Ring-chain equilibrium polymer

Figure I. Typical molecular distribution for a ring-chain equilibrium polymer...

This is a typical equilibrium reaction, which proceeds relatively quickly. Next to the water-polymer equilibrium, a ring-chain lactam-polymer equilibrium... 

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. 

Although some interesting synthetic processes of cyclic polymers have been addressed, the cyclization principle can be classified into two main methods as indicated in Fig. 2. One is the utilization of the ring-chain equilibrium that occurs in many polycondensation and ring opening polymerization. Another is the end-to-end cyclization method that can be used for synthesizing cyclic polymers from a/o-difunclional linear precursors. The ring-closure reaction by the end-to-end cyclization is further divided into intermolecular reaction and intramolecular reaction, i.e., bimolecular process and unimolecular process, respectively. 

Presently available information is insufficient to permit deciding whether the high molecular weight polymers obtained in ordinary polymerizations of cycloolefins consist of macrocyclics alone or mixtures of rings and chains. Nevertheless, the intentional introduction of known amounts of chain ends via addition of vinylenic olefins should result in ring-chain equilibrium of the type presented in Reaction 11. For this type of equilibrium, the weight fractions and size distributions of rings and chains present in the system may be compared with the Jacobson and Stockmayer theory (8). Experiments to test theory in this area are currently underway. 

The most important equilibrium is the ring—chain equilibrium (3) which expresses the relative thermodynamic stabilities of the lactam and the open-chain monomer unit inside the polymer chain. We have... 

Removal of water in the later stages is facilitated by reduced pressure. Since the ring-chain equilibrium for the 12-membered ring lactam is much less ring favorable than it is for the seven-membered lactam used to prepare nylon 6, there is only about 0.5% of the residual lactam left in crude nylon 11 product. This percentage of monomer has so little effect on the properties that it is normally left in the polymer. 

The ring-opening polymerization of D4 is controlled by entropy, because thermodynamically all bonds in the monomer and polymer are approximately the same (21). The molar cyclization equilibrium constants of dimethylsiloxane rings have been predicted by the Jacobson-Stockmayer theory (85). The ring—chain equilibrium for siloxane polymers has been studied in detail and is the subject of several reviews (82,83,86—89). The equilibrium constant of the formation of each cyclic is approximatdy equal to the equilibrium concentration of this cyclic, Kn [(SiRjO) J. Thus the total concentration of cyclic oligomers in the equihbrium is independent of the initial monomer concentration. As a consequence, the amount of linear polymer decreases until the critical dilution point is reached, at which point only cyclic products are formed. 

Heath RE, Wood BR, Semlyen JA. Equihbrium ring concentrations and the statistical conformations of polymer chains part 16. Theoretical and experimental investigations of cycHcs prepared from poly(decamethylene fiimarate) and poly (decamethylene male-ate) ring-chain equilibrium reactions. Polymer. 2000 41 1487-1495. 

Li SL, Xiao T, Xia W, et al. New hght on the ring—chain equilibrium of a hydrogen-bonded supramolecular polymer based on a photochromic dithienylethene unit and its energy-transfer properties as a storage material. Chem EurJ. 2011 17 10716-10723. 

Xiao T, Feng X, Ye S, et al. Highly controllable ring—chain equilibrium in quadruply hydrogen bonded supramolecular polymers. Macromolecules. 2012 45 9585-9594. 

Chen CC, Dormidontova EE (2004) Ring-chain equilibrium in reversibly associated polymer solutions Monte Carlo simulations. Macromolecules 37(10) 3905-3917... 

The first synthetic route explored to produce cydic polymers made use of ring-chain equilibrium. This approach involves the natural equilibrium that occurs between linear and cydic polymers during condensation polymerizations although, inevitably, this yields linear byproducts and broad polydispersities. As a result, precipitation or preparative gel-permeation chromatography (GPC) was required to obtain cyclic polymers of sufficient purity for further study. This approach is amenable to a broad range of polymerization chemistries, induding the preparation of cyclic polyesters [7,8], polyethers [9], poly(dibutyltin dicarboxylates) [10,11], and poly(siloxanes) [12-15]. 

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. 

Macrocydes are present as a more or less important fraction in polymer systems exhibiting ring-chain equilibrium. Macrocydes can also be synthesized on purpose by using different synthetic strat es such as ring-expansion polymerization (REP) or end-to-end coupling of a linear polymer. The advantages and limitations of these different methods will be examined and illustrated... 

There is, of course, an equilibrium between the monomers and polymer in the lipase-catalyzed polycondensation of dialkyl esters and glycols. In the lipase CC- or MM-catalyzed polymerization of dimethyl succinate and 1,6-hexanediol in toluene, adsorption of methanol by molecular sieves or elimination of methanol by nitrogen bubbling shifted the thermodynamic equilibrium (101). When dicar-boxy lie acid dialkyl esters and a , >-alkylene glycols were used as monomers, cyclic oligomers were formed from any monomer combinations examined (102). The yield of the cyclics depended on the monomer structure, initial concentration of the monomers, and reaction temperature. The ring-chain equilibrium was observed and the molar distribution of the cyclic species obeyed the Jacobson-Stockmayer equation. 

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. 

The formation of knotted macromolecular rings, was also claimed in polymer systems exhibiting a linear to ring-chain equilibrium or during the end-to-end ring closure of linear-chain precursors (McKenna ef aZ., 1987). However, the presence of catenaned and knotted rings could not be clearly demonstrated. 

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