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Polymer synthesis

Jon Eggert Carlé

Polymerization is the process of combining many small molecules (the monomers) into a covalently bonded chain or network. Three polymerization reactions, especially useful in connection with organic solar cells are described here, see also figure 1. The polymerization reactions are Stille cross coupling, Suzuki cross coupling, and Direct arylation polymerization.

Figure 1. A schematic representation of copolymerizations using Stille, Suzuki and direct arylation cross coupling polymerizations.

Stille cross coupling polymerization

The Stille cross coupling reaction can generate new carbon-carbon bonds, through transmetallation of an organotin compound with a halogenide as coupling partner, with the use of a palladium catalyst system in an a solvent, see Figure 2. The reaction takes place under mild conditions and normally protection groups are not needed as the reaction is tolerant towards other most functional groups.DOI:10.1002/anie.198605081DOI:10.1002/anie.198605081 The tin-derivates are in general fairly stable and not sensitive to air or moisture which makes them particularly attractive but a drawback is their high toxicity.DOI:10.1002/anie.200300638 The reaction mechanisms are not fully understood but the four steps in the catalytic cycle are commonly accepted. These are: 1) ligand dissociation, 2) oxidative addition, 3) transmetallation and 4) reductive elimination, see Figure 2. When the starting materials are setup for polymerization, then the product can enter the cycle repeatedly and hereby forming a polymer. The only major side reaction associated with the Stille reaction is the oxidative homocoupling of the tinderivates reagent. DOI:10.1002/anie.200300638

Figure 2. Top) A schematic representation of a copolymerization by Stille cross coupling reaction. Bottom) A simplified catalytic cycle of the Stille cross coupling, showing the four generally accepted steps: ligand dissociation, oxidative addition, transmetallation and reductive elimination. The product can enter the cycle repeatedly and hereby polymerized.

Suzuki cross coupling polymerization

Another much applied polymerization reaction that likewise generates carbon-carbon bonds is the Suzuki cross coupling reaction, see figure 3. This comprises an organoboron functionalized compound together with an organic halide compound and also apply a palladium catalytic system. The reaction can take place under mild conditions and the starting materials tolerate towards many functional groups. Compared to the tin-derivative applied in the Stille reaction the boron containing compounds are less toxic and are suitable for industrial scale. The mechanism of the Suzuki cross coupling is analogous to the Stille and proceed in four steps: 1) oxidative addition, 2) metathesis, 3) transmetallation and 4) reductive elimination. One important difference although is the requirement for an additional base in the Suzuki coupling compared to the Stille coupling. The base acts as an accelerator of the transmetallation step.DOI:10.1039/c1cs15114bDOI:10.1016/S0022-328X(98)01055-9

Figure 3. A schematic representation of a copolymerization by Suzuki cross coupling reaction.

Direct arylation polymerization

Recently a reaction named “Direct arylation polymerization” has been applied for preparation of conjugated donor-acceptor copolymers, see Scheme 4. This is a Heck-type coupling and compared to Stille and Suzuki reactions the direct arylation polymerization does not require preparation of stannylated or boronated reagents which eliminates one of the normally more complicated steps in the monomer preparation. The cross coupling take place between a halogenated, normally bromine, aromatic monomer and non-functionalized aromatic monomer, still using palladium as the catalyst. The polymerization has so far only been used on a limited number of systems but could be a future alternative to Stille and Suzuki cross coupling polymerizations.DOI:10.1021/cr0509760DOI:10.1039/b805701jDOI:10.1021/ar3003305

Figure 4. A schematic representation of a copolymerization by direct arylation.

Polymerization rates

Common for all these types polymerization is that they are examples of step reactions prepared from having two di-functional monomers. In a step-reaction polymerization the monomers will react with each other rapidly in the being but as the monomers are consumed the polymerization rate will decrease. In this manner the weight of the polymer will only increase slowly even when great deals of the monomers have reacted, see Figure 5. The polymerization will continue until the concentration of functional groups becomes so low that the polymerization rate will be insignificant or that the mobility of the functional chain ends are reduced due to precipitation or increased viscosity of the reaction medium. Another factor that affects the molecular weight of the final polymer is the stoichiometric balance of the two monomers. This means among other that only highly purified monomers should be applied in the polymerization if high molecular weight polymers is to be realized.Malcolm P. Stevens, Polymer Chemistry: An Introduction, ISBN: 9780201073126

Figure 5. Left) A representation of step reaction polymerization where white dots symbolize monomers and black chains symbolize oligomers and polymers. Right) Molecular weight as a function of monomer conversion of step-reaction and living chain reaction polymerization.
http://en.wikipedia.org/wiki/Stille_reaction http://en.wikipedia.org/wiki/Suzuki_reaction3 http://en.wikipedia.org/wiki/Heck_reaction http://en.wikipedia.org/wiki/Step-growth_polymerization#cite_note-Cowie-1

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