-catalyst and some new Pd(0)-phosphane catalysts which are bulky and electron-rich are employed to displace Pd(Ph3)4 and have a good result in high yield percent.

Recent Applications
SP has become an useful tool for connecting aromatic units directly to form polymers which are widely used in industrial synthesis. A vast majority of polyarylenes and related unsaturated polymers published in the open or patent literature are synthesized through SPC. Conjugated polymers synthesized through SP are important materials have been used in many fields. For instance, Conjugated polymers are basic materials to create new kinds of solar cells which makes great progress on organic photovoltaic technology. In addition, conjugated polymers and field-effect transistors are closely linked.

OLED Applications
Polyfluorenes have gained significant interest in recent years for organic light emitting diode applications (OLEDs). A common method of synthesizing these polymers is Suzuki Polycondensation, but other coupling methods have been employed. There are an extraordinary number of derivatives of polyfluorenes published in the open and patent literature, and the field continues to see susbtantial development. Fluorenes are generally commercially available in disubstitued boronic acid or halide form, so the most common SP reaction for synthesizing polyfluorenes is the AA/BB reaction type.



It is noteworthy that the exclusion of palladium from the polyfluorene is important, as any palladium leads to non-radiative quenching of the excitation which would otherwise lead to emission. So optimizations of SP must consider the ease of separation of the polymer from the catalyst.