Ring-chain equilibrium mechanism

In the ring-chain equilibrium mechanism, Hnear polymers are in equilibrium with cycles. There is a critical polymerization concentration below which virtually only cyclic polymers are present however, above this critical concentration linear polymers are formed while the concentration of cychc polymers remains constant. The fraction of cyclic polymers present is largely dependent on the length and rigidity of the monomers. Especially at low concentrations, the presence of cyclic polymers can have a drastic influence on macroscopic properties such as viscosity. 

In the polymerization of cycloalkenes, not only linear polymers, but also cyclic oligomers are formed. These reactions are thus examples of ring/chain equilibriums, but their mechanism is not yet fully clarified. 

Brown And Slusarczuk Base Catalysis. The Brown and Slusarczuk [4] method of preparing cyclic PDMS involves an anionic i.e. base catalysed ring-chain equilibrium. The mechanism for the anionic polymerisation of D4 is shown in Figure 3. 

Both of the above mechanisms are complex processes with pol3rmerisation, depolymerisation, repolymerisation and chain transfer all occurring. Since all the siloxane bonds present in the reaction mixture will stand an equal chance of undergoing reaction whether they are present in D4, oligomers or polymer a ring/chain equilibrium is established. 

A particular mechanism of barbiturate ring opening has been observed for some barbiturates hydroxylated on the side-chain. The mechanism and relevance of this tautomeric lactam-lactone equilibrium are discussed in Chapt. 11. 

The polymerization process for nylon 6 consists primarily of the three types of reaction illustrated in Fig. 23.6. Each of the reactions is reversible, tvith the equilibrium of the products being controlled primarily by the concentration of water in the reaction vessel. The reaction is initiated by the hydrolytic ring opening of caprolactam to form 6-aminohexanoic acid, as shown in Fig. 23.6 a). Chain extension of the type shotvn in Fig. 23.6 b) dominates when water is abundant (10 to 20%) in the reaction mixture. At lower water levels (2 to 5%) chains grow primarily by the mechanism shown in Fig. 23.6 c). In order to limit the average molecular... 

However, the possibility of equilibrium between triazoline 4-carboxylic esters and the ring-opened diazo compounds (Section II,A,4,a) poses a problem as to which entity is actually oxidized. In fact, the open-chain tautomers formed from the addition of phenyl azide to acrylic and cinnamic esters are oxidized in good yield to the triazoles by permanganate (Scheme 119).32-28" 398 When the diazo compound is formed with ease, the oxidation is very smooth284 thus the diazo compound from the addition of phenyl or methyl azide to benzylideneacetone is oxidized rapidly in air to a triazole-4-carboxylic ester.284 A similar mechanism may be proposed to... 

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