Acceptor Alternating Copolymers

In a bulk-heterojunction photovoltaic cell with methanofullerene [6,6]-phenyl C61-butyric acid methyl ester (PCBM) as an electron acceptor, alternating copolymer 19 (Fig. 9), derived from 2,7-fluorene and 2,5-dithienylsilole, can show impressive performance as the electron donor.31 In a device configuration of ITO/PEDOT/active layer/Ba/Al, the dark current density—bias curve shows a small leakage current, suggesting a continuous, pinhole-free active layer in the device. Under illumination of an AM 1.5 solar simulator at 100 mW/cm2, a high short-circuit current of 5.4 mA/cm2, an open-circuit voltage of 0.7 V, and a fill factor of 31.5% are achieved. The calculated energy conversion efficiency is 2.01%. 

Donor-Acceptor Alternating Copolymers COOH COOH... 

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." ... 

Ha JS, Kim KH, Choi DH (2011) 2,5-Bis(2-octyldodecyl)pyrrolo[3,4-c]pynole-l,4-(2H,5H)-dione-based donor-acceptor alternating copolymer bearing 5,5 -di(thiophen-2-yl)-2,2 -biselenophene exhibiting 1.5 cm s hole mobility in thin-film transistors. J Am Chem Soc 133 10364-10367... 

The radical-catalyzed polymerization of furan and maleic anhydride has been reported to yield a 1 1 furan-maleic anhydride copolymer (89,91). The stmcture of the equimolar product, as shown by nmr analyses, is that of an unsaturated alternating copolymer (18) arising through homopolymerization of the intermediate excited donor—acceptor complex (91,92). 

Alternating copolymers of chloroprene have been prepared from a number of donor acceptor complexes in the presence of metal haUdes. 

In this section wc consider systems where the radical formed by propagation can eyclizc to yield a new propagating radical. Certain 1,4-dicncs undergo cyclocopolymerization with suitable olefins. For example, divinyl ether and MAH are proposed to undergo alternating copolymerization as illustrated in Scheme 4.19.167 These cyclo-copolymerizations can he quantitative only for the case of a strictly alternating copolymer. This can be achieved with certain electron donor-electron acceptor pairs, for example divinyl ether-maleic anhydride. 

Copolymerization, on the other hand, is very easy with maleic anhydride. It copolymerizes by a free-radical reaction with a wide variety of monomers and many of the copolymers are perfectly alternating. This tendency of MA to form alternating copolymers derives from the participation of a donor-acceptor complex formed by the two reacting monomers. The term is used to describe... 

Lewis acids have long been known to influence free radical polymerizations [117]. They have been particularly important in copolymerizations of hydrocarbon olefins with electron-poor monomers such as acrylates or acrylonitriles. In this way strictly alternating copolymers can be synthesized from monomer pairs which in the absence of Lewis acids would give more random copolymers. The Lewis acid complexes with the electron pair of the acceptor group of the acrylate or acrylonitrile to form the more electrophilic complexed monomer, which then copolymerizes in alternating fashion with the electron-rich hydrocarbon olefin. 

The outcome of charge-transfer polymerizations has been systematized by Iwatsuki and Yamashita in their penetrating early review [130]. They arrived at a correlation of polymerization behavior with the value of the EDA complex equilibrium constant, Keq, With weak donor and acceptor olefins, no spontaneous polymerization takes place, while the addition of a radical initiator results in a random or an alternating copolymer depending on the value of Keq. As the donor and acceptor strength of the olefins increases, spontaneous initiation rates for radical copolymerization increase and with even stronger donor and acceptor olefins, ionic homopolymerization takes place (cationic and/or anionic). 

Another acceptor is fumaronitrile (FN). In spite of its lack of absorption above 300 nm, the partners or the CT complexes may absorb light and thus bring about polymerization. In studying 2-VN/FN system, Shirota obtained not only 1 1 alternating copolymer but also several cycloaddition products [26] ... 

It was further indicated that copolymerization of two monomers which were close together in the donor-acceptor series gave a random copolymer, while monomers which were well separated in the series had a marked tendency to give an alternating copolymer. 

In contrast, the donor monomer-acceptor monomer interaction involves a one-electron transfer from the donor monomer to the acceptor monomer to form a charge transfer complex. The latter undergoes homopolymerization through a radical mechanism to give an alternating copolymer. 

Iwatsuki and Yamashita (46, 48, 50, 52) have provided evidence for the participation of a charge transfer complex in the formation of alternating copolymers from the free radical copolymerization of p-dioxene or vinyl ethers with maleic anhydride. Terpolymerization of the monomer pairs which form alternating copolymers with a third monomer which had little interaction with either monomer of the pair, indicated that the polymerization was actually a copolymerization of the third monomer with the complex (45, 47, 51, 52). Similarly, copolymerization kinetics have been found to be applicable to the free radical polymerization of ternary mixtures of sulfur dioxide, an electron donor monomer, and an electron acceptor monomer (25, 44, 61, 88), as well as sulfur dioxide and two electron donor monomers (42, 80). 

Similar to the above discussed processes, the photoinitiation of the copolymerization between cyclohexene and AN in the presence of pyromellitic dianhydride or phthalic anhydride is based on the sequence of PET and proton transfer [30]. Consequently, the copolymerization rate with the former acceptor (Rp = 1.6x 10-4moll-1 s-1) is higher than this of the latter (Rp = 1.4x 10-4moll-1 s-1 [AN] = 4.5 mol l-1, 30°C). Interestingly, the average-molar weights of the alternating copolymers lie between 1000-2000 g mol- The reason for this very small value is possibly an efficient primary radical termination due to the formation of the two radicals IV and V see Eq. (4). Exact polymer characterization data are not available, so far. 

Alkylaluminium chlorides form complexes with methyl acrylate which can react with styrene yielding alternating copolymers [164], The donor-acceptor complex of acrylonitrile (donor) and potassium persulphate (acceptor) makes possible the homopolymerization of acrylonitrile in polar media (in water, dimethylformamide, dimethylsuphoxide, dioxan) at low temperatures [165]. The initiating radicals are formed according to the scheme... 

When a monomeric donor of suitable structure is encountered by a monomeric acceptor, a donor-acceptor (DA) complex is formed. It is characterized by a reorganized electron structure accompanied by charge shift. Therefore such formation are also designated as charge-transfer (CT) complexes. Until recently, the tendency to the formation of alternating copolymers has been ascribed to a special kind of monomer reactivity and of the resulting... 

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