Tertiary chlorine

If tertiary chlorine atoms are indeed critical to heat resistance, then reactions that consume them should improve polymer stabiUty. This is indeed the case. Post-reaction of polychloroprene with dodecyl mercaptan (111), use of higher levels of ethylene thiourea for curing (112), and inclusion of reactive thiols such as mercaptobenzimidazole in cure systems (113) all improve heat resistance. This latter technique is especially effective in improving the heat resistance of mercaptan modified polychloroprene. 

A) can react further to give a branched polymer with a tertiary chlorine at the branch point [Eq. (5)]. This is also an important structural feature from the point of... 

Hjertberg and coworkers [38-41] were able to correlate the amount of labile chlorine, tertiary and internal allylic chlorine, to the dehydrochlorination rate. They studied PVC samples with increased contents of labile chlorine, which were obtained by polymerization at reduced monomer concentration. According to their results, tertiary chlorine was the most important defect in PVC. In agreement with other reports [42,43], the results also indicated that secondary chlorine was unstable at the temperatures in question, i.e., random initiation would also occur. 

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. 

Using thiophenol instead of phenol, Michel et al. [49] found a new selective reaction that takes place exclusively with allylic chlorines and not with tertiary chlorines. A single product of thioether structure is formed [Eq. (11)]. 

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]. 

Figure 5. Formation of tertiary chlorine sites in the polymerization...

Before the development of living cationic polymerization in the 1980s, Kennedy and his co-workers devised another way to synthesize end-functionalized polymers, which uses special reagents called inifer, or initiator-chain transfer agents [129]. The method is primarily for the synthesis of polyisobutene with a tertiary chlorine terminal, which is, however, a synthon for a variety of other functional groups. These developments have been reviewed extensively [1,3,130] and fall outside the scope of this chapter. 

Alkyl halides are hydrolyzed to alcohols by water or dilute bases, the order of reactivity of the halogen atoms being tertiary > secondary > primary and iodine > bromine > chlorine. By heating 1,2-dichloro-2-methyl-propane, (CHj),CClCH,Cl, with an aqueous suspension of calcium carbonate, the tertiary chlorine atom is replaced to give 1-chloro-2-methyl-2-propanol (48%). ... 

On the other hand, H NMR spectroscopy focussed at 1.65 ppm (protons belonging to geminal methyl groups next to the terminal tertiary chlorines) provided evidence for the presence of two Cl atoms per macromolecule, i.e. for termination according to the reaction below, applied to both active ends ... 

In all samples, the content of tertiary chlorine is considerably higher than the content of internal allylic chlorine. It is obvious that tertiary chlorine contributes most to the instability of PVC, while the contribution from internal allylic chlorine is of the same order as that of random dehydrochlorination. 

Reductions with tributyltin deuteride (BujSnD) showed that the microstructure of the butyl branches is 2,4-dichlorobutyl with a chlorine attached to the branch carbon, i.e. a tertiary chlorine. 

It was also shown that the major part of the long chain branches (LCB) also contain tertiary chlorine but the presence of hydrogen at the LCB branch carbon could not be excluded. 

Formation of butyl branches takes place by a back-biting mechanism via a six-membered ring, while transfer to polymer from macroradicals is a reasonable source to LCB with tertiary chlorine (7 9). We have suggested an alternative mechanism which also explains the formation of internal double bonds and LCB with tertiary hydrogen (7, 8). This mechanism is based on transfer to polymer from chlorine atoms produced in the mechanism for transfer to monomer ... 

The rate of dehydrochlorination in nitrogen at 190internal double bonds (correlation and coefficients 0.97 and 0.88, respectively). With reference to the better correlation obtained with tertiary chlorine and to the fact that its concentration is roughly 5 times higher than that of internal double bonds, we considered tertiary chlorine to be the most important labile structure in PVC (7. 8). 

In the present investigation, we have assumed that all long chain branch points are associated with tertiary chlorine. The content of tertiary chlorine can therefore be taken as the sum of ethyl, butyl and long chain branches. 

Earlier, tertiary chlorine was generally considered to be less reactive than internal double bonds. This was based on experiments with low molecular weight model substances, see e.g. ref. 45. However, using copolymers between vinyl chloride and 2-chloro-propene, Berens (46) stated that the presence of 1-2 tertiary chlorine per 1000 VC per se would account for the thermal lability observed in ordinary PVC. Furthermore, our previous investigation indicated that the thermal reactivity of internal allylic chlorine is of the same order as that of tertiary chlorine (8). We, therefore, consider it justifiable to use the total content of labile chlorine atoms. As shown in Figure 10, there is a very good relation between the rate of dehydrochlorination and the amount of labile chlorine obtained in this way. For comparison, it can be mentioned that the degradation rate of commercial samples is found in the interval 1.5-3.5 10 %... 

In all samples the amount of tertiary is considerably higher than that of internal allylic chlorine. This is also valid for ordinary PVC where typical values are 0-0.5 internal allylic and 1-2 tertiary chlorine per 1000 VC. We have, therefore, suggested that the latter structure is the most important labile structure in PVC (8). In a recent paper, Ivan et. al. ( 2.) have instead claimed internal allylic chlorine to be most important. For a commercial sample, the content of this structure was given to 0.1 per 10(J0 VC and the rate constant of the dehydrochlorination to about 10 min They did not measure the amount of tertiary chlorine but suggested the presence of an unidentified labile structure. Its rate constant of degradation should be only somewhat less than that of internal allylic chlorine. Furthermore, the content of the unknown structure was claimed to be about four times higher than the concent of... 

If the lability of internal allylic chlorine is in fact higher than that of tertiary chlorine, the low stability of the 45°C series can be explained qualitatively. A statistical evaluation of the data given here might reveal a difference in reactivity. This will be discussed in another paper. 

The minimum in degradation rate found for subsaturation PVC obtained around 55°C becomes less obvious if the monomer concentration at the reaction site is used as variable instead of the relative monomer pressure, P/PQ. The observed behavior is mainly due to the influence of the polymerization conditions on the formation of thermally labile chlorine, i.e. tertiary chlorine and internal allylic chlorine. Tertiary chlorine is associated with ethyl, butyl and long chain branches. The labile structures are formed after different inter-and intramolecular transfer reactions. Generally, the content increases with decreasing monomer concentration and increasing temperature in accordance with the proposed mechanisms. The content of internal double bonds instead decreases with increasing temperatures. 

The content of labile chlorine can be calculated as the sum of tertiary and internal allylic chlorine. The relation between this measure and the rate of dehydrochlorination is very good. Extrapolation -to zero content indicates the presence of random dehydrochlorination. The total contribution from this type of initiation is of, the same order as that from internal allylic chlorine. However, tertiary chlorine must be considered as the most important labile structure in PVC. 

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