Poly with electron-withdrawing substituents

Poly(p-Phenylene Vinylene) Copolymers with Electron-Withdrawing Substituents... 

A series of terthiophenes substituted with an aryl group at the central thiophene ring (3 position) were polymerized, and the properties of the monomers and the polymers were studied [523, 524], An improvement of the properties compared with the unsubstituted PTT is achieved. The best results are obtained with electron-withdrawing substituents, especially with a 4-cy-anophenyl substituent [524], The redox potentials of PBT derivatives decrease the order poly(3,3 -dibromo-2,2 -bithiophene) > poly(3,3 -dichloro-2,2 -... 

Electron-deficient heteroaromatic systems such as 1,2,4-triazines and 1,2,4,5-tetrazines easily undergo inverse electron demand Diels-Alder (lEDDA) reactions. 1,2-Diazines are less reactive, but pyridazines and phthalazines with strong electron-withdrawing substituents are sufficiently reactive to react as electron-deficient diazadienes with electron-rich dienophiles. Several examples have been discussed in CHEC-II(1996) <1996CHEC-II(6)1>. This lEDDA reaction followed by a retro-Diels-Alder loss of N2 remains a very powerful tool for the synthesis of (poly)cyclic compounds. 

The remaining polymerization route involves zero-valent nickel complexes and dihalide monomers. Variations of this route most often arise where different sources or regeneration methods of the active nickel species are utilized [82,199, 200-204]. A typical example is shown below in Scheme 51 in which poly(3-phe-nylthiophene) 50 is synthesized from the parent 2,5-dichlorothiophene. As with the Ullmann reaction, polymerization appears to be most compatible with ring systems containing electron-withdrawing substituents. 

Most polymers that are positive resists tend to depolymerize via a monomer-unzipping action when degraded, and poly(methyl methacrylate) (PMMA) is typical of this type. Unfortunately, the sensitivity of PMMA to electron beam radiation is low, and in an attempt to improve this feature, PMMA derivatives have been prepared by replacing the a-methyl group with more polar electron-withdrawing substituents, e.g.. Cl, CN, and CF3, to assist electron capture (Figure 16.5). Modification of the... 

Fig. 12.23. A Mukaiyama aldol addition (-> C) and its reaction mechanism (bottom row). As shown here, this method can be exploited to obtain the poly-unsaturated aldehyde D. Under the conditions of the first reaction step the primary product C—which, like the substrate A, is an acetal—does not compete with A for still unconsumed enol ether B. This is due to the fact that the methoxy substituent in the oxocarbenium ion G, which would have to be regenerated from Cin order to undergo further reaction with B, destabilizes G because of its electron-withdrawing inductive (-1) effect.

A significant difference between poly(RCOT)s and unsubstituted polyacetylene is that the former bear substituents which may perturb their electronic properties. The optical absorbances (Table 10-3) probe this to some extent, but electrochemical data are more sensitive. For example, whereas polyCs ec-butylCOT) and poly(Me3SiCOT) have similar absorption spectra, both the formal reduction and oxidation potentials of the silyl-substituted polymer are shifted positive of the alkyl-substituted polymer (Fig. 10-27). This is expected, based on the more electropositive nature of the silyl substituent [139]. Also, while the effects are not large, the substitution of either an electron-donating (para-methoxyphenyl) or an electron-withdrawing (para-trifluoromethylphenyl) substituent do perturb the polymer s electronic properties, with the latter material being harder to oxidize and easier to reduce than the former. 

A heterogeneous arylation of ethyl acrylates and styrene (2) was successfully catalysed by CU/AI2O3 (15). This catalytic system proved applicable to aryl iodides with both electron-withdrawing and -donating substituents [24]. Subsequently, a silica-supported poly-y-aminopropylsilane Cu(II) complex was used for Mizoroki-Heck-type reactions of three aryl iodides with methyl acrylate (1), acrylic acid (16) and styrene (2) [23, 25]. 

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