Defect groups

Such problems have led to a recognition of the importance of defect groups or structural irregularities.12 16 If we are to achieve an understanding of radical polymerization, and the ability to produce polymers with optimal, or at least predictable, properties, a much more detailed knowledge of the mechanism of the polymerization and of the chemical microstructure of the polymers formed is required.16... 

Defect groups or structural irregularities need not impair polymer properties, they are simply units That differ from those described by the generalised formula 1 f Initiating radicals arc formed from those Initiator- or transfer agent-derived radicals that add monomer so as to form propagating radicals (see 3.1). 

Establishment of the detailed microstructure of PVC has attracted considerable interest. This has been spurred by the desire to rationalize the poor thermal stability of the polymer (Chapter 1). Many reviews have appeared on the chemical microstruclure of PVC and the mechanisms of defect group formation.56 60... 

It is well know that the zeolite materials synthesized in alkaline systems usually have a high number of silanol groups (=SiOH) named defect groups [10] which possess a moderated Bronsted acidity [11]. Oppositely, Silicalite-1 synthesized in fluorine media are relatively defect-free [12] and the fluorine ions remain in the small cages of the MFI structure even after the calcination process [12]. The 29Si-NMR analyses carried out on samples Na-Silicalite-1 and F-Silicalite-1 confirm the presence of silanol groups only on the SI support surface (results not showed). Delaminated zeolites (ITQ-6) are obtained by exfoliation of as-synthesized lamellar precursor zeolites [13]. After this process, the final structure of the delaminated zeolite results in a completely hydroxylated and well-ordered external surface [13]. 

Fig. 1.3 Functionalization pathways for SWNTs (a) defect-group functionalization, (b) covalent side-wall functionalization, (c) noncovalent exohedral functionalization with surfactants, (d) noncovalent exohedral functionalization with polymers, and (e) endohedral functionalization with, for example, C60. For methods (b)-(e), the tubes are drawn in idealized fashion, but defects are found in real situations. From [103] with kind permission of Wiley.

Fig. 1.2 Different possibilities for the functionalization of SWCNTs (a) noncovalent exohedral functionalization with polymers (b) defect-group functionalization (c) non-...

Newkome-type dendrons were attached to the carbon scaffold of SWCNTs and MWCNTs by defect group functionalization [108], First- and second-generation amine dendrons such as those depicted in Fig. 1.5 were condensed with the carboxyl groups of purified and opened SWCNTs and MWCNTs according to the car-bodiimide technique [108], These CNTderivatives can be expected to combine the characteristics of carbon nanotubes with those of dendrimers, potential building blocks for supramolecular, self-assembling and interphase systems. 

The actual amount of Si-O-D groups (D= H, M or R ) can be fairly quantitatively estimated by combining l3C-NMR and TG data (JJ)but it was not the purpose of this work to compute the actual amount of all defect groups, nor to identify them to a particular Si-O-D species. 

Figure 2. Variation of the number of SiO- defect groups, as a function of the number of A1 atoms per unit cell of Nu-10 (numbers refer to the composition given in Table II).

In all cases, the stable frameworks generate a number of SiO defect groups, necessary to neutralize the excess of positive charges brought by the pore fillers. It was indeed demonstrated that even larger amounts of such defects do not stabilize a zeolitic framework to a measurable extent (33, 46). 

A lower Al content definitely leads to the Si richer Beta phase, that incorporates TEA+ counterions to Al negative charges and TEAOH ionic pairs, that occupy the maximum of the intracrystalline free volume, while Na+ ions partly neutralize the Si-O" framework defect groups. 

HM-MOR was dealuminated with phosgene between 673 and 973 K. Chemical analysis of samples combined with A1 NMR and Si NMR data allowed us to quantify the Si/Al ratio, the concentration of extra-framework aluminium and framework vacancies. The Si NMR spectra reveal progressive dealumination with increasing temperature, accompanied by formation of SiCl and Si(OH)2 defect groups. The former yields quantitatively SiOH groups. The result showed unambiguously that framework reconstruction occurred during dealumination. 

The relative intensity of the Si(IAI) configuration is computed from the Si/AI ratios determined by chemical analysis. The SiOM values are then computed and reported in Table 2. It can be seen that the amount of SiOM is higher in presence of TEA+ ions. As the Al content is smaller than the (TAA + Na) content, SiO defect groups have to be created to neutralize the excess positive charge. 

Secondly, it is possible to control the morphology and the crystal size of the MTW by varying the amount of MTEA+ and TEA+ cations in the reaction mixture. The introduction of TEA+ cations leads however to an increased amount of defect groups in the zeolite framework. 

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