Conjugation defects reactions

The experiments summarized above for reeu tions of bromine with poly-DCH indicate that bromine has reeu ted with the PDA chain in the formation of the materials which have gained ca 6 or 8 Br atoms per repeat. It is thus appropriate to inquire into the types of mechanistic processes which might be involved, and three possibilities come to mind. They are (1) reaction via electron transfer, (2) the "classical addition mechanisms, and (3) reaction via conjugation defects. 

The reactions and their mechanisms have been discussed and reviewed in detail. For the application in the field of electronics, it is desirable that the polymer formed is perfectly r-conjugated. Defects are responsible for a reduced performance of light-emitting devices. 

Parameters such as, synthesis medium (HCl, camphor sulfonic add (CSA) or dodecylbenzene sulfonic acid (DBSA)), molar ratio aniline/oxidant, method for oxidant addition, temperature, polymerization duration, dedoping conditions will have effects on polymer properties in terms of yield, chain length, effective conjugation, defects rate, and electrical properties. Temperature was kept to 0 C, -30 C or -40 C and reactions were stopped after various durations in order to control the molecular weight of samples. Polymerization durations were varied from 1 hour to 5 days and depended on the reaction temperature. The inherent viscosities of the polyanilines (Pani) were determined at 25 °C in 0.1%w solutions in concentrated sulfuric acid (95 %), using an Ubbelohde viscometer. For instance, high viscosity samples (1.4 dl/g) were obtained after 3 days at -40 °C vsMe low viscosity samples (0.6 dl/g) were obtained after 1 hour at 0 A relationship was found between polymerization duration and inherent viscosity for polymers synthesized at low temperature (- 40 °C). Inherent viscosity increases from 0.6 to 1.4 when duration of polymerization increases from one to five days. In the case of synthesis at 0°C no correlation was obtained between duration of polymerization and inherent viscosity. A careful control of other parameters (synthesis medium, molar ratio aniline/oxidant, method for oxidant addition) have permitted to get samples with inlierent viscosities ranging from 0.55 to 2.1 dl/g in a reproducible way. 

The first conjugated polymer to be made by the ADIMET process was dihexyl-PPE (12a) [13]. Reaction of l,4-dipropynyl-2,5-dihexylbenzene (10a) with (tBuO)3W=CtBu (11) at 80 °C in vacuo in a high boiling solvent furnished dihexyl-PPE 12a as a yellow powder in 80 % yield and with a degree of polymerization (Pn) of 94 repeat units (Scheme 6). The material was characterized by NMR spectroscopy, and did not show any defects that could be attributed to... 

There have been only a few reports of mesostructured metal sulfides. Mesoporous cadmium sulfide was prepared from polyethylene oxide surfactants and cadmium salts exposed to hydrogen sulfide [35], A study of the effects of the counter-anion on the formation of CdS mesostructures led to the conclusion that the use of cadmium nitrate and perchlorate salts improved the degree of order of the mesostructure over the chloride, sulfate and acetate salts. This effect was attributed to the stronger acidity of conjugate acid by-products of the reaction in the case of nitrates that leads to the dissolution of high-energy defects and enhances structural order. 

The synthetic organic chemist, on the other hand, who is used to deal with structurally defined, but small molecules should be aware that avoiding structural defects in conjugated polymers is by no means a trivial task. A side reaction in any of the polymer forming steps does not lead to a separable side product, but produces an inherent structural defect of the macromolecule. 

Some important aspects of topochemical polymerizations can be understood by inspection of Eq. (1), All reactivity comes about by very specific rotations of the monomers and by 1,4-addition of adjacent units and an extended, fully conjugated polymer chain is formed. The unique feature of the topochemical polymerization of diacetylenes is the fact that in many cases the reaction can be carried out as a single phase process. This leads to macroscopic, defect-free polymer single crystals which cannot be obtained, in principle, by crystallization of ready-made polymers by conventional methods. Thus, polydiacetylenes are ideal models for the investigation of the behaviour of macromolecules in their perfect three dimensional crystal lattice. 

Table 9 Domestic animal species with defects in certain conjugation reactions...

---