Oligomer formation, equilibrium cyclic

Ito, K., Hashizuka, Y., and Yamashita, Y., Equilibrium cyclic oligomer formation in the anionic polymerization of e-caprolactone, Macromolecules, 10, 821-824, 1977. 

It has to be pointed out, however, that the values of K, K2 and K3 depend on the initial concentration of water [2, 3, 5, 12, 221, 225, 232, 234, 235] (Figs. 29 and 30) even when the formation of cyclic oligomers is taken into account [15]. Giori and Hayes [14] demonstrated that the variation of the equilibrium constants with changing initial composition is due to the variation of the activity coefficient of water. Substitution of the water activity for the molar concentration in eqn. (132) yields much lower but fairly constant values of K2 (Fig. 30). Besides this, the variation of the dielectric constant with changing water content will affect the degree of ionization of end groups and, consequently, the proportion of the catalysed and uncatalysed reactions (115)—(117). 

A side reaction often occurring in polycondensation reactions that take place in the molten state is the formation of cyclic oligomers. As already mentioned (Section 5.2), this is an important group of reactions in the preparation of poly(ethylene terephthalate). The reactions that lead to cyclooligomerization are, of course, not degradative the cyclic material is in equilibrium with linear polymer molecules. 

The kinetics and thermodynamics of this process show close similarities to the polymerization of D4 initiated by CF3SO3H [3,4]. Both processes involve simultaneous formation of cyclic oligomers and polymer and lead to equilibrium, with similar proportions of cyclic-to-linear fractions [3-5], They also show similar thermodynamic parameters and a similar effect of water addition on the initial rate of polymerization. The specific feature of the polymerization of >2 is that cyclic oligomers 03 and D4 are formed simultaneously with the polymer fraction, but they equilibrate with monomer much faster than the polymer fraction. This behavior is best understood assuming formation of the tertiary oxonium ion intermediate, which isomerizes by ring expansion-ring contraction [3]. These kinetic features of the polymerization of make this monomer an interesting model for deeper studies on the cyclic trisilyloxonium ion question. 

Formation of cyclic oligomers in cationic polymerization of highly strained four-membered cyclic ethers, oxetanes, seems to proceed in parallel with propagation and virtually stops when monomer is consumed. The equilibrium is not attained probably because the active species of propagation are not stable and their terminated forms, like protonated polymer, formed after finished polymerization, are not effective in starting any cyclization reactions, unless a new portion of monomer is added. 

The equilibrium ROP of cydosiloxanes has also some limitations. The main restraint concerns the formation of cydic oligomers. Since the polymer yield in equilibrium depends dramatically on the size and polarity of substituents at silicon, the process is usually applied to the s)mthesis of dimethylsiloxane and methylsiloxane polymers and (their) copolymers. The equilibrium ROP is carried out in bulk as the dilution of the system favors formation of cyclics. The reartion cannot be used for the synthesis of polysiloxanes with a narrow MW... 

In 1976, Hory et al. [7-9] returned to the problem of macrocyclization equilibria, presenting a new mathematical approach called Direction Correlation Factor theory, to improve the agreement between calculated and experimental data for rings of DP < 15. Cyclization equilibrium constants ( ) for the formation of cyclic poly(dimethyl siloxane)s, PDMS, were calculated for 6 < c < 31 and compared with experimental data. Similar calculations were performed for cyclic oligomers of Nylon-6, but the agreement with the experimental data was not satisfactory for the smaller rings. Remarkably, Hory presented a positive citation of two papers of Andrews and Semiyen (see below) who reported that equilibrated Nylon-6 contains around 12 % of cyclic species in the melt at 500-560 K. Unfortunately, he did not comment, how this relatively large value fits in with the much lower values he had previously calculated from the J - - S theory (see text above and p. 328 in Ref. [5]). 

The effect of initiator type on the formation of cyclic oligomers manifests itself very markedly in the nonactivated anionic polymerization of CL [24]. When the process is initiated by the sodium salt of lactam, the formation of macrocycles is kinetically controlled the concentration of the cycles pronouncedly exceeds the equilibrium values, in similar fashion to cationic polymerization. In contrast, when the magnesium salts of CL or ethyl magnesium bromide are used as initiators, a considerable suppression of cyclization reactions was demonstrated for... 

An important source of experimental and theoretical studies of equilibria in ring formation is represented by the field of so-called macrocyclisation equilibria (Flory, 1969). Interest in this field appears to have been restricted so far to chemists conventionally labelled as polymer chemists. Experimental evidence of cyclic oligomer populations of ring-chain equilibrates such as those obtained in polysiloxanes (Brown and Slusarczuk, 1965) may be delated to the statistical conformation of the corresponding open-chain molecules (Jacobson and Stockmayer, 1950 Flory, 1969). In these studies experimental results are expressed in terms of molar cyclisation equilibrium constants Kx (14) related to the x-meric cyclic species Mx in equilibrium with the... 

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