Chlorine structure

The presence of allylic chlorines and tertiary chlorines and their influence on the thermal stability of PVC has now been established with some degree of confidence, and together they are considered to constitute the labile chlorine structures in the polymer. Numerous chemical modification methods involving the selective nucleophilic substitution of labile chlorines in PVC with other chemical moieties for identifying and quantifying labile structures have been reported in the literature. 

Organic metal salts retard the development of color in the thermal treatment of PVC, and their ability to react selectively with allylic and tertiary chlorine structures according to Eq. 23 has been demonstrated with model compounds [19,32,113,115]. 

Dahlman O, Morck R, Ljungquist P, Relmann A, Johansson C, Boren H, Grimvall A (1993) Chlorinated Structural Elements in High Molecular Weight Organic Matter from Unpolluted Waters and Bleached-Kraft Mill Effluents. Environ Sci Technol 27 1616... 

Flodin C, Ekelund M, Boren H, Grimvall A (1997) Pyrolysis-GC/AED and Pyrolysis-GC/ MS Analysis of Chlorinated Structures in Aquatic Fulvic Acids and Chlorolignins. Chemo-sphere 34 2319... 

There are numerous varieties of monofunctional fluorinated compounds which exhibit a wide range of linear or branched, hydrogenated or chlorinated structures. 

The Cl(2p) and Mo(3d) x-ray photoelectron spectra (XPS) of 15 were kindly recorded by Prof. R.E. McCarley and Mr. M. Luly. The chlorine XPS, displayed in Fig. 12, clearly shows two types of Cl atoms (each peak is a doublet composed of the 2P3/2, 2Pi/ spin-orbit pair) separated by 1.2 eV. The higher energy peak (198.0 eV referred to C(1s) at 285.0 eV) may be attributed to a bridging Cl-atom and the lower energy peak (197.6 eV) to a terminal chlorine. Structure XII is assigned to 15, even though only one Mo(3d) peak could be resolved in the Mo XPS (the energy separation may be too small to resolve due to the broad peaks). 

Dahlman O., Morck R., Ljungquist P., Reimann A., Johansson C., Boren H., and Grimvall A. (1993) Chlorinated structural elements in high molecular weight organic matter from unpolluted waters and bleached kraft effluents. Environ. Sci. Technol. 27, 1616-1620. 

The content in the present material varies between not observable to about 1 Cl per 1000 VC. In agreement with the results published by Starnes and his coworkers (9. 40) we have also found nonallylic primary halogen, i.e. CH Cl- groups in long chain ends and methyl and butyl branches. For the polymers studied here it is especially chloromethyl branches which can be observed, although butyl branches with chlorine residues were observed in some polymers as well. When necessary, the content of branches (see below) has been corrected with respect to the presence of these chlorinated structures. 

At a temperature above 80 °C, poly (vinyl chloride) eliminates hydrogen chloride and allylic chlorinated structures appear, with 4-chloro-2-hexene being considered as a model. At the processing temperature (180-200°C), the main problem of poly (vinyl chloride) stabilization is preventing the zip dehydrochlorination that induces discoloration and cross-linking of the polymer. 

Rodin, C., Ekelund, M., Boren, H., and GrimvaU, A., Pyrolysis-GC/AED and pyrolysis-GC/MS analysis of chlorinated structures in aquatic fulvic acids and chlorohgnins, Chemosphere, 34, 2319-2328, 1997. 

The laboratories <98MI1> <94CRV327> of Battersby and Montforts developed selective methods for the total synthesis of chlorins on model systems, which contain the characteristic dialkylated parts in the saturated rings of the chlorin structure. The experiences gained from these studies were later used to synthesize naturally occurring chlorins. 

Novel chlorin structures have been isolated from various marine organisms and their substitution patterns suggest their origin from chlorophyll a. 

It is interesting to compare the rate-determining transition state of the nitration reaction in Figure 4.4 with the corresponding transition state for chlorination in Figure 4.5. It is clear that whereas the nitration transition state resembles the oriented jc-complex, the chlorination structure is much closer to the o-complex. Thus, Olah s original hypothesis is essentially confirmed [35]. 

Because the thermal stability of PVC is substantially lower than one may expect from its nominal structure, numerous studies have been initiated to identify the defect sites responsible for this anomaly. After considerable controversy concerning the role, nature, and importance of possible irregular structures, the most active sites for initiating PVC degradation were finally identified by H- and C-NMR as internal allylic (Figure 15.5,1, II) and tertiary chlorine structures (III, IV) [14]. 

It was believed for a long time that head-to-head radical addition to monomers is a major route for formation of labile structures. Kinetic studies, in association with NMR measurements, reveal that formation of internal allylic and tertiary chlorine structures actually proceeds through an intramolecular or intermolecular chain-transfer reaction to polymer [Eqs. (31), (32) VC = vinyl chloride]. 

These resins were designated as A, B, and C, and their halogen content and epoxy equivalent increased from A to C. In Figures 3.5 it can be seen that, for the same curing agent (BF3MEA), as the halogen content increased, the thermal stability decreased. This effect may be due to the presence of chlorinated structures, such as structure VI (Equation 3.31) ... 

As illustrated above, some side reactions occur in vinyl chloride polymerization due to rearrangement effects in the polymer chains, unsaturated structures in the polymer chains (1.5-3.0 double bonds per 1000 monomer units), and the tertiary chlorine structures formed by chain transfer to polymer. The chlorobutyl groups (2-3 per 1000 monomer units) formed by backbiting reactions contribute to the thermal instability of the polymer through the tertiary chlorine on the backbone [63]. 

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