Local chain configuration dependence of the mechanisms of analogous reactions of PVC. I. A conclusive study of the microstructure evolution in SN2 nucleophilic substitution

Synthesis of PVC-graft-PMMA Through Stereoselective Nucleophilic Substitution on PVC

“…Firstly, the disappearance of mm triads is linear throughout the interval of substitution studied (around 3%) which is understandable in the light of the mechanisms proposed in prior work. [22,26,33,34] Actually, the decrease of isotactic triad content was found to occur steeply up to conversion of roughly 7%; then, it becomes slower. The slope in each reaction period was demonstrated to be a measure of the superiority of the mmr tetrad in reacting, relative to the rrmr pentad.…”

“…However, in this case the very low maximum degree of substitution reached, around 2%, is indicative of the low nucleophility of the ethyl xanthogenate group compared to other nucleophiles studied in preceding work. [22,26,33,34] Nevertheless, this limitation does not affect the present work. On the contrary, it can guarantee of a high stereoselectivity of the reaction, because only the most reactive chlorines are engaged in the reaction.…”

“…Thus, mechanism C is to be viewed as a specific case of mechanism B because both may be assumed to depend mainly on the associated syndiotactic sequence. [26] One implication of these results is the ability to apply the aforementioned results to prepare graft copolymers of well-defined structure, based on PVC. The method of production of thiols from halides via xanthates through hydrolysis under basic conditions is well known.…”

“…In addition, the conformations of higher potential energy that are likely at mmr and rrmr configurations often exhibit higher volume and local motion facilities. [21,26] As to the associated tactic sequences, in particular those of a length above a certain threshold, these are chain segments of a more regular nature; consequently, they are liable to exhibit enhanced interchain interactions relative to the atactic parts. [21,26] As a consequence, the so-understood microstructure is to be assumed to create strong changes in the space arrangement of the chain thereby strongly influencing the interchain and the intrachain interactions, the local free volume, the local and cooperative motions, etc.…”

“…Also, these local discontinuities, especially when related to specific conformations cause the reactivity of a chemical group to be radically altered. [21,26] Basically, it has been demonstrated that: i) the reaction proceeds exclusively by the central CHCl group of either the last isotactic triad (mm) of isotactic sequences or the heterotactic triad (mr) at the end of syndiotactic sequences. Thus, as extensively argued, the very reactive sites are necessarily the local configurations mmr and rrmr at the end of isotactic and syndiotactic sequences, respectively; [26] ii) in order for these structures to react, the mm and rm triads have to take the GTTG -or GTTT conformation respectively, which involves the occurrence of conformational changes between the likely conformations in each structure, so as to ensure the availability of the above reactive conformations throughout the reaction process [21,26,27] and iii) although the GTTG -isotactic triad conformation is extremely reactive compared to the GTTT heterotactic triad conformation, the evolution of the ratio of mmr to rrmr with the degree of substitution may be easily controlled by changing the nature of the solvent or of additives, for the reactions carried out in solution [28] or in the melt, [29] respectively.…”