Donor-Acceptor Type Alternating Copolymers

These D-A alternating copolymer are often used as efficient photoactive donor materials with low bandgap (leading to efficient harvesting of major portion of solar spectrum especially those towards the IR and NIR regions) and good transport properties (due to inherent high mobility). It has been found that, in such D-A copolymer systems, the HOMO 

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Conjugated copolymers where suitable donor and acceptor segments or even blocks are chemically linked, have been demonstrated to fine tune the optoelectronic properties (e.g. bandgap, transport properties) and morphological features (e.g. phase segregation at a scale to match exciton diffusion length) to improve the optoelectronic performance and stability of related OPV devices. In particular, the donor-acceptor type alternating copolymers and donor-acceptor type block copolymers have special relevance in the context of above discussions. 

Figure 5.21 Structure of low-bandgap donor-acceptor type alternating copolymer-based acceptor. Reprinted from Ref [161] with permission from Wiley.

Figure 1.7 Schematic representation of a donor-acceptor (D-A)-type alternating copolymers (left) and bandgap engineering using such system.

Zwitterions are the propagating species in a number of spontaneous polymerizations that occur upon mixing electron donor and acceptor monomers. Several reviews of spontaneous polymerizations via zwitterion intermediates are available [317-323]. In this novel type of polymerization reaction there is no need to add an initiator or catalyst. Alternating copolymers have been obtained in some cases when at least one of the monomers contains a heterocycle. In these polymerizations a nucleophilic monomer (Mn) and an electrophilic monomer (Me) react to form a zwitterion [Eq. (89)]. This zwitterion is referred to as a genetic zwitterion. ... 

Until recently, catalyst-transfer polycondensations were limited to the preparation of p-type 7i-conjugated polymers and block copolymers comprising relatively simple electron-rich building blocks (such as thiophene). On the other hand, well-defined conjugated donor-acceptor alternate copolymers and donor-acceptor all-conjugated block copolymers are needed for practical applications. Very recently, controlled polymerizations of a few electron-deficient monomers were also reported." ... 

For the many alternating copolymer systems briefly discussed in this chapter, MA (Mi) is adjacent to some comonomer M2, and M2 is adjacent to Ml. The respective MiMi and M2M2 segments, resulting from Eqs. (16) and (19), are essentially nonexistent. Only the cross-propagation reactions, Eqs. (17) and (18), are observed. This very special copolymerization reaction occurs readily between the strong electron-acceptor MA and many different types of electron-donor monomers. 

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