Szwarc Termination

This model was introduced by Frisch, Schuerch, and Szwarc (38) who discussed the chirality of vinyl polymers assuming that effects due to the chain length and the nature of the terminal groups were negligible. Arcus (58) spoke of nonterminal chain segments. The infinite chain model was explicitely used by Natta, Danusso, Corradini, Farina, and others (30-32). Natta, Pino, and Mazzanti cited this model in 1957 (35). It represents a simplification of the one with different terminal groups used by Natta in his first paper on isotactic polymers. 

Some cationic ring-opening polymerizations take place without termination and are reversible. Oxirane and oxetane polymerizations are seldom reversible, but polymerizations of larger-sized rings such as tetrahydrofuran are often reversible. The description of reversible ROP is presented below [Afshar-Taromi et al., 1978 Beste and Hall, 1964 Kobayashi et al., 1974 Szwarc, 1979]. It is also applicable to other reversible polymerizations such as those of alkene and carbonyl monomers. The propagation-depropagation equilibrium can be expressed by... 

This attribute is justified by the fact that the growth of the macromolecules does not show any termination it stops when the monomer is removed, but is resumed immediately at the same rate when the monomer concentration is restored to its initial value. In some cases (e.g., the case of living polymers of Szwarc, obtained with anionic catalysts), it is exactly the same macromolecule which continues to grow, yielding polymers whose molecular weight increases with the polymerization time. 

Living polymers have been extensively investigated by Szwarc and coworkers [3], who have shown that the formed polymer can spontaneously resume its growth on addition of the same or different fresh monomer. Block copolymers are easily synthesized by this technique. The lack of self-termination is overcome by the addition of proton donators, carbon dioxide, or ethylene oxide. 

Similar instability of conjugated anionic systems has already been observed Lagendijk and Szwarc have reported that di(a-naphthyl)ethane dianion ( - ) in ether solvent undergoes scission of the central C-C bond vith formation of two monoanion (NN 2 —> 2N ) (26) To suppress the deleterious effect of conjugation and to obtain a stable dianion terminated PS or PTBS, at least one isoprene unit is to be inserted between the polyvinyl aromatic chain and the naphthalene end group this opportunity has been successfully applied. 

The configuration of each monomer unit in the chain is thus fixed at its point of entry. A similar transition state was suggested by Szwarc (106) where the incoming monomer molecule approaches above the plane of the last unit of the polymer chain. Only isoprene in the cis-form would be capable of reaction. 1,2 addition is also possible with this mechanism but its occurrence is considered less likely owing to the greater electron availability at the terminal carbon atoms, and to the lower stability of the olefin formed. 

The beauty of the Szwarc procedure is that the chains can be terminated by hydrolysis, oxidation, carboxylation with COz, and so on, to give polymer with the same kind of groups on each end of the chain. Also, it is possible to form chains in which different monomers are present in blocks. The only requirements are that the different monomers polymerize well by the anion mechanism and contain no groups or impurities that will destroy the active ends. Thus one can start with ethenylbenzene (S), and when the reaction is complete, add methyl 2-methylpropenoate (M) to obtain a block copolymer of the type... 

There is no doubt that the methyl methacrylate used by Rembaum and Szwarc (9) was not sufficiently well purified and the termination observed by them was due to some impurities. 

Another series of closely related reactions for which transition-state calculations have greatly helped in providing an understanding of the observed trends is the addition to deuterium-substituted alkenes. Szwarc and co workers (Feld et al., 1962) have determined secondary deuterium isotope effects for methyl and trifluoromethyl radicals by comparing the rate of addition to a terminal alkene with the rate for the deuterium-substituted alkene (25). Isotope effects for cyclopropyl radical addition have been measured by Stefani and coworkers (1970). For these three radicals a small inverse isotope effect (kJkK)... 

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