Labile 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. 

As discussed in Section III.G, the possibility of simultaneous initiation of degradation from normal units in the PVC chain is receiving increased attention. The work of Millan and coworkers who showed that the normal secondary chlorines in isotactic triads are also labile and can act as points of initiation for thermal degradation was also discussed. This favors simultaneous participation of normal chlorines along with defect sites in the thermal degradation of PVC. The content of nondefect labile structures in PVC is much higher than that of the defect structures [125]. 

Thus, the role of the defect labile structures in the thermal degradation of PVC, which is the key to the Frye and Horst mechanism of stabilization, is itself debatable. The Frye and Horst mechanism—the substitution of labile chlorines by more stable groups—has also been criticized. 

A similar unravelling of the heterocyclic ring occurred in intramolecular reactions with furans,79b,159 161 leading to cyclic 1,4-diacyl-1,3-butadienes (204 equation 42)159 and (205 equation 43).79b The ideal size for the tether was five, but less efficient capture of the carbenoid was also possible with six- and seven-membered rings. With the 3-furanyl derivative (206), the acid labile structure (207) was formed, which readily rearranged to p-hydroxyphenylacetaldehyde (208 Scheme 43).161... 

Polymerization in microemulsion systems has recently gained some attention as a consequence of the numerous studies on microemulsions developed after the 1974 energy crisis (1,2). This new type of polymerization can be considered an extension of the well-known emulsion polymerization process (3). Hicroemulsions are thermodynamically stable and transparent colloidal dispersions, which have the capacity to solubilize large amounts of oil and water. Depending on the different components concentration, microemulsions can adopt various labile structural organizations -globular (w/o or o/w tyne), bicontinuous or even lamellar -Polymerization of monomers has been achieved in these different media (4-18),... 

Amorphous carbon is characterized by a highly imperfect structure and high reactivity. This shows by a considerable amount of mobile carbon atoms at a surprisingly low temperature. Besides, a vast number of defects and small sizes of graphene sheets make the carbon matrix very labile. As a result, it may be deformed under the action of adsorbates. For example, granules of amorphous carbon swell [88,89] in water with concomitant changes in the carbon substructure and porosity [90,91]. These properties of the support weaken rapidly as its crystal structure becomes more perfect. The labile structure of amorphous carbon is responsible for at least two mechanisms of blocking of the surface of supported metal particles. 

Labile structures initiating polymer decay [215] are formed during the process of thermooxidation in the air. Thermooxidation rate is defined by the rate of oxygen diffusion into polymer. Constant of destruction rate in the air compared with inert medium increases, and activation energy decreases [216]. However, in some cases active energy increases this is connected with the contribution of physical phenomena of heat and mass transition together with chemical processes into the total kinetics of destruction. 

The rate of dehydrochlorination in nitrogen at 190tertiary chlorine and internal 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). 

VC, respectively. At these concentration levels the accuracy of the analytical techniques is not too good. In combination with the expected narrow concentration range, correlations between thermal stability and labile structures are uncertain when ordinary PVC is used. 

According to Berens (27). the concentration of monomer in the gel is only influenced by the relative pressure in the P/P -range of interest. However, by using polymers prepared at 11, 50 and 90°C, Nilson et. al. (28) could show that the monomer concentration is a function of both P/P and the polymerization temperature. By interpolating their data, it can be estimated that an increase in polymerization temperature from 45 to 80°C should increase the monomer concentration with about 25% at saturated conditions. This will tend to counteract the expected faster formation of labile structures due to the temperature alone which qualitatively is in accordance with the observation of an optimum polymerization temperature with respect to the thermal stability. 

If the labile structures are the main reason to the low thermal stability of PVC, Figures 3 and 4 should also reflect the concentration of the defects. In our previous work (7. 8). we showed that the rate of dehydrochlorination could be related to the amounts of tertiary and internal allylic chlorine. However, it is also likely that random dehydrochlorination will contribute to a certain extent (2. 3. 33. 34). According to our estimation, random dehydrochlorination could account for 10-15% of the initiation during degradation of ordinary PVC C2). It has been suggested that the stereo-structure should influence dehydrochlorination from ordinary monomer units (33. 35-37). The present samples also cover a change in polymerization temperature, AS-SO C. A comparison between the content of labile structures and thermal stability might therefore reveal an eventual influence of the tacticity. 

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... 

In a series of papers, Minsker et al. (see, e.g., ref. 48) have claimed that all actual labile structure should be ketoallylic ( (C=0)-CH=CH-CHC1 ). The formation of this group is suggested to occur by oxidation of allylic methylene groups during the production and storage of PVC. As an alternative, Svetly et. al. (49-51) have suggested that this structure should exert its adverse effect by... 

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. 

Reduction of the sulfoxide to the sulfide gives an acid-labile structure. 

ESI is very popular due to the absence of critical temperature. It is a soft technique since very labile structures can be carried as ions. 

Seitz and Kunz advanced the art of solid-phase synthesis by developing a novel allylic anchor (Hycron) whose virtues were exemplified in a synthesis of protected and unprotected O-glycosylated Mucin-type ycopeptides. Anchoring though allyl esters not only allows peptide derivatives to be detached without affecting acid- and base-labile structural elements, but also provides orthogonal... 

This reagent has become increasingly popular over the last 20 years for the nucleophilic introduction of the Si(SiMe3)3 group. A number of interesting, kinetically labile structural units have been stabilized with this substituent. However, it was found that, in some cases, even the bulk of the Si(SiMe3)3 group is not sufficient to provide the necessary steric protection.An example... 

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