Ring-opening polymerization zwitterion

Suzuki has shown that vinylcyclopropane 143 behaves both as an electrophile and a nucleophile and thus undergoes palladium-catalyzed ring-opening polymerization as shown in Equation (66). Vinyl cyclopropane 143 first reacts with palladium(O) to induce ring opening of the cyclopropane ring and forms zwitterionic TT-allylpalladium/molonate anion species. Repeated intermolecular attack of the malonate anionic moiety to the 7r-allylpalladium part through bond formation of an r/i -carbon atom affords finally the polymer 142. ... 

Vishwa Prasad, A., Yinghuai, Z. Syntheses of cyclic poly(lactones) by zwitterionic ring opening polymerization catalyzed by N-heterocyclic carbene. I. Annl Pnivm.. 2013,128, 3411-3416. 

Zwitterionic ring-opening polymerization for the synthesis of high molecular weight cyclic polymers 13ACR2585. 

Zhang Z, Cui D. Heteroscorpionate rare-earth metal zwitterionic complexes syntheses, characterization, and heteroselective catalysis on the ring-opening polymerization of rac-lactide. Chem Eur J. 2011 17 11520-11526. 

In parallel, NHCs were also used as initiators for the ring-opening polymerization of epoxides. This could be executed for the synthesis of polyethers of ethylene oxides. The living zwitterionic intermediate could further react with 8-caprolactone to generate di-block co-polymers in one pot. ... 

Kficheldorf et al. have reported that pyridines can be directly used as organic nucleophilic initiators for the metal-free - but noncatalyzed - zwitterionic ring-opening polymerization (ZROP) of pivalolactone (Scheme 7). Linear zwitterionic aliphatic polyesters, with a pytidinium and a car-boxylate group, in a- and co-position respectively, are formed in this way. 

Imidazole is a well-known initiator for the anionic ring-opening polymerization of epoxides. The photogeneration of imidazole and its subsequent use as an initiator for the polymerization of multifunctional epoxy-novolak resins have been reported by Nishikubo et Photolysis of the nitro-benzyl derivative 119 as shown in Scheme 36 generates imidazole, 120, which forms zwitterionic intermediate, 121, by reaction with the epoxide. The latter species is a rather weak nucleophile and the application of heat (120 °C) was required to drive the polymerization to completion. 

However, cyclic sulfonium zwitterions without two molecules of water are unstable and polymerize due to the nucleophilic attack of the anion (0 ) on the tetra-hydrothiophenium ring followed by ring opening with consecutive polymerization. Products of polymerization are low molecular weight linear polymers and cyclic dimers and trimers (6,7) ... 

Water opens the caprolactam ring producing aminocaproic acid, which is believed to exist as the zwitterion. The zwitterion interacts with and initiates the step polymerization of the monomer, with the ultimate generation of linear polymer molecules. In other words, the polymerization process involves an initial ring-opening of the monomer that is followed by the step polymerization. 

The compounds formed are converted to zwitterions with sodium methylate or ion exchangers, and these are then polymerized to polysulfide ethers with ring opening and loss of charge in what is known as death-charge polymerization, i.e.,... 

Weymouth and coworkers carried out kinetic and mechanistic studies of heterocyclic carbene mediated zwitterionic polymerization of cyclic esters [96]. Based on their results they proposed the following ring-opening mechanism ... 

A zwitterion that forms first is the key intermediate for the polymerization. The onium ring from 2-oxazoline is opened by a nucleophilic attack of the carboxylate anion at carbon 5 ... 

The mechanism of homopolymerization of epoxy resins initiated by imidazoles is now well established. A first nucleophilic attack by the unsubstituted nitrogen atom of the imidazole ring forms zwitterion 57 (Fig. 7, path C), which rearranges to an adduct by internal proton transfer. This is followed by a nucleophilic reaction of the newly formed unsubstituted nitrogen, opening a second epoxy group to give the 2 1 adduct that promotes the anionic polymerization of the epoxy. FTIR... 

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